I've been searching posts trying to find the right info to fix my wife's 96 C280 (140k miles) but I've got some follow up questions.

The CE light is on and the engine code is the generic P0400. I ran the engine up to operating temp and she's idling smooth at about 750-800 rpm. I hooked up a hand vacuum to the EGR and when I hit about about 5-7 in. Hg vacuum the engine idle drops by about 50-100 rpm. I then ran the same test at 2000 rpm with about the same rpm drop when I get to about the same 5-7 in. Hg vacuum. When the rpms drop the engine runs a little rough, but it's not too bad (I imagine these Benz engines are well dampened?).

The incoming vacuum line to the EGR already appears to be rubber and in good condition all the way to the hard plastic line that runs somewhere else under the hood. I hooked the hand tester up to that incoming line and once I hit about 3000-3300 rpm I get a spike of vacuum pressure to about 20 in Hg VERY briefly (<2sec) then it drops back to 0. When I keep the rpms up in that 3500-4000 range I get a horribly rough shakeing.

So, a little advice please. What are my next steps?

Do these results mean the EGR valve is good? Should I replace the short rubber vacuum line to ensure it is OK, or the hard plastic one as well? Or should I just assume it's the EGR feeder tube (from the manifold to the EGR) and clean/replace it?

Thanks in advance! Any help is greatly appreciated.

EGR needs 12" of vac for flow test. Should also hold vac.
Also , check vac at plastic line before SOV valve.. that should have full engine vac. [ they are infamous for cracking]

Good info, thanks. I did give it up to 15in of vacuum. To clarify my earlier post the EGR value appeared to open when I hit somewhere around 6in. And it held pressure well.

What is the SOV value? I am unfamiliar with that. Where can I find it?

Thanks so much!

Let me clarify..
If one opens an egr at 5/6 " vac , he is not fully opening the egr .. so, the results are an incorrect flow test . a partially opened egr results in the same mis-diagnosis as a partially restricted egr tube at engine intake.. LOW FLOW.
so, we need to know that you have at least 12" vac when performing the test..see?????

The SOV is simply the electrical valve between the egr and the plastic engine vac feed line that, when opened , allows engine vac from the mentioned plastic line to the egr port you hooked your hand pump to.. This is operated from a 12v sig from the Engine management ECU sig.

So, forget the SOV for now and see if you have FULL engine vac at the line BEFORE the SOV.. that tells you that side of the feed is OK
Many guys who do not have a hand vac simply put a hose on the plastic line and bring it directly to the egr to activate it...[ this eliminates/by-passes the SOV valve/circuit] ..that will tells if the egr is opening and if the tube is blocked...if both pass that test , then the SOV circuit is suspect..
The rough running you mention at 4k is simply the rev limiter for torque converter protection at N/P trans positions

Looking at the problem & your results..I believe that the metal pipe between the valve & intake is 90% plugged..

I would replace that pipe.

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After about 30 minutes of removing various shrouds & covers I hit a stopping point because of not having the correct tools to remove the intake manifold to try this fix. So I'm left with only two options - suck it up and have someone install the part for me, or buy the correct tool to remove the intake manifold and give it another try next weekend.

So a quick question for ya: Is the intake manifold the only major part I am going to need to remove to get the EGR tube out and replaced? It looks to be the case, but I can't tell for certain. I am assuming that I just need to unbolt the 8-10 hex-head bolts that attach the intake manifold to the engine? Is there anything else I need to know before I get started?

Thanks so much for chiming in!

Matt

Matt, can you take a pic of this part? I want to investigate mine

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OK, since I've last chimed in....

I replaced the EGR Tube (not an easy job for a duffer like me) and a vac line that looked a little 'iffy'. Cleared the code and whamo, it came back after 2 trips in the car (1st one was < 5mi, 2nd trip when it came on was about 12 miles).

So then, out of desparation, I replaced the EGR valve, although my tests seemed to indicate it is OK.

I re-read the code and now I have a P0400 and P0411.

What next? I am at my wits end. Thanks for ANY help you can offer.

Matt