I'm getting OBDII check engine code P0400, EGR flow malfunction. Here is what i've done/checked:
1. Reset the check engine, but it comes back
2. Applied vacuum to the EGR valve and it opens and closes properly. Engine stumbles a bit when it is open.
3. Measured intake manifold vacuum at idle, 20"Hg. With EGR opened (with my vacuum pump) the manifold vacuum drops to 17" Hg.
4. Tested the EGR switchover valve with 12V and it switches and ports vacuum correctly. Same with the changeover valve that is used for the EGR line to the intake maniflod pressure sender unit.

Note: In order for the EGR flow malfunction to be triggered no failure in the electrical activation of the EGR switchover valve can detected so i can assume that electrically everything is OK. I also noticed with my OBDII scanner that the MAP fluctuates with different throttle positions.

Is there something else to test? Also is it a sign that the EGR pipe is partly clogged if the intake vacuum only rises from 20" to 17"Hg with the EGR open? The engine stumbles noticeably as expected with the EGR valve open but no where near stalling out as may? be expected. I don't look forward to taking the EGR pipe apart for nothing especially if the info above points to that not being a problem.

Any thoughts?

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1972 450SL
1982 300D Turbo

a partially clogged egr is very common, why not pull the pipe off the egr valve and run the engine, see how strong the vacuum is, do you have another car to compare?

I'm starting to suspect a partially clogged EGR valve as well. I guess going from 20" to 17"Hg intake vacuum when the EGR opens is not considered enough of a change for the OBD to say that the EGR system is functioning correctly. You suggested detaching the EGR pipe and then checking manifold vacuum. Would this be comprable to running the engine with the EGR valve open?

Finally, disconnecting the pipe on this 1996 SL320 looks like a difficult job. The pipe wraps around the back of the engine and i think it has to be completly removed in order to clean it. Too risky cleaning it attached on the manifold side with the EGR side opened cause loosened debris could gert sucked into the engine. If I take the plunge are there any gaskets etc I may need when putting it back together?

Did I understand you correctly when you wrote:
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I think engine manifold vacuum would get weaker with the EGR valve pipe disconnected? if the vacuum goes much lower than 17" would that suggest a partially clogged pipe.

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1972 450SL
1982 300D Turbo

I have same problem with 600SEL. Both EGRs and pipes were cleaned, but the fault comes back. I don't think the pressure drop at idle is relevant. We need to check it during the test when all conditions met.

Here is fault P0 400 description copied from the MB OBD2 technical training notes:

Activation requirement Check Engine Light: After one "performed" test with failure, at start of next trip.

Test to detect fault:
The EGR logic chain is checked if the following conditions are simultaneously satisfied:
- No failure in the electrical activation of the EGR switchover valve Y27;
- Deceleration fuel shut off operating;
- Close throttle position recognized;
- Engine speed approx. 1900 - 2600 rpm and constant;
- Intake manifold pressure less than approx. 325 mbar;
- Momentary intake air temperature >10C

After approx. 1 second, the diagnostic module determines the intake manifold pressure. Initiated by the diagnostic module (via the CAN data bus), the engine control module then briefly activates the EGR switchover valve. The intake manifold pressure is evaluated for approx. 3 seconds.
The pressure in the intake manifold must drop by at least approx. 15 mbar while the EGR switchover valve is activated.

The EGR valve is opened with vacuum and closed by internal spring pressure. This code can not only be set by the failure of vacuum to reach the EGR valve but by a mechanical failure of the EGR valve itself. The valve must fully open and close with an effective seal.
Check vacuum line integrity, switchover valve mechanical, EGR valve vacuum and mechanical, EGR passage ways for carbon blockage.

check the vacuum at the pipe,

Thanks for that info myarmar. Intake manifold pressure at idle is a condition for indicating an EGR malfunction. But intake manifold pressure is evaluated before and after the EGR valve is open at 1900-2600RPM. When i perform the test at idle I am looking for a change in intake manifold pressure to verify that in fact the intake manifold is increasing in pressure (less vacuum) because the EGR is opening the intake to exhaust. My test was to determine that indeed the EGR is allowing exhaust into the intake. This is observed by:
1. Engine stumbles (as it should when the EGR valve opens)
2. By reduced engine vacuum when the EGR opens.

Myarmar, your conclusion that cleaning out the EGR pipe did not fix the P0400 check engine code is why i don't want to jump to any conclusion either. regarding your case: did you test the other parts of the EGR system, e.g. the switchover valves and vacuum lines going to the EGR control port? Although your valve and pipe are OK you still need the EGR vacuum controlled system to work.

__________________
1972 450SL
1982 300D Turbo

Carson356,

OK, understood. I'll try your test with my thumb over the open EGR pipe. not sure what enough vacuum will feel like and have no reference car to compare. will report back.

__________________
1972 450SL
1982 300D Turbo

Here is something I found in an old posting. My engine stumbles as well so the pipe is not clogged?

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Arthur Dalton
Registered User Join Date: Aug 1999
Location: Florida
Posts: 2,887

Although there is a reasonable chance that the egr tube is blocked,[common
104 prob], an easy test for vac. flow is to simply apply some vac [ hand pump or vac source] to the egr while at idle. This opens the egr and will cause the engine to stumble..[ go lean]...if it does ,the tube is not blocked and other parts of the egr system are suspect...
You may know about the EGR/TSB on this chassis...[ archieves]
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__________________
1972 450SL
1982 300D Turbo

I checked EGRs same as you did with same results - engine stumbles. Did observe pressure drop, but don't remember the values now. Also I have Snap On scanner and I activated EGRs OK, so I know switchover valves are working. Next would be check if they open without delay as a test runs only for a few seconds.
Reg. Mike

Don't expect an egr hand activation test to stall the engine .. it goes lean and stumbles , but the ECU will automatically enrichen the mix before stall..
See if egr snaps shut with hand test...