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Garage => Mechanicals => Topic started by: dnaspazz on 23 November 2019, 04:22 PM

Title: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 23 November 2019, 04:22 PM
All,
This is a great site and I appreciate all of the info in it. I have read through so many posts, and it has been incredibly helpful for learning about my first Mercedes.

So, I bought a 1980 W116 280se, which is in very good shape. It was laid up awhile, and I have been having fuel problems that I have been slowly working through. I have replaced the pump, screen, accumulator, and injectors. For a bit, the car ran great with exception of  a very rough idle till the engine warms up. Then, the car lost all power (felt choked) and I cleaned the K-jetronic fuel distributor. Fixed that problem, but then then I could not get the idle set. Really rough start idle, but would run well on freeway and than stall the second I came to a stop. Started again fine, but needed more gas. Anyhow, anytime I would adjust the mixture it would be fine for a bit but  then go out of adjust. So, I am having the fuel distributor rebuilt currently.

Anyhow, I am wondering if I am going down the right road. At a start, the car spews black particulate/spray from the exhaust and I am sure it is running way too rich at a start. So, I while I have the distributor out, are there other points that make sense to tackle now? I  am not  an expert, but in the process of enjoying learning. My read of many of the posts seems to suggest that a bad WUR could explain the rough idle at start, but after that I am out of ideas. Any suggestions?
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 23 November 2019, 04:24 PM
Forgot to mention that it starts fine, just  idles very, very rough until it is warmed up. Also, at times it feels like it "surges" and I can hear the pump seems to pick up at that the same time.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 23 November 2019, 06:32 PM
Well, you're definitely on the right track.  A rebuilt fuel distributor AND warm up regulator are in order.  Unless is an odd situation always have those two things rebuilt at the same time. I hope you send them to Larry Fletcher.  There's just no one else in the U.S. that can do it right. ......even though many claim they can.  Anyway, While you're waiting on those to be rebuilt I'd definitely address the following:

-Check and correct valve lash
-Renew EVERY piece of rubber on the intake manifold....boots, idle air hoses, hose connectors, etc. 
-Remove distributor to clean/lubricate the centrifugal weights and check for excess wear in the lower housing bushing
-Make sure you have the correct NON RESISTOR sparking plugs (still available from Mercedes)  If there is an "R" in the spark plug nomenclature then they're the wrong
  ones
-Address the high tension leads if necessary, but do not buy Bosch or BERU
-Check ignition coil or just buy a new one if you're suspicious of anything
-Check and clean all electrical connections that look dirty or suspicious
-Test your coolant temperature sensor to make sure it's not bad.....or just buy a new one given the age of the original
-Make sure that your cold starting valve isn't sticking open
-Make sure that the catalytic converter isn't plugged up (if equipped)
-Make certain that both the supply and return fuel lines are unobstructed

The surging of the engine is likely what's causing you to hear the fuel pump surge, (alternator current surging) but if the new fuel pump is a Bosch you shouldn't be able to hear it unless you're close to it.  Also, did you inspect the inside of the fuel tank for rust and debris?  These fuel tanks are bad about rusting inside when left to sit. I'm certain someone will chime in and make mention of something I've left out.....and I hope they do so we can be thoroughly helpful to you!   
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: TJ 450 on 24 November 2019, 06:43 AM
Yes, spewing black soot out the exhaust is a good indicator that it's running too rich, surging and black smoke might also occur when driving.

Once you have the FD back, make sure the airflow meter plate is centered/ not binding and the arm moves freely.

When reinstalled you will need to adjust the mixture screw to get it to idle smoothly if this can be achieved.

Another trick if it’s running roughly and you suspect a mixture issue, you can manually lift the plate (if running rich) or push it down if running lean, if it smooths out then you know what the deal is.

For further troubleshooting you need to have a suitable fuel pressure gauge to measure the control/ system pressure. This is why you should ideally have both the FD and WUR rebuilt at the same time as this means they can be calibrated to achieve the proper pressures.

Try the rebuilt FD and see how it goes.

