W126 500SEL advice

[h17n dj]

Good day all,

My 500SEL has been at a reputable Mercedes specialist for a while now and they have rebuilt the engine. They are struggling to get it running sweet again so I hope you all can shed some light please... Symptoms include, running fine when cold and idling. When engine warms up and it is put under load, it is hesitating and running lumpy. I have been told the following has been changed/looked at after the full bottom end up rebuild using genuine parts-
Dizzy cap and rotor
HT leads
Cam sensor at the front of the engine
New injectors
New fuel filters
New spark plugs
Fuel pumps are working as they should be

Hopefully you can help sort this so I can get my car back. I’m sure I’ve missed a couple of things but if they are pointed out, it’ll jog my memory.

Thanks in advance.





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[Oldbarrack]

Which specialist ?

 

[h17n dj]

Which specialist ?

I’d like to keep them anonymous for the moment.



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[johnsco]

The problem in diagnosis is that a lot of things have all been changed at the same time.
Items like the dizzie cap, rotor arm, HT leads and spark plugs are critical with these cars.
Also injectors can have a significant effect.
Have original MB parts been used for these items ?
The fuel metering system is also seriously important.
You need a MB specialist with some serious depth of knowledge for these lovely cars.
Jaymanek (advertises on this Forum) is not a million miles from you.
I highly-recommend them from personal experience.

 

[Bellow]

From the reported symptoms.... I'd pull a spark plug after shutting it down from its lumpy running and look for soot blackening due to richness. If richness found, I'd investigate the temp sensor. Does the car have a MAF sensor? - they seem to be troublesome. Basic stuff that your garage should be aware of.

It can't be far off if it's idling well (min idle speed is a marginal condition that requires spark and fueling to be pretty much spot on and with good compression. Try reducing idle speed further and it dies - marginal).

 

[h17n dj]

Thanks for the replies so far. I believe they have looked at the fuel/air mixture several times. as previously mentioned, all items removed and replaced are genuine MB.

I have been informed they are removing the gearbox to check the ring with the teeth in between the gearbox and engine to check this. also there is a an electrical connection(?) that they are checking...

Not sure if there is a MAF sensor (I haven't seen one).

the car is with a local quite well-known MB specialist, who also owns a couple of W126's. he has his senior chap with a lot of knowledge (so I am told) looking at this and even he is puzzled with this one.

 

[ivan1983]

What year is your car?
I think the early models used Bosch D jet fuel injection whilst the later ones used k jet.

No MAFs on these engines.

You could have an issue with the cold start injector/ warm up regulator or as stated above temp sensor.


In any case you need someone who understands k/d jet systems , they are not complex but often misunderstood

 

[h17n dj]

My car is a 1991... Im not sure if its Bosch or K Jet...

 

[ivan1983]

It will be K jet, Bosch is the manufacturer.

 

[h17n dj]

Right thank you. so you are saying look out for the cold start injector/ warm up regulator or temp sensor?

 

[Bellow]

It sounds as though the engine is getting all the fuel it needs for cold start but isn't leaning off as the engine warms - like leaving a manual choke on. (There should also be a whiff of petrol at the tail pipe).
If however, the engine idles well when fully up to temp - I'd look elsewhere ie, to whatever controls transient (throttle) response. If it is Bosch K - I'd check the metering plate for freedom of movement. But, it is in the hands of those who know so maybe not quite so simple.
BTW, I can't imagine it idling well if the trigger wheel is somehow out of synch. Idling is too marginal a condition to tolerate large (signal, etc) errors.

 

[h17n dj]

Thank you for your response Bellow.

I know they've fettled with the metering unit and apparently that is set and correct. they thought that he metering unit was the problem so even swapped and reset it from another car (same type). They are trying everything at the moment but I guess I was hoping for a miracle suggestion from the forum in case the specialist are missing something...

 

[Bellow]

. When engine warms up and it is put under load, it is hesitating and running lumpy.

