OM606 in 97 S300TD - intermittent "no boost"

[Llewelyn]

interesting issue on the W140. The wastegate is operated by vacuum, switched by a signal from the ECU. So is the EGR valve. There's a second pipe from the EGR which heads up to a little unit on the bulkhead - my guess is this is monitoring the EGR to see that it is in fact operating.

Problem is, sometimes, the wastegate is being held open, so resulting in no boost. It does seem from googling and other forums that if the ECU detects EGR problems, it disables boost by opening the wastegate. Stopping the car and strarting it again mostly seems to reset this, and it then works fine - however, after running a bit it will recur. Current thinking is either the EGR doesn't operate when triggered, causing the low boost mode, or maybe the EGR operates and sticks, also causing an error condition.

Beside removing and checking the EGR, anyone done anything about disabling it? There are several threads on other forums about W210, but it sounds like they have other management systems, as nobody mentions this additional pipe from the EGR.

I could of course bypass the wastegate operating valve and supply it with vaccuum full time, but that could result in over-boosting the engine which likely isn't good for it. Wonder what happens if I bypass the EGR and connect the 2 pipes which attach to it together?

 

[Llewelyn]

Should add now: the EGR has a position sensor on it, which might be faulty and would account for the intermittent failure. I think the pipe from the EGR to the gizmo on the bulkhead is measuring manifold pressure or something, as it's on the inlet path not the EGR operator.

 

[Llewelyn]

Should add now: the EGR has a position sensor on it, which might be faulty and would account for the intermittent failure. I think the pipe from the EGR to the gizmo on the bulkhead is measuring manifold pressure or something, as it's on the inlet path not the EGR operator.

(edit) well I hope it can be persuaded to work, as the EGR valve