Tim
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: Peter on 24 November 2019, 05:46 PM
Also check and clean the EGR and congrats on your first W116 - the late model 280Se is a wonderful car,  ;D
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: daantjie on 26 November 2019, 03:22 PM
Also replace the fuel pressure damper at the fuel distributor.  You should see a small can shaped vessel in the circuit with the Warm Up regulator.

Here is a pic:

Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 27 November 2019, 08:29 PM
All, t

Thanks for the input, it has been helpful to figure out what I can tackle immediately vs having to figure out how to do it etc. I really appreciate the help and you taking the time bring a newbie up to speed. As I mentioned, I love the car- already did the injectors, shock absorbers, FD, seatbelts.  Gotta get the engine running tops, then the screaming speedometer, etc. Would be great to get it back to top condition. Super enjoyable to drive already!

A few quick follow questions- where is the location of the EGR and cold valve start on a 1980 280se? Ive been poking around trying to figure which they are, but not so simple. Once i find the EGR I am going to clean it, but it seems like there are not any new ones available if it has failed. Is that true?  I am going to get the WUR rebuilt, but have to get the numbers of the  current one to see if I can find a core so that I can keep driving while it is being rebuilt.


Also, what is the best way to confirm/clean the return fuel line from the distributor? I read just putting the fuel intake and return on loop and cycle carb cleaner through the lines for an hour? Is that the best way to clean that out?

One last question- to the right of the engine bay, there is cylinder shaped unit wrapped in foam, mounted on the top of the wheel well. It has multiple hoses going into it, including from the water pump and the fuel distributor.  What is that called?

Again- thanks!



Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 27 November 2019, 10:29 PM
The EGR valve will be located in one of the exhaust manifolds.  It'll also have a metal line attached to it from the intake manifold.  The EGR valve itself, part number
000-140-58-60, is sadly no longer available.  That's not to say that someone doesn't have a new one lying around somewhere.  They usually don't fail unless the diaphragm inside ruptures.  Then again, it's better that the EGR system is disabled, as they cause a lot of carbon build up in the intake manifold.  I know you're in CA, but given that your car is a 1980 you shouldn't have to worry about passing any kind of an emissions test. 

The cold start valve is located in the intake manifold.  It'll have a metal fuel supply line going to it from the fuel distributor.  There will also be a bi-polar electrical connection that plugs to it, as well.  It'll look like the electrical connection that connects to the warm up regulator.  The cold start valve should also have some blue or black plastic on it right below where the electrical connection goes. 

If you suspect varnish build up in the fuel lines then the looping of carburetor cleaner, lacquer thinner, or methyl ethyl ketone will definitely clean the lines.  Otherwise just ensure that they're not blocked or restricted in any way. 

The device on top of your inner fender well with the cooling lines and fuel lines going to it is a fuel cooler.  It was put in place to prevent vapor lock, and rarely gives any problems.  If any of the cooling lines or fuel lines going to it are suspect or hard/cracked definitely replace them.  I'm at odds about it being connected to the cooling system though, as the cooling usually came from the fuel cooler being connected to the A/C system's low side line. 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: rumb on 29 November 2019, 01:22 PM
I'd like to see a picture of the fuel cooler.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 30 November 2019, 08:44 PM
Guys, thanks a lot for the help so far. I got the new fuel distributor on, the car is idling at start much, much better and seems to be rounding smoother.  On flats, the car runs incredibly well. I still have the issue with losing power going up hill ( nochugging, backfiring, missing etc , the car just loses power and goes much slower), so I’m still trying to sort that out. It’s not the cat, I had that checked already.  Any ideas greatly appreciated.

Here is a picture with the fuel cooler in it.

 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 30 November 2019, 11:29 PM
Well, wait until the warm up regulator has been repaired before anything else is done.  And don't forget to check the most important things beforehand......things like valve lash adjustment, ignition timing, and insuring that there are absolutely zero vacuum leaks.  If these things aren't in order beforehand you'll chase your tail over and over again.  And don't forget that after a warm up regulator and fuel distributor rebuild the fuel mixture/co must be checked/reset as needed.  It's not a plug and play ordeal.  Each fuel distributor is tuned to each individual engine.  Granted, when new they were all adjusted alike (according to the car's domicile).  However, as engines wear, and as the cars find their final region of residence, the fuel mixtures are adjusted accordingly.  We could almost say that now two are alike.....or tuned alike, that is. 