Without knowing if the above is due to mixture strength going rich or weak (if indeed it is varying) it's hard to know where to go, what to check.
I did a quick check and 'electronic ignition' is as much as I can find for ignition details. If it is still a clockwork system (probable) with timing control administered by the dizzy, I'd hook up a strobe and see that A) it is retarding correctly under throttle opening; B) isn't going full advance prematurely (weak/broken springs) - or at least that the variations are broadly in line with expectations. The more so if either richening or weakening isn't being detected. (Rich, they are 'bumbly' and tend to recover; lean they are more hollow sounding and prone to backfiring, dropping out and just quitting. Hard to tell from here, 500 miles away, and not in the same room.
That it idles serenely is a big clue to work from. The base settings are there.

 

[h17n dj]

Without knowing if the above is due to mixture strength going rich or weak (if indeed it is varying) it's hard to know where to go, what to check.
I did a quick check and 'electronic ignition' is as much as I can find for ignition details. If it is still a clockwork system (probable) with timing control administered by the dizzy, I'd hook up a strobe and see that A) it is retarding correctly under throttle opening; B) isn't going full advance prematurely (weak/broken springs) - or at least that the variations are broadly in line with expectations. The more so if either richening or weakening isn't being detected. (Rich, they are 'bumbly' and tend to recover; lean they are more hollow sounding and prone to backfiring, dropping out and just quitting. Hard to tell from here, 500 miles away, and not in the same room.
That it idles serenely is a big clue to work from. The base settings are there.

Thank you so much for the detailed help. I will be going to the garage this Friday and will pass on the information.

 

[ivan1983]

Without knowing if the above is due to mixture strength going rich or weak (if indeed it is varying) it's hard to know where to go, what to check.
I did a quick check and 'electronic ignition' is as much as I can find for ignition details. If it is still a clockwork system (probable) with timing control administered by the dizzy, I'd hook up a strobe and see.

D jet is for sure, k jet I'm not sure about but I think so.
Should be easy to confirm if a vacuum pipe can be seen going to the dizzy.

I'm fact it could even be a vacuum leak developing when hot. I assume all pipes and connectors are new?

 

[Lewis 2.3 16]

A friend of mine had a piece of the starter fly off and hit the crank shaft sensor, this caused a mis fire under load. But as you say they are remiving gearbox to inspect, would change the cps for good measure.

Also EHA valve could be a culprit

 

[Bellow]

D jet is for sure, k jet I'm not sure about but I think so.

I think, it will be clockwork.
Back then, 'electronic' meant eliminating the contact points (hurrah!) and fully mapped ignition (as we now know it) didn't really surface until electronic fuel injection was well established.

Should be easy to confirm if a vacuum pipe can be seen going to the dizzy.

Providing said pipe isn't missing....

 

[grober]

There are several things it could be-- either fuel/air or ignition related. One common suspect in these older mercedes is the EZL ignition unit. They rely on being heatsinked to the inner wing by a heat transfer compound and this eventually dries out leading to overheating. This usually manifests as a misfire as the unit warms up and eventually ends in complete ignition failure. Its difficult to detect because the EZL heating up often "mirrors" the engine heating up. The cars are often best diagnosed by an old crypton type oscilloscope engine analyser where all the HT firing lines can be displayed.
see:-
https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf

 

[h17n dj]

Cheers to everyone for their help so far. I have noted all of the above down and will mention it to the specialist when I go and see them on Friday and report back.

 

[Bellow]

It's going to be this....

There are several things it could be-- either fuel/air or ignition related. One common suspect in these older mercedes is the EZL ignition unit. They rely on being heatsinked to the inner wing by a heat transfer compound and this eventually dries out leading to overheating. This usually manifests as a misfire as the unit warms up and eventually ends in complete ignition failure. Its difficult to detect because the EZL heating up often "mirrors" the engine heating up. The cars are often best diagnosed by an old crypton type oscilloscope engine analyser where all the HT firing lines can be displayed.
see:-
https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf