1994 E200 W124 idle problem when hot

[marcintosh]

Here's the problem.
The engine starts fine (16 valve, 2-litre unit), idles at 2K for a few seconds, then drops to 1K. Quite smooth. When I put in gear, it drops to around 600 revs and is still very smooth.
After I have driven the car for a longer period of time and then pull over and put it in neutral, the engine revs at around 1K but it is quite rough so the entire car shakes. Again, put in gear and it drops to 600 revs and is smooth again. The car drives fine, no problems with accelaration, excessive fuel consumption or backfiring. It only happens when the engine is hot. I think it gets worse when the car has been stood for a while with the engine switched off and has cooled down to just below 80 centigrade.
I have replaced the air filter, fuel filter, plugs, checked the leads, just to exclude anything obvious.

Has anyone had a similar problem? Any ideas what I could check myself before I start dragging it from garage to garage? Any suggestions will be much appreciated.

 

[Ian B Walker]

Engine mountings or maf would be a guess from me.

 

[Parrot of Doom]

When you turn the engine off, does the car shake? I agree with Ian.

 

[marcintosh]

What I meant was that it idles so rough that it makes the car shake. Me English speak no good ;-)
When I switch to neutral, it runs smooth for a few seconds, then the engine starts to idle rough.

 

[marcintosh]

Initially I suspected the temperature sensor (the one for the ECU) as the problem with thr rough idle only occurs when the engine is hot. I replaced it as the part cost me only 12 pounds but it was a wrong guess, the problem still exists.

 

[jaymanek]

sounds like you have an air leak to me... dont think that engine had a MAF as we know it...

 

[grober]

sounds like you have an air leak to me... dont think that engine had a MAF as we know it...

If its a twin cam 111.* SERIES engine, I think it does have a MAF/MAS. see http://www.detali.ru/cat/oem_mb2.as...717.412413&CT=M&cat=19K&SID=07&SGR=040&SGN=01

 

[marcintosh]

If its a twin cam 111.* SERIES engine, I think it does have a MAF/MAS. see http://www.detali.ru/cat/oem_mb2.as...717.412413&CT=M&cat=19K&SID=07&SGR=040&SGN=01

Indeed mine is the 111. series. Does MAF stand for airflow meter? Is there any other way of checking it than by substituting for another unit?

 

[grober]

MAF is short for mass air flow meter. It measures the volume/density of air entering the cars injection system. usually tested by substitution.You have 3 choices
1. cleaning the existing sensor element with an appropriate solvent.
2. replace the sensing element with a new one.
3.replace the entire unit.
there are lots of detailed posts about this if you do a search. http://mbclub.co.uk/forums/showthread.php?t=25262
1 and 2 involve removing the unit from the car and partially dismantling it.
3 is just a straight swop but is more expensive.
I would try 1 first and then 2.
The MAF unit varies slightly from car to car (connector different for a start)so you need to get the exact replacement for your car. That engine injection system is very similar to the one found in the C Class 94-2001 so any posts relating to those should help. Thats if the maf is the problem??

 

[marcintosh]

Thanks for the tip.
It is mentioned in those threads that a faulty MAF should result in loss of acceleration and power. Mine doesn't feel slow or underpowered. It revves up nicely and it accelerates all the way up to just above 115 mph.
Then again, I have no comparison as I haven't driven another E200...

 

[jaymanek]

checked the leads,

how did you do that? i had a similar problem on my sec when i first bought it, i changed plugs, leads and caps and all was ok...

 

[marcintosh]

I checked them visually for cracks etc. I also checked that they were sitting properly on top of the plugs and in the distributor (or whatever that thing on the other end is called, coils?). I considered replacing them but they were around 50-70 pounds as far as I can remember and I decided to hold off.
What I was hoping to avoid was replacing the entire car bit by bit until I would find out what was wrong with it.
Is there any way of checking the HT leads properly?

 

[marcintosh]

One thing I forgot to mention. Occasionally when I stop and the car is in gear, revs will go up and down. Not sure how to describe it but it looks like the rev meter niddle bounces up and down. Then on the next stop it will disappear and the rev meter will be steady at around 600 rpm.
Perhaps it is connected with the sympthoms I descibed earlier?

 

[marcintosh]

So I decided to buy new leads. To install them I had to remove the tube connecting the air filter to the throttle body. When putting everything back together I noticed that it must not have been sitting properly inside the throttle as the bottom part of the tube was bent. As I was tightening the worm gear clamp, the end of the tube was sliding off the throttle creating a small gap and letting some extra air into the throttle.
Anyways, I attached everything as firmly as I could and took the car for a little road test. So far so good, it is driving smoother and it is idling nicely at 1000 revs when in N or P. Let's hope it was just that and nothing more serious.

 

[grober]

Sounds as if Jamanek was right that an air leak was the problem. Difficult to tell since you changed the ignition leads at the same time but if the problem has disappeared then a good result.:bannana:

 

[marcintosh]

I am not sure how long the previous leads were on the car. Could be that even since the car was new... so it did not hurt replacing them.
Thanks everyone for your suggestions and tips.

 

[marcintosh]

It just seems like one problem after another...
Driving to work this morning. Lot's of traffic. Whenever I put the car into neutral on traffic lights to see if it would idle better than before, the revs went up to 2000-2500. The problem did not disappear even when the engine reached it's normal temperature.
Revving it high in neutral for a second seemed to help but only till I stopped again.
What can it be this time round?

 

[Dieselman]

Sticking accellerator cable or throttle flaps. Possibly the cruise control/idle actuator sticking.

 

[marcintosh]

Actually, it turns out it was my own stupidity. When installing new leads last Sunday, I accidentally pulled out one of the vacuum pipes that is attached to the throttle body. I have no idea what it's called or what it does to the engine but after re-attaching it, the car ran fine this morning. The tricky bit was that I managed to pull it out not from the throttle body but from, what looks like another sensor, deep down inside the engine bay. Good job I spotted it sticking out...
I also had a closer look at the MAF. It is surprisingly small compared to pictures I of other MAFs I found on this site. The manufacturing date stamp on it says 98, which indicates that it was replaced after 4 years since the car was new. It also means it has now been 8 years since it was last replaced. From what I understand, MAFs don't last long and it made me think that it would be a good idea to replace it again, depending on the price obviously.
After all the fixes I have done, the car runs much better than before, however it doesn't idle exatly smooth. I decided to disconnect the MAF and run the engine without it for a couple of minutes. Then, after reconnecting MAF, I couldn't tell the difference, the engine seemed to idle exactly the same...

 

[marcintosh]

I have just been to MB in Sheffield to enquire about the cost of a new MAF. They told me that what I thought was the MAF, was in fact the intake temperature sensor. But this just gets better...

There is no MAF on E200 made in 1994!

Aparently all the magic happens inside the throttle body. The good news is that it is very reliable. It also means I don't have to spend 260 pounds in case my non-existent MAF decides to pack up.

The pipe that I accidentally disconnected is called "regeneration pipe" or "regeneration valve pipe" and aparently it has something to do with MOT checks. No clue what exactly it means.