I have a 1994 E320 sedan and have had the wiring harness replaced. After a couple of months the "Check engine" light came on (again). I had it checked and it indicated an EGR problem. The mechanic told me not to worry about it because the car has 155,000 miles on it and it would be expensive to replace.

Why would it be expensive to replace and where is this valve located?

Because the valve cost $200 plus labor to change. If the car is running right, why bother? It's on the left rear side of the engine.

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Techs have frequently posted that an egr problem is not usually caused by an inoperative valve (a sticking valve, or a valve stuck in the open or closed position), but rather a problem with a clogged hot inlet pipe that goes from the egr valve to the intake manifold.

This problem is well documented on M104 engines, and a mechanic who specializes in Mercedes should be well aware of the problem.

Steve Brotherton (Stevebfl), has an article posted written under the DIY link at the top of the page under Evaluating Electronic Engine Controls that illustrates the problem, with a picture, at the end of the article.

The plug is caused by a build-up of carbon from oily pcv vapors, and always occurs where this pipe enters the intake manifold.

There are two options to solve the problem:

1. remove the build up by inserting a long wire, like a speedometer cable, and reaming it out with a drill motor.

2. replace the hot inlet pipe with a new one. They have been redesigned, and the pipe is a little longer where it enters the intake manifold, solving the problem permanently.

There is still a chance your egr valve is bad, and it can be designed by pulling a vacum with a hand held pump.

Use the search function to find posts on the procedure to check if a valve is bad or not.

There are also numerous postings on guys that have gone the speedometer cable route.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I think your mechanic is wrong in that the valve is not really an expensive part, and it is not hard to replace.

If it is bad, the valve is located on the passenger side of the engine, near the back. It looks like a flying saucer with a rubber hose going to it, and a small diameter metal pipe exiting to toward the back of the engine.

Last, if you don't fix the problem, you will not be able to pass a smog inspection, should your state require one.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Thanks a bunch. I will check this out this weekend. Strange thing happened this morning with it. It was 13 degrees F. and the check engine light did not come on as it has done the past few weeks. However, there was very little power and took an effort to accelorate. It would miss intermittently at idle. After a while that cleared up and the light stayed off.

This car has a few miles on it but the interior, exterior and engine bay look like new. The engine normally runs so smooth and quiet, you cannot hear or feel it run. This problem it not like the normal car.

Again, thanks for all your help. It is appreciated.

One question, my mechanic (not really mine, just one I used once) also told me that there was an internal engine valve dealing with the EGR system. What could that be?

I had a similar problem with a 96 G320. The harness had 2 identical connectors for the EGR control and unused air pump. The factory harness had a wire tie over the EGR connector to encourage the wrong connector to be used.

From your description, it actually sounds like your egr valve is stuck in the open position. There is no "internal engine valve dealing with the EGR system". The only component of the egr system is the egr itself.

Here is how the system works.

There is a way to check if your EGR valve is not working due to a plugged up hot inlet pipe, or if the valve itself is sticking (sticking in the open position or sticking in the closed position.):

The ECU sends an electric signal to the switch-over valve which opens the EGR valve to engine vacum. The vacum pulls open the diaphragm on the EGR, allowing some of the exhaust gas to recirculate back to the intake manifold (thus the name, Exhaust Gas Recirculation). The purpose is to reducing emissions.

The ECU will not send this electric signal at idle or wide open throttle, or until the engine has warmed up. So, if you hand pull about 12" of vacum on the EGR at idle with a warm engine, you are creating a very lean condition (vacum leak) and the engine will attemp to stall and run very rough. This tells you not only that the EGR is opening, but also that the hot inlet pipe is not completely plugged up. If, when you pull vacum and nothing changes, then you know that either the valve is not opening up, or the hot inlet pipe is completely blocked.

Now, release the hand vacum and the EGR should snap shut. Engine should smooth out.

In your case, your car is idling rough when cold, and the problem cleared up when the engine warmed up. This describes an egr that is stuck open.

There is another possibility that the diaphragm in the EGR itself may leak and cause an internal leak. The test for this is to see if the hand vacum will hold the EGR open for a period. It is common on vacum leaks to not be as noticeable with a cold engine as the engine is running rich and that helps off-set the added intake of air caused by the leak.

Last, when you clear the check engine light, it takes the same fault to occur over a specific period of time for the check engine light to come on again. It may take a few days, or even a week, but it will come on again.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".