I have been living with the problem for a couple of months - but I want to fix it before I go out of town in two weeks. CEL came on in August, took to AutoZone and read a code of P0400 - EGR flow. I removed the EGR valve and tube (nut on tube was frozen so I had to remove the who thing loosen nut out of the vehicle). The EGR tube was completely clogged up, I cleaned it and re-installed the whole thing. Went back to AutoZone and had them clear the code. Clearing the code lasted two days, and it came back on (P0400), I had it cleared again and it came back on a day later. I'm puzzled. Any ideas? I'm thinking a bad EGR valve would have triggered the code instantly. Tube shouldn't have clogged back up that quickly. No other known problems with engine - the only other code I have ever found was the one generated when the engine is left running during refueling (don't remember the code number).

After you clear the code, it'll depend on when the EGR system gets tested by the ECM, it all depends, it takes certain parameters before the engine will test it. You need to get it to a shop where they can activate the egr valve with the ECM itself to fully test the system.

Gilly

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I remember reading now that the EGR only opens when the engine is up to operating temperature and in a certain RPM range. I'm not a car tech, but I wrench on CNC machining centers for a major pump manufacturer (it kind of spoils me - most problems go back to switches and sensors with binary inputs that can be tested through the computer in real-time, I don't have this privilege working on cars in my driveway). Are there any tests that I can do? I don't mind taking it to a shop, but I know SOP is to replace parts if in doubt since it is on the customer's dime anyways. I have to dump a couple thousand into this ML anyways (got to put rear struts on, replace oil level sensor, have front bumper painted where it is peeling) but wanted to save a little money if I can (still paying hospital bills for our newest addition to the family born in May).

As an aside, I picked up a '76 230.4 a couple months ago (kind of stupid on my part but I REALLY missed my '83 240D - except for the lack of acceleration) and have enjoyed good vintage MB wrenching. Found I still had the knack for timing "by ear", adjusting carbs, and learned a little about point style ignition. I wish they still made those cars brand new - I would trade the ML and buy a new one in a heartbeat.

The signal to open the EGR valves comes from a small vacuum switch on the front of the engine. On mine, the vacuum tubing had cracked open so no vacuum was reaching the EGR valves. Very hard to spot the break, just at the inlet to the switch.

They put the MB computer on it, and couldn't get the EGR to open at all. Diagnosis - bad EGR valve. I'm probably going to order one off of Fastlane and put it on myself, as I had to remove the old one to get the EGR tube out. Its not too bad at all to do.

Gilly,
Is there any reason why I should take it back to get the code cleared, or will the computer take care of it in twenty key cycles?

It'll shut the light off eventually without clearing the code. I wouldn't sweat it. Especially if you think the shop doing the work will charge you for it. Got an autozone or someone who'll clear it for free?
Gilly

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I'll take it to AutoZone to clear - that is how I found the codes to begin with. I changed the spark plugs today - not a bad job at all. I bought the KD spark plug wire pliers (the ones made out of heavy stamped steel and rubber-coated jaws). For $77 (Bosch +2 platinum spark plugs, pliers, and some Armorall wipes for the interior), I think I came out for a lot cheaper than the $225 quoted for the plug change. I'm not knocking the shop - they are way cheaper than MB on their service and I like going there when I get stumped on something, but I like to save a buck and DIY whenever it is feasible to.

Gilly, I just want to thank you again for being a great moderator on this site. I always learn something when you post, and your knowledge and friendliness are outstanding.

Got a MB EGR valve from local shop for $89.10 (a STEAL!) and put it on last night. What I found on the old EGR valve was interesting. I could not get the EGR valve to open by pushing up on the diaphragm with my finger. I pried up on it and obviously broke loose the carbon seizing the shaft and it moves freely now. I left the new EGR on as it is nice to have a new part, but the old one will be kept as a spare. Had Autozone clear the P0400 - I drove it last night and wife has driven it a few times today and the code hasn't come back.

Where can I find information on how to remove the EGR valve?

The Mercedes Service Manual for the M112 engine would be the best place, if it is anything like the manual that I have on the M115. Very detailed with pictures. The procedure is really easy, though. The EGR valve is located on the rear of the passengers side of the engine. The valve is held by two torx-head bolts and the flexible tube is attached to the intake manifold via a flange and two torx-head bolts. You have to remove the engine cover, the flexible intake ducting between the mass airflow sensor and manifold, and the wire to the EGR valve to gain access. I also found it easier to get to by removing the wire to the nearest fuel injector to gain access to the bolts and make removing the wire to the EGR easier. After that, you have to loosen the tube connection to the EGR valve, pull the wire to the valve, and the vacuum tube going to it. After that, remove the two bolts to take the valve off. Remove the two bolts in the tube flange and snake the tube out of the intake with a twisting motion (it is a complex shaped snorkel going down into the manifold probably about 8 inches). If it is clogged, you will see the carbon plugging the end after it is extracted. While you have everything off, take a screwdriver and delicated pry up on the diaphragm in the EGR valve and make sure it is free to move. That was what was wrong with my EGR valve itself. I put a new one on because it was in my hand and I got it cheap, but I could have cleaned the old one after freeing it up and probably been OK.

ARKIE got it right. I just performed the operation today. I used a 5/16 6 point socket to remove the torx-head bolts. Also a 1" open end wrench works just fine on the nut on the tube. Hardest part was working the tube out of the back of the engine. Now to tackle the spark plugs.

There's an electric motor in the M112 EGR system that sometimes fails and generates the 0400 code. That is, there can be more to fixing the problem than simple cleaning.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles