Author Topic: A few q's regarding my engine.  (Read 3392 times)

oscar

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A few q's regarding my engine.
« on: 05 July 2006, 02:45 AM »
I recently posted some q's re igniton woes.  Well the other day I checked the dwell angle (after I opened the gap) and found a marked improvement in power by reducing the dwell to 11.  The recommended is 30-34 and I would've assumed that at 11, perhaps the spark may not be around long enough to ignite the fuel mix.  Then again, the spark may be more powerful and whilst less sustained, igniting the mix faster.  Any thoughts?  I'm at a happy medium at 28 at the moment.  The points' contact with the distributor's shaft was worn on the leading edge as well.

I also finally managed to aim a timing light and view the markings on the crank adequately thru the 1" square space available thru the fanbelts, distributor, oil pump etc.  I found the igniton was about 10o BTDC.  I changed it to 5o.  I also had a bit of a play moving the distributor backwards and forwards trying to guage the effects in the difference in timing.  ??? It went faster than slower.  Slight difference in sound too but I couldn't hear a "ping".  I've never heard it before and couldn't make it happen at 3000rpm.

Re: the MPS, is this it and should it make a noise when starting the engine and after turning it off?  Never noticed it before and it's audible especially after the engine stops.


Before I go, some terminology. 
With timing, is "advanced" the same as "before"?
Does ATDC= "Advance of Top Dead Centre" or "At Top Dead Centre" or "After Top Dead Centre"?
Which way does "retard" refer to?
Thanks.
1973 350SE, my first & fave

s class

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Re: A few q's regarding my engine.
« Reply #1 on: 05 July 2006, 04:41 AM »
BTDC = before top dead centre (in other words "advanced")
TDC = top dead centre (in other words zero timing at idle)
ATDC = after top dead centre (in other words, "retarded")

What ever your timing is, to "advance" it means to make it "more BTDC" or equivalently "less ATDC"
"retard" is the reverse.

Generally, engines require to have a timing somewhere BTDC, because the combustion process isn't instantaneous.  In other words, you must start the combustion process (fire the spark plug) BTDC - before the engine reaches top dead centre, so that it works out that the maximum combustion pressure occurs just after TDC, as the piston starts moving down again. 

Your distributor provides for mechanical (or centrifugal) advance as well as vacuum advance.  THis is so that at higher rpm, you get an even higher BTDC firing angle.  This is needed because the engine is going faster, but the combustion still happens at the same speed, so you need to start the combustion even earlier in the engine rotation so as to still get the maximum combustion pressure at the right instant.  With vacuum and mechanical advance working, you should see the timing increse from say 5deg BTDC up to about 20 to 30deg or even more at 4000rpm.

Ping will result if you combust the fuel too early - in other words timing too advanced.  A tendency to ping is exasperated by the following : high engine load, high combustion chamber or valve temperature, too lean fuel ratio (which raises temp), or too high a compression ratio, or usually a combination of several of these.

You are unlikely to hear ping at idle because there isn't enough engine load.  If you were to time your engine at say 20deg BTDC and try driving it, you would definately hear pinging when accelerating.  And don't try this - pinging damages piston faces and valves. 

If you play around with the timing in the 5 to 10 degree BTDC range, the most obvious difference you will feel is the acceleration performance.  Generally (if your mechanical and vacuum advance functions are correct), as you increas ethe advance, the performance will be better, until you start to overdo it (usually around the 8 to 10 degree BTDC region) you will notice a drop in performance (or pinging) because the combustion is happening whilst the piston is still moving up.  Yo dont want to run in this region. 

I cant help youwith that photo - it looks like a D-jet peculiarity.  I only do K-jet  ;D

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

oscar

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Re: A few q's regarding my engine.
« Reply #2 on: 05 July 2006, 05:52 PM »
That's tops Ryan.

Simple questions I know but I had to make sure.  :) It's been about 15 yrs since I've timed an engine due to everything having transistorized ignition or being new with servicing included.

The MPS is that and is a Manifold Pressure Sensor.  I reread Denis' posts re megasquirt.  I'm hoping Denis might tell me what the MPS should do and whether the "flap-like closing" sound is normal.  That is of course if Denis sobers up, puts down the wine and stops watching football and cycling.  ;D   I can't see that happening soon, he's got too much to celebrate at the moment.

As far as the dwell angle's concerned.  I've only set points gap before with a feeler gauge.  It's got me stumped as to why a smaller angle lead to an increase in  power.  I just may leave well enough alone and take heed in knowing the posted recommendation is 30-34.  Then again if someone can convince me that 11 wont do no harm I'll happily change back.
1973 350SE, my first & fave

Denis

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Re: A few q's regarding my engine.
« Reply #3 on: 06 July 2006, 04:41 AM »
Hi Oscar and all


Hey-hey, we won against Portugal last night ! and on sunday, we will take up Italy. If we WIN, we become the BIG ONE - World soccer champion...which usually creates a huge party, as in 1998 §


Ok then, the MPS is a great device but at the age that they have reached, any used device is metal-fatigued. The device should hold a vacuum and not leak - use a vacuum pump - or suck on its end  ::) and you "may" hear the noise that oscar is mentioning. I am not sure that the noise is a good sign  ??? the metal diaphragm inside is quite delicate and if you remove the MPS, handle it very carefully.


The problem with this device is the replacement cost should yours die ... in some cases, it costs more than the car is worth  :P

A master tinsmith ??? could make a part for it but I really mean an EXPERT craftsman. And its calibration is very difficult. Electrically, this is a type of variable transformer that feeds the ECU depending on engine vacuum.


