My 96 C280 w/143k miles got a CEL and a P0400 code. I replaced the EGR tube & EGR valve (following some advice and posts from others on this site) then cleared the code.

After about 10 miles the CEL came back on w/a P0400 and P0411 this time.

What am I missing here? What's next to test? Replace? Any likely culprits?

ANY help would be greatly appreciated. Thanks!

Matt

Put a T in line with the vac line to the egr and put a vac gauge on the windshield where you can see it . See if you are getting soorce vac when you drive around a parking lot or somewhere safe. You may have no vac going to the egr...meaning no signal from ECU or bad/plugged SOV...common fault on these is the plastic vac line in front of engine cracks from heat fatique ..the line feeding the SOVs... if so , replace w/rubber line.
The reason I suspect this line is b/c you also have an AIR code[411] coupled with EGR code [400] ...and this feed line I mention is a COMMON feed for BOTH the EGR SOV and The AIR SOV..so, if both are popping a code . then it looks like neither one is getting acuating vac suppy...follow me ??????

I would also do a hand vac test at egr while engine is at idle to check flow.

PS ..if you were near these SOV when doing the other work on the car , make sure you did not swap the lines or plugs at the SOVs ... I have seen this happen to several others and the results would be the dual codes..............

One more tidbit on this. I just noticed (probably because I usually have the radio on) that on cold starts (especially in the am when the car has sat overnight) there is a hi-pitched whining sound for several minutes. It sounds like a turbocharger spooling up. I don't recall this sound in the past. Any possible link to the codes I am getting?

Possible vac hose leak..or engagement of the AIR pump

Arthur - you're right on the money. I removed the shroud that covers the fan belt and found this (is the the SOV?).



It appears that the incomming vaccum line was cracked and broken about 6 inches from the SOV (assuming that is what the above part is) - the vacuum line terminated into thin air - i.e. it was just broken and not connected on the other end.

So - the million dollar question: Where did the other end of this vacuum line go? I was unable to locate the other end anywhere! Can I just buy some rubber tubing at the local auto store and hook it back up? Where sdo I find the vacuum source for the other end????

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Get a piece of vacuumhose that is a snug fit over the nylontubing, and use it to re-connect.
You're good to go.
Yes, that is the SOV ( Switchover Valve ) in your picture.

__________________
2007 C 230 Sport.

The feed end will be at the Intake manifold, just in front of the throttle assembly. There are 4 vac nipples on the intake . You want the most foward one .. you will see where the other end of the plastic line has broken off.
Use rubber vac. hose for the entire length.
The parts of plastic that are missing are usually just gone from hitting the fan blades.

Try the diagrams here:

http://www.detali.ru/cat/oem_mb0.asp?TP=1

You have to buy tubing that is stiff enough so that it doesn't collapse or expand due to any pressure/vacuum.

I must be the least observant, or least intelligent person that has graced (or lacked grace) on this site, but for the life of me I can't find the other end where this hose was supposed to connect. I searched - visually and by running my hands over the underside of the intake manifold and what I could see of the throttle body, but I still cant find it! I found three other vacuum lines - all in good shape - all still faithfully connected to the other stuff they've always been hooked up to.

I even looked at the link to the schematic diagrams - no luck.

Any more descriptive on where this mystery fourth vacuum nipple is located. I am befuddled!

Matt

Should be right in front..can see it looking under the runners arch...it is next to the one that goes to the vac monitor switch.
Might have a purple check valve where it hooks to.