So, I reiterate, check and adjust your valve lash, ignition timing, and repair any/all vacuum leaks, check and double check your fuel delivery/return system.  Dampers and accumulators can and will cause issues if they're compromised. Get your warm up regulator rebuilt.  Then an ONLY then can you start making adjustments. 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: raueda1 on 01 December 2019, 07:27 AM
Also replace the fuel pressure damper at the fuel distributor.  You should see a small can shaped vessel in the circuit with the Warm Up regulator.

Here is a pic:
Can you or anybody explain what this thing actually does?  And why there's a vacuum line on it?  I've been wondering for the last 2 years and others may wonder as well.   ???
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 01 December 2019, 11:40 AM
Working on the vacuum system now- and found the green and white vacuum check valve is blocked. I’ve looked for replacements but can’t find any. Advice on where to source?

Also found that the frequency valve isn’t “buzzing or clicking”. It tests at 3 ohms, but no sign of activity. Trying to figure what it actually does!

Update 1 hour later- So, I had removed the instrument cluster at the same time as having the FD rebuilt (because of the speedometer cable screaming)  Turns out, once I realized the frequency valve wasn’t firing but still at 3 ohms, I checked the fuse #4.  It was blown- a simple replacement and the car power is now way better ( at least manageable). Now back to getting the wur rebuilt, fixing the vacuum lines and figuring out the valve lash adjustment.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: daantjie on 01 December 2019, 04:39 PM

Dave to my knowledge this is in fact not a vacuum line but a leak line, in case you get diaphragm rupture inside this accumulator, so fuel does not go spewing out over a hot engine but rather just gets dumped into the valve cover.

Maybe more learned friends will chime in but I am pretty sure there is no vacuum working in on this unit.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 04 December 2019, 04:50 AM
Also replace the fuel pressure damper at the fuel distributor.  You should see a small can shaped vessel in the circuit with the Warm Up regulator.

Here is a pic:
Can you or anybody explain what this thing actually does?  And why there's a vacuum line on it?  I've been wondering for the last 2 years and others may wonder as well.   ???

^^What Daniel said.  It's not a vacuum line.  It's a leak line in the event of a ruptured diaphragm inside.  While the leak line will prevent raw fuel from spraying all over the engine bay and exhaust preventing potential fires, the other issue is that the inside of the engine will fill with fuel.  So if you ever notice your oil level rising it'll likely mean the damper has ruptured. Also, the oil will smell of petrol and be very thin in consistency. 

The damper itself is there to absorb pulsations from the fuel pump so there is a constant pressure on the fuel distributor and injectors.  If pulsations aren't alleviated then the engine will run rough, drive worse, and deliver poor fuel economy.  Some of the earlier models (D-Jet cars and some early K-Jet) had a primary damper at the fuel pump, as well.  On this one there is no diaphragm inside.  It's only a metal housing shaped to lessen pulsations. 

Regarding your valve lash adjustment, you'll need a valve cover seal kit, part number 110-010-08-30, and you'll need a special wrench, part number 110-589-01-01-00, to turn the adjusters on the pivoting assembly that the rocker arms pivot on.  The wrench is still available new from Mercedes for around $60-$65.  While doing this job, do take note of the rocker arm surfaces where the camshaft strikes them.  The rocker arms are notorious for becoming score due to the top metal layer being too soft (a factory mistake).  I make note of this because if this issue is left to it's own devices the scored rocker arms will damage your camshaft lobes.....and then you have to buy new rocker arm AND camshafts.  Rocker arms from Mercedes, part number 110-055-09-01, are $206 each as of right now, and if you had to buy all of them you'd have $2,500 just in rocker arms.  By all means seek out aftermarket if any of them need replaced.  Thankfully, any aftermarket rocker arms made for an M-110 engine are old stock and are made well.  Anything new or recently made be very suspect of. 

