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wrenched on my 450SEL yesterday

Started by Papalangi, 01 February 2007, 12:45 AM

Papalangi

I worked on my poor running '76 450SEL yesterday.

It's had a lumpy idle, sometimes backfired when pulling away from a stop and would run on when shut off.  Warm starts were hit or miss.

The timing is at 5BTDC.  I've fixed all the vacuum leaks I can find except one in the climate control behind the dash.  The master cylinder was leaking between the booster and the cylinder.  There was a big leak in the fuel vapor recovery system. I've pulled plug wires one at a time looking for a misfire and found nothing.

I finally had a chance to get out my new fuel pressure tester and the CO meter yesterday.

Cold control pressure was .8 bar and ramped right up as the heater element warmed up but it stopped at 2.5 bar.  Disconnecting the full load enrichment vacuum line would change it a bit but I couldn't get it anywhere near the 3.4 - 3.8 bar spec.  By closing the pressure tester valve I can read system pressure which is 6.2 bar, a bit higher than spec but I don't think it's a problem.

A quick check of CO showed 8.5% at idle. Yikes!  No wonder it runs like crap.  By throttling the pressure tester valve I could simulate a control pressure of 3.8 bar which would bring the CO to spec (0.5% - 1.5% at idle).  Since I don't have a spare fuel filter or warmup regulator, I adjusted the mixture screw to bring it to 1.5% at idle.  WOW, what a difference.  Smooth idle, maybe a bit more power.  Starts right up warm or cold.

I haven't done a fuel flow test but assume I should change the fuel filter because I don't have enough flow to make proper control pressure.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

oscar

Nice work Papa,

Is the 76 a d-jet or k-jet (can't remember the change over year).  If it's a d-jet I assume you just turn the mixture knob on the ECU? Which way to reduce? If it's a k-jet what did you do?

I experimented with turning my CO control on my d-jetted 350 when I had running issues last year but was hesitant to muck round too much and ended up leaving it where it was. I'll have to get my mechanic to check with a sensor, I assume the CO sensors are expensive?
1973 350SE, my first & fave

Papalangi

It's a K-jet but before the O2 sensor came onboard.

I have this CO tester. It works great but seems a bit cheap when you first pull it out of the box.  I worried about it blowing away it's so light.

http://www.aep.bigstep.com/digital1.htm

I also have the eezibleed and it works great also.

They are made in the UK and I don't know if they are available down there.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

Papalangi

I changed the fuel filter on the 10th and didn't notice much of a difference.

Yesterday I hooked up the pressure tester and CO meter again and started investigating.  Long story short, it turned out that the vacuum lines to the warmup regulator were backwards.  There was a change in the regulator in 1979 and I must have a later regulator.  I had them connected for a 1976 regulator.

When the vacuum line is pulled, the control pressure should go down, making the mixture richer.  Mine was opposite.  Lot of head scratching about that until I found a picture that showed the difference.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

oscar

That's a significant discovery.

What was the result after you changed it over?  Big difference?
1973 350SE, my first & fave

michaeld

Papalangi,
People who have their own CO meters are my heroes (it used to be Superman, but, you know, I never saw the guy anywhere). 

I'm glad you were able to use it to locate your issue.  That oughter help you pass smog.  when your 800 times over the limit, some people start to frown.

Vacuum issues really DO suck, don't they?

Papalangi

A guy at work calls me Superman every time he sees me.  Hmmm.

I haven't gone for a test drive yet but expect it to run better.  It was down on power after I set the CO before I replaced the fuel filter.  I kind of expected that since the system is designed for a specific pressure.

It's old enough that it does not need to be tested but I still like to do my part and keep my cars running their best.  My 1976 280C set off the vent fans and nearly set off the evacuation alarm at the test place before I got it dialed in.

I still have a vacuum leak problem.  It is running about 12"Hg at idle and I'd like to see closer to 15"Hg.  The upper and lower halves of the intake are connected by rubber tubes and there is a Y shaped tube assembly that runs from the aux air valve to under the fuel distributor.  The Y shaped tubes are as hard as ceramic and I can't see the connecting tubes.   By putting the end of a 3' length of 1/4" rubber hose in my ear (very attractive by the way), I can hear it leaking just behind the throttle valve.


Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

john skene


Michael, I have a '76 450SEL, the first of the antipollution models having retarded ignition etc, and this week when trying to find a leaking door locking module measured the vacuum on a number of lines at 12.5 inches Hg. I then measured the manifold vacuum direct at 12.5 inches too, so it wasn't a steady vacuum module rubber leak causing the low (?) reading. These readings were at idling speed.

I thought 12.5 inches was a rather low reading after having my earlier cars give readings between 17 and 21 inches (MG, Chrysler Plymouth Jaguar, Holden etc) but the reading was absolutely dead steady, so what vacuum level should I expect with my '76 model?

Papalangi

John,
I don't think a Mercedes would be all that different from any other car.  That's why I'd like to see at least 15"Hg and maybe even 17"Hg at idle.

My door locks are very slow so I have them disconnected for now. I also have the dreaded automatic climate control and it can be painful listening to the slow creaking as the flaps move around.  Sometimes it moans behind the dash until I bring the RPMs up a bit.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

Papalangi

OK, I've driven it for two days and it's back to what I expect a 31 year old car to drive like.  Much better pickup, no farting or hesitation and good cold starts.  Now to address the vacumm leaks under the intake.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8

Andrew116

Question... vacuum leaks can affect idle speed if the engine is Jetronic and not carburated?
I know it's off topic and sorry about that...

Andrew

Papalangi

Quote from: Andrew116 on 27 February 2007, 06:15 AM
Question... vacuum leaks can affect idle speed if the engine is Jetronic and not carburated?
I know it's off topic and sorry about that...

Andrew

Andrew,
I don't think it's off topic as part of getting a stable idle and CO reading is making sure there are no vacuum leaks.

My engine uses the K-Jetronic or mechanical fuel injection.  It "measures" the air flow via the plate in the fuel distributor.  At the other end of the arm is a pin that uncovers calibrated slots that allow fuel to the injectors.  Any air that sneaks in after the plate is unaccounted for and makes the mixture lean which affects idle and drivability.

In a carburated engine, as the air passes through the venturi in the carburator, it creates a low pressure that draws fuel through the idle and main jets.  Again, any air that sneaks in after the venturi is unaccounted for and causes the engine to run lean.

The D-Jetronic injection uses electronics to measure the air flow and fires the injectors according to a prerecorded scale.  Same thing here, any air sneaking in after the measurement will upset the balance.

A modern fuel injection system is closed loop, meaning that it measures both the air in and the resulting emissions.  It can compensate to a small extent for leaks but they will still cause trouble.

Michael
'83 300SD, I'm back!  It's the son's new car (12/2020)
1976 450SEL, 116.033  Sold it to buy a '97 Crown Vic.  Made sense at the time.
1971 250C, 114.023
1976 280C
1970 250/8