Nick U

My 1999 E300TD starts and idles fine. When the car gets up to normal operating temperture it idles rough in "Park". When in "Drive" it is barely noticeable but still there. My thought is something which is affected by a warm engine such as a gas recirc valve, emission control or some such thing. The rough idle is very consistant predictable and only when the engine is warm. Ideas?

mi benz

I have the same issue on mine. I did a search and found that this is sometimes causes by improper torquing of the delivery valves in the injection pump. I haven't tried re-torquing mine yet, because I need to buy the seals and am thinking I'll do it when I replace my nozzles (car has 210K mi on original's).

EdzBenz

You may want to check the operation of your EGR valve. These are known to gum up and get stuck. This is done by removing your intake manifold.
If you haven't already done this, check with Speedtuning USA regarding a repogramming of your engine computer to delete the EGR valve. This will eliminate the EGR's operation and no longer cause you any problems.
There are also some threads on here discussing a DIY of EGR delete. I'm not electronically inclined, so I optioned with the tuning program.

DavidW210

Hello Nick U. Your problem sounds the same as the problem I have with my 1997 E300TD. The motor idles fine until warmed up, then begins to idle roughly. If I turn on the air conditioner, or put it in gear with my foot on the brake to stop movement (which puts a little bit of load on the motor) the idle becomes good again.

I have replaced the EGR valve, Mass Air Flow Sensor, air temperature sensor, fuel filter, accelerator sensor, reconditioned the injectors, checked the tappet spacing, and maybe some other things I have forgotten, without altering the problem.

Did you solve your problem? If so, can you please let me know what you did. Thanks.

Carsy

In the distant dim past I had a similar problem which was caused by worn timing gear. All ran well with a little load but was rough with out it.

DavidW210

Thanks Carsy. The mechanic currently inspecting the problem has said he suspects the problem maybe caused by a stretched timing chain. This is of course similar to what you have written.

I forgot to mention previously that I have also replaced the fuel pump and alternator. These also made no difference.

Nick U

An update: After almost 2 years I still have the problem of rough idle when warm. I took it to the mercedes dealership and their top mechanic spent two days of hard work trying to figure out what is going on. Their solution is to replace the computer ($1600) as the "think" it is the problem. Not going to happen. A second service center tried to find the problem. No luck.

After checking the engine computer diagrams, the ONLY thing that is affected by the computer in relation to air/fuel mix is the recirc valve. Since the rough idle happens exactly at 80C every time the computer senses temp of 80C SOMETHING happens which affects the idle.

I asked the MB shop what happens in the programing code of the computer when the engine reaches 80C and the looked at me with a glassy eyes....

Since a diesel is a simple engine I dont see how a airflow sensor, timing belt etc would affect the fuel/air mixture which I believe is the source of the problem. Since the EGR valve is the only thing which affects mixture AND is controlled by the computer, my plan is to have it replaced next week. Both mercedes shops say it works as it is supposed to, but my gut is that of all the solutions it seems the most plausible.

EdzBenz

Nick, sounds like a good solution. I'd suggest the route of having your computer reprogrammed to delete the EGR. Better performance, and you will completely eliminate this as a problem instead of just replacing parts. It won't cost you much money. Plus, you can option for a "performance" tune while the computer is away getting programmed.

TMAllison

If yours is a 606.962, you can delete the EGR with some simple off the shelf stuff from Radio Shack. About $10 worth of parts.

MB makes a blocking plate for the 606.910 and .912's.

When MB doesnt have a clue whats wrong on a 606 they almost always suggest replacing the ECU or doing a compression test. Good money for them which rarely resolves anything.

DavidW210

Hello Nick U. If this helps. I concluded that the problem was the EGR valve, and an MB agreed with me. Perhaps i shouldn't have made any suggestion as it may have affected his thinking.