I make mention of these part numbers so you can research them on Google or take them to your local dealership and have them order the part numbers.  Most dealerships will have no knowledge of the older models or that they can even still get parts for them.  Heaven forbid you ask for a special tool.....they'll instantly tell you that it's no longer available.  So use the aforementioned part numbers to your advantage and outsmart the others. 

One other thing I want to touch on is sparking plugs.  Be certain that you have non resistor type sparking plugs.  You can still buy the correct, non resistor, sparking plugs from Mercedes for your car. Every place else will sell you the wrong plug.  There are two choices you can use for your car, a Bosch W 9 DCO or a W 8 DCO.  The W9 will be a hotter running plug, and is meant for normal driving because it has high heat dissipating characteristics.  Short jaunts to and from destinations in which the engine might not get fully up to operating temperatures would require the W8 plug since it won't dissipate as much heat.  The W8 part number is 003-159-11-03, and the W9 plug is part number 003-159-10-03.  In essence, if when reading the nomenclature on the side of the sparking plug you see an "R" in the description then it's the wrong plug.  The "R" stands for resistor.  Your high tension leads already have a resistor in the metal housing that goes over the sparking plug.  The ignition system does not need another resistor to overcome.  It causes rough idle, lessened fuel economy, and puts a strain on the coil and switchgear box (Ignition control module) long term.

Regarding your check valve, which one are you referring to?  There are a handful of check valves on your car all in different locations.   
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: rumb on 04 December 2019, 08:33 AM
Speaking of spark plug wire if you have aftermarket ones on they need replaced with correct ones.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 04 December 2019, 07:30 PM
U guys are amazing, thanks for all of the help!
Regarding the green and white vacuum check valve, it is located on the vacuum hose coming off the lower portion of the air intake. At idle, that hose is pulling constant air, but the g&w valve is completely blocked and there is no vacuum going from there into the 3 in 1 connector.

What ( who from) are the correct spark plug wires wires?
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 04 December 2019, 07:34 PM
I owe a picture of the fuel cooler
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 05 December 2019, 07:48 AM
You're fuel cooler has low side air conditioning lines running to it, not engine coolant lines. 

Regarding your check valve, in the first picture below it shows it, but it shows to be in a different location.  I' not certain that the parts catalog is always correct. So take it with a grain of salt.  The part number is 000-140-68-60, but that part number is no longer available.  It has been replaced by part number 001-140-16-60.  It's still available from Mercedes for around $10.00.  However, do make sure that someone didn't reverse the check valve first.  I've seen that happen on numerous occasions. 

In the second picture you'll notice highlighted parts.  Make sure all of these rubber hoses are new, as well. 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 10 December 2019, 08:53 PM
Yep, still working. I m getting stuck, since I cant find the info about how/where the original vacuum lines went. So, anybody have pictures of  1980 280se engine bay or a link the the manual that maps out where everything is supposed to go? The 2nd picture above looks almost like how  things are laid out, but it is also off- doesnt show everything that  is in my car!

Also, I am going to take my first shot at adjusting the valve lash this weekend. I am going to do it in the morning, on a cold  engine. A bit nervous, since I doubt this car has been adjusted in many years. Any advice is  greatly appreciated
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 11 December 2019, 07:49 PM
Since you have a CA emissions model you'll have to seek out an M-110 and/or W116 "Introduction into service" manual.  CA emission models are always a class all their own, and no other car in the world had the amount of unnecessary vacuum lines and connections.

Regarding you valve lash, make sure you have the wrench, a new valve cover gasket kit, and try to inspect the rocker arms when making the adjustments.  Each camshaft lobe has to be pointing away from the rocker arm before lash is measured.  Obviously, this involves having to spin the engine.  To do this either hook up a remote starter (the squeeze trigger kind), take a chance and do it by turning the ignition key, or put a 27mm socket on the crankshaft to spin the engine over by hand.  Just be sure you remove the socket before you start the engine......bad things will happen other wise. ;) 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 18 January 2020, 08:25 PM
Got some questions for next steps, but will give a complete update about what Ive done so far.  \
Just a reminder, I recently bought this CA 1980 280se (K-jetronic with lambda) and had a goal of fixing a very rough start idle, spewing soot on startup, and weak power/accelerations. 

Work done so far (learning on the go)-
1) new fuel injectors, 2) reman fuel distributor 3) reman WUR 4) Removed air intake manifold, had it cleaned (looks like new), 5) cleaned the throttle body, 6)put on a new intake manifold gasket (note- the car had an aftermarket gasket (Victor Reinz) that blocked the crank case breather hole  >:() and throttle body gasket, 7) a new fuel damper behind FD, 5) new fuel pump, filter, accumulator, and fuel tank strainer, 6) adjusted valve lash, 7) new 9 DCO plugs (old plugs were pretty fouled) Also cleaned the idle assembly, diverter valve, fuel lines, and anything else I could get to. Blew out fuel return to fuel tank (super easy!) Put on new hoses (had a hole in the breather hose!), all new rubber vacuum pieces, mostly new vacuum lines. I hooked up all of the original vacuum lines, per the CA emission tag on the car.  I did not clean the AAV ( >:(stupid of me, didnt figure out I shouldve until the mani was back on), install appropriate plug wires yet, nor have I replaced the coil. I measured the warm control pressure at 49 PSI/3.4 Bar and valve closed system pressure at 75 PSI/5.3 Bar. I didnt do the cold pressure right, but will do tomorrow. I am also going to try to use a digital oscilloscope on X11 Ports 2 and 3 to try to optimize the setting of the mixture. Then I will work on the timing.

Observations after this work- Overall better, feels like a well running car is in sight 1) Smoother start, 2) Idle not drifting all over from day to day, 3) When I start the car cold, it chugs a bit for 1-2 minutes, then speeds up quite a bit, 4) Is shifting gears early around 2200 rpm from 2nd to 3rd, 5) more power, but still not great, 6) in the driveway, the engine will randomly speed up or slow down ~100 rpm.

So here are my questions for the learned members...
1) How/when do you set the nylon idle screw, and how /whenis it adjusted relative the mixture adjustment screw and should the nylon screw be set after a cold start, or warm?
2) Timing- I know to set it at 3000 rpm first, but I am unsure what to set it to since we 93 gas in california? Any advice here for an old CA car?
3) Timing- On the harmonic balance there is a mark/pin around 8. What is that for, Is it TDC 0, or is it TDC at 0? (Stupid question I know, but I got confused by the manual...
4) Has anybody else worked with the lambda system? Is the oscilloscope the way to go, any advice appreciated...



Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 19 January 2020, 03:38 AM
Early shift issue....renew nylon propylene bushing at transmission end of linkage going from accelerator linkage to transmission, and then adjust this linkage until you're happy with how it shifts.  The book isn't always right. 

The nylon idle screw is used to maintain engine RPM.  Sometimes you might have to adjust idle in the middle of a procedure, but for the most part it'll pretty much be the last thing you do.  Always adjust the idle with a warm engine.  When the engine is cold it'll idle high until warmed up, so setting the idle when cold is useless. 

The pin on your harmonic balancer is only for the round diagnostic socket on the left inner fender well.  When the factory tool was connected to this diagnostic socket this pin would provide an RPM reading on the factory test equipment.  It has nothing to do with setting/checking your ignition timing.  Pay attention only to the numbered graduations on the harmonic balancer and the pointer mounted to the clock opposite the graduated numbers. 

Regarding fuel, all U.S. model M-110 engines were low compression engines.  They require regular unleaded (87 octane) only.  Putting a higher octane rated fuel in will cause the car to run poorly and/or consume an inordinate amount of fuel.  If anything you'd be wasting money on expensive gas. 

If you're good with an oscilloscope then go for it.  It'll be the most accurate, but remember that the car and it's systems are too antiquated to respond to any minute readings that are off one way or the other.  If an oscilloscope is used then close is good enough.  Otherwise, use any on/off ratio detector.  They give you the dwell, or duty cycle, of the o2 sensor.  You can even use a volt/ohm meter set to the mV position. 

Did you adjust your fuel mixture after the rebuilt warm up regulator and rebuilt fuel distributor were installed?  A lean mixture can cause idle fluctuations. 

Bear in mind that a U.S. 280se was only rated at 142 horsepower at 5,750 RPM.  At the time that was somewhat tolerable, but compared to anything new we'd drive today it'll seem like a slug. 

Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 20 January 2020, 06:58 PM
All,
UTN, thanks for the reply. California cars are bit hard to get info, so Im still trying to sort through everything. I got K jetronic with transistorized ignition.
Your right the transmission bushing is shot, and quite sloppy.
Regarding timing
Ive read all about set the timing at 3000 rpm @ 30 degrees (w/o vacuum), check at 1500 w/o vacuum, and then idle with vacuum should be TDC+/- 0-3.  With the car warm/hot I  got it set 30@ 3000rpm (the harmonic balancer was jittery?), 1500 looked right (HB was still bouncing around). I attached the vacuum lines, and 3000 rpm was about 38, but idle was still 10 BTDC.  I went through each of lines and there are no leaks. It just seems there is no retard vacuum at idle. At the distributor, I have 2 vacuum lines 1) is directly to throttle body to the back of of distributor, and 2) a yellow line that comes from a  3 way connector that is pulled from side of the manifold (below the throttle body). The other 2 lines go to the wur and a sequence of a 50 degree and 17 degree thermo valve. Any chance the CA car spec at idle is 10BTDC? I think the harmonic balancer shouldnt be jittery, so I am thinking about replacing the coil.

Regarding the mixture screw, yes I have been turning coutnerclockwise leaning it out bit by bit. In total, I have turned it counterclockwise about 3/4- 1 whole turn.  I found a point where it is running rougher/missing and went back a bit. I  played around with the oscilloscope, going to be harder than I thought. The issues are
1) i dont know if the oxygen sensor is any good- it was replaced before I got the car, but the 02 light is on in the dash.
2) the other site says that a regular multimeter with a duty cycle does not accurately measure the frequency valve modulation for 80-85 MBs. If I set it to a 50% duty cycle, (measured by the + probe in Port 3 of the X11) the car seems way too lean.

Has anybody else set the K Jet with lambda? How did you do it?

Still, lots of progress but every step is new learning experience!
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: rumb on 21 January 2020, 07:31 AM

You really need to remove the AAV and clean and test it in hot water.  They get all gummed up and will not close all the way. This is what raises the idle when cold and reduces when warmed up. As mentioned adjust after it warms up. There are directions on the interweb\utube on how to do.

You have a vac advance and retard distributor. The retard works if the AC is off and comes off when the AC is on. there should be a vac line going to a little dome shaped valve with some wires going to it. Just to insure the disti is fine put a mitivac on each line and verify the plate/ timing changes. Make sure the other vac line is going to the correct port on the throttle body, there s/b one above the plate and one below, I dont remember which one you should be using.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 21 January 2020, 08:33 AM
If you're seeing jitters on the harmonic balancer then there are either vacuum fluctuations in the engine (due to ill-seating or burned valves....quite common), or the distributor itself is worn....another common issue. 

Under the hood somewhere, probably on the drivers side, there is a little metal box with a bi-polar plug on it that the speedometer cable runs through.  It'll have a button on it you have to manually push in to turn off the 02 sensor light.  Otherwise there is an issue with the Lambda system's electrics or the new 02 sensor wasn't plugged in.

As far as how to test the Lambda, you're looking to see what voltage it's giving off after it get hot.  The frequency valve has nothing to do with the voltage the 02 sensor is giving off.  The voltage given off can be mathematically converted into a percentage duty cycle.  If you have specific questions about something in particular regarding Lambda please ask.  The chapter on Lambda is 30-40 pages long.  I don't want to post all of those pages......it's too time consuming. 

Below are lot of pictures that should answer a lot of your questions.  Do make it a point to buy yourself an M-110 engine repair manual.  I've been working on these for around 25 years, and even I have to use the manuals often.  It's a worthy investment.  I believe this same manual is also in this forum's library. 
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: dnaspazz on 26 January 2020, 10:24 AM
Again, thanks for info! I had bought an electronic version of the service manual, but it turns out not to be complete with all of the info for the 1980. Urgh. I went through the forum library also, but didnt finid the lambda specific stuff. I would greatly appreciate it if you could post the formula for calculating the duty cycle from the voltage. I am going to have to keep going on that- the car is running well (and not spewing soot anymore) but I know I dont have the mix right yet.