Touch wood


Denis

Paris, France

Denis

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Re: A few q's regarding my engine.
« Reply #4 on: 06 July 2006, 07:52 AM »
Hi Big Richard

You are absolutely correct BUT  ::)  ::)  ::)

It isn't that simple  :(

That MPS incorporates several interacting functions that unfortunately are not easily implemented with say, a MAP sensor.

For starters, the MAP sensor emits a voltage while the MPS deals with current  >:(

Worst still, I dont know where the operating theory is written out ... Bosch is secretive.

Denis

Paris, France

alabbasi

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Re: A few q's regarding my engine.
« Reply #5 on: 06 July 2006, 03:38 PM »
I think Nutz has a bunch of info on D-Jet trouble shooting at his site and at the benzworld forum. You may want to contact him offline. Everytime I go to a junk yard and find a MAP sensor a Computer for a D-Jet I grab it. I learnt that the hard way when my W108 gave me such a hard time.
With best regards

Al
Dallas, TX USA.

oscar

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Re: A few q's regarding my engine.
« Reply #6 on: 09 July 2006, 12:37 AM »
Thanks all! 

Alabassi, I've seen some of Nutz's posts thru links or a web search but have neglected to save any.  Thanks for the reminder, I'll source out his info .

Another point worth mentioning is the way the car started last winter and twice this winter.  The car will rev up to <2000rpm then drop to below idle and build up to <2000rpm again for a second or so then drop again.  It does this until ?? the engine warms up or the vacuum builds up. (Or both).  On a normal cold day the engine revs to that same <2000 rpm mark then slowly comes back to normal idle at approx 750 rpm after the engine warms up.  Temp's outside have been between 0-5oC.  Sometimes the car is garaged, sometimes not.
What's your guess? 
MPS, cold start valve ?? 

I'll have to try that MPS test (cautiously) you mention Denis to see if it holds a vacuum.

I'll be cheering on the French despite my town's strong Italian influences (Griffith).   If for no other reason because the Italians knocked us out in the last few seconds of regulation time.  Actually Denis, I wish the French were playing a squad of referees.  They became the Aussie's biggest opponents in this tournament.  >:(
Go the French.

1973 350SE, my first & fave

oscar

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Re: A few q's regarding my engine.
« Reply #7 on: 09 July 2006, 04:06 AM »

I'll have to try that MPS test (cautiously) you mention Denis to see if it holds a vacuum.


 :D :D :D  Listen to me laugh :'(     My MPS is cactus!!!! 

A suck or blow and air passed thru the diaphragm.  I could hear it and feel it escape thru the two slotted vents on the opposite side of the MPS.  My guess is the hole in the diaphragm is small enough to allow it to operate but it doesn't hold a vac.  Mmmm, what to do.  Nothing yet.  i think it's been this way for quite some time.  Before getting hold of a replacement, do you think solder could be used to repair the hole?  I'm not forking out the $$ for a newbie until this one is completely battered and rendered ratshit.  Nothing's deemed doomed until its replaced. :) :-\
1973 350SE, my first & fave

Denis

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Re: A few q's regarding my engine.
« Reply #8 on: 09 July 2006, 04:08 AM »
Hi oscar and all

Quote
The car will rev up to <2000rpm then drop to below idle and build up to <2000rpm again for a second or so then drop again.  It does this until ?? the engine warms up or the vacuum builds up. (Or both
)

Hmmm...I know this symptom, unfortunately it is MPS-related. My car would not run at all when the MPS membrane broke. Having nothing to loose, I used a dremel to cut the four pins that hold the two halves together, removed the membrane in the front part and carefully soldered the broken copper diaphragm. I then consolidated the "repair" (the solder does not last long by itself as the diaphragm just cracks elsewhere) with a thin film of RTV sealant.

The car runs now, uses too much fuel (calibration of MPS is out) and does surge as you describe at cold startup. That is WHY I purchased the Megasquirt kit.

Got to go now, soldering Megasquirt is planned for today.

Hang in there ! touch your zebrano  :P

Denis

Paris, France

Allez les bleus !!! (bleu is blues, the colour of the french soccer team)

Denis

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Re: A few q's regarding my engine.
« Reply #9 on: 09 July 2006, 04:14 AM »
Oscar, my condolences  :(

Here are your options :

1 - a new MPS (sit down and take tranquilizers before calling MBZ.
2 - get a used one (these are model-specific items) - but NOBODY can tell how long tehy will last - rmember they are as FATIGUED as yours was ...
3 - Be daring and solder one up as I suggest (stopgap measure)
4 - Get a Megasquirt and benefit from my experience in setting this up for a 350 but step  3 or two may still be required since this is a "July" project.

I have seen W116s in the car graveyards due to the MPS dying.

Good luck


Denis

Paris, France

oscar

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Re: A few q's regarding my engine.
« Reply #10 on: 09 July 2006, 04:36 AM »
Thanks Denis.

$750 US for a new one is the cheapest I've seen.  Maybe one day, especially if it would be the last MPS the car would need if this first one lasted 380,000km.

I don't know enough about EFI to be able to upgrade the lot.  Although my knowledge base has picked up somewhat since your dog350 thread.   But like everybody else, I'm waiting to see how you go with your install.

So soldering is the only option at the moment.  I'm looking forward for the challenge believe it or not.  A thin copper diaphragm?  Perhaps a low melt solder.  I may attempt it this week and make a thread of it. We shall see.  Thanks for your advice.  :)

1973 350SE, my first & fave

Denis

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Re: A few q's regarding my engine.
« Reply #11 on: 09 July 2006, 11:26 AM »
Hi Oscar

Email me privately for details.

WE are going through the same problem area.

If you can scounge a use done somewher for 50$ it light help you...

Denis

Paris, France