After taking the old EGR valve out so that the new one could be put in, the mechanic said that the old one appeared to be working OK. The new EGR valve did not make any difference. This was disappointing because of the time involved, and also the cost of the valve. $650 in Australia(which I actually got from Germany because it was a lot more expensive for one in Australia).

It is difficult to understand how nobody can definitely identify the solution to this problem, including in my case, 2 different MB mechanics.

The mechanic who is currently working on it now suspects the problem is caused by incorrect timing. He is going away on holiday until the end of January. So I suppose the saga will continue when he returns. The car has been like this for a number of years.

Please keep me posted on any progress. Thanks.

nelbur

When you say you have a "rough idle" are you talking about a miss, or a surge. My 606 NA has what I would call a rapid surge at warm idle. I have heard it in quite a few old diesels, and while I don't know it's cause, I have assumed it is due to wear either in the timing chain or the governor, allowing the idle speed to sort of hunt for the correct RPM. But I am no diesel mechanic, which is unfortunate because I have four of them at present.

DavidW210

'Rough idle' is at normal revs, but uneven and irregular. It is not a regular miss as if a cylinder is not firing. It doesn't slow down or speed up. It idles smoothly until the motor warms up (after about 5 minutes), then begins to idle roughly.

Car has 392,000 kms on it, and goes well!

Carsy

Valve timing I reckon. OK with cold thick oil on timing sprockets, chain & tensioner but as the oil heats up & thins the wear shows up .

Will be interesting to hear your news at the end of Jan,

mi benz

If it were valve timing, then why would the problem go away under load? Seems like it would be much worse under load.

Carsy

With the chain tight on the sprockets the timing is near correct but with no load the loose chain slack retards the timing when on the back side of the sprockets .

I had it happen on geared OHV .

DavidW210

The problem was actually a faulty injector pump.

Before I went further along the incorrect timing path, another mechanic recommended a diesel mechanic he thought was good. Both were non Mercedes Benz. He was of the opinion that the problem had to be in the injector pump. I had already bought a used injector pump from a wrecker as another contributor to this forum had replaced his injector pump and fixed a similar problem.

The mechanic tested the used pump, and it was good. He fitted it to the motor; and the rough idle was gone!!! It is now delightful to stop somewhere and the motor just idles along smoothly.

It has taken nearly two years to identify the solution and fix the problem. The car has been to two Mercedes mechanics and six other mechanics.

The following has been replaced in the hope of fixing the problem: EGR valve, MAF sensor, accelerator sensor, Air temp sensor, fuel pump, alternator, Fuel filter, injectors reconditioned, glow plugs, copper washers in fuel lines, and a fuel shutoff solenoid. None of these altered the problem.

I hope this is of assistance to anyone with a similar problem.

Carsy

Thanks for the feedback.Much appreciated.

Did you hear why the injector pump was faulty ?

DavidW210

I sort of know what the problem with the pump was.

The problem was with the electronic part of it. The signal from the 'brains' of the car was being delivered OK, but the message was not getting through correctly from the electronic section of the pump to the mechanical action. The mechanical action was good.

The mechanics said they were surprised that the pump had an electronic part on it rather than being vacuum operated.

It was also difficult to fit because it had approximately 50 teeth on the drive shaft, and no TDC marker, rather than the regular type of drive with much fewer teeth and a TDC marker.

Carsy

Thank you.

In my opinion manufacturers rely too much on electronic controls.

Its absolutely crazy to see a modern diesel engines control wiring diagrams.There are 5 -15 inputs to every process.

In 1994 my Peugeot diesel had just one electronic control on the engine ... the fuel shutoff valve. It regularly topped 50 mpg ( Imperial)

OJ's DNA

Great thread, love reading the history and other owners experiences. I have the same issue of rough idle when motor is warm and put it in park with my new to me 98 E300, other than that all appears good. Still interested in the EGR delete TMAllison - I did this on my 95 E300 for the less than $10 in parts you recommended. Anyway after reading this I will not start replacing many parts and chasing this issue, thanks for the history and education guys.