In other news, I put  the bushing on the transmission control rod lever. There was no old bushing left, and the new one made an such an enormous difference. Most impactful $2 spent yet. Also found me leak in the trunk- water is going in around the tail lights. So, going to be doing that next.
Title: Re: New to me 1980 280se, rough start, changing idle
Post by: UTn_boy on 26 January 2020, 03:02 PM
So.....were any of the previous pictures any use?  Were you able to straighten out what was amiss an/or understand better the set up for your particular model? 

Reading the o2 sensor duty cycle isn't how you set the mixture....it's what you adjust last as a fine tune to the mixture.  The O2 sensor’s sensing ability comes about by producing a small voltage proportionate to the exhaust oxygen content. For example, if the oxygen content in the exhaust is low the o2 sensor will produce a high voltage (0.90 Volts - Rich mixture) and if the oxygen content is high it produces a low voltage (0.10 Volts - Lean mixture).  Theoretically, the O2 sensor should cycle between 0.00 volts and 1.00 volts, but in reality it cycles between 0.10 volts and 0.90 volts.  It is easier for an O2 sensor to go from rich to lean than vise-versa.  O2 sensors tend to fail on rich bias. In other words, they tend to shift their cycling to the upper side or rich side of the voltage scale. 

An O2 sensor with a high voltage reading does not necessarily mean that the mixture is rich or high in fuel content. An EGR valve problem will send the O2 signal high as well.  So ensure that the EGR system is in good working order or completely disabled.  A stuck open EGR valve will create a lack of oxygen in the exhaust, since the re-circulating exhaust has all its oxygen already burnt . Your Lambda control unit sometimes uses the O2 sensor to check for proper EGR operation and sets a code if necessary. On a W116 thi "code" would be the illuminated o2 sensor light. So be aware of the fact that a vehicle might be running lean because the Lanbda control unit sees a rich O2 signal due to a defective (stuck open) EGR valve. Since the Lambda control unit sees a rich signal, it will try to correct with a lean command and try to lower the O2 sensor’s high voltage signal.

To directly answer your question, and to use the formula, you're going to have to know the pulse width the o2 sensor is giving off.  If you don't know it you'll have to hook an oscilloscope up to it to determine the pulse width.  To do this, connect the output of the signal to the input of an oscilloscope. (Be sure that the lead to the oscilloscope you're using is attenuated with a 10MΩ lead input) The oscilloscope screen will show a series of pulses oscillating at the frequency of the signal. Note the width, in seconds or microseconds, of each pulse. This is the pulse width, or PW, of the signal.  Next, you'll need to calculate the period (T) of the frequency (f) using the formula T=1/F.  For example, if the frequency is 20 hz, then T = 1/20, with a result of 0.05 seconds.  You now know what the pulse width (PW) and the period of the frequency (T) is.  At this point you can now determine the duty cycle (D) through the formula D = PW/T. As an example, if PW is 0.02 seconds and T is 0.05 seconds, then D = 0.02/0.05 = 0.4, or 40% duty cycle, or on/off ratio. 

Bear in mind that the period of the frequency will change with engine RPM, so do one test at idle (750 RPM) and one at, say, 2,000 or 3,000 RPM.  You'll have to recalculate the period of the frequency with the different RPM's. 

The aforementioned is a very time consuming way to do what you're wanting to do, but will work just fine.  If you want to save time maybe consider using a hand held duty cycle tester by tapping into the pins 2, 3, or 6 of the diagnostic test plug.  I don't remember which of those is ground, hot, and from the o2 sensor.  You'd have to play around and figure it out.

Last, When you are adjusting mixture and go a certain amount (rich or lean) it'll stumble making you think you've gone too far.  Sometimes that's true and sometimes it's not.  When it stumbles goose the throttle and see if it clears up.  If it does then you haven't reached a maximum or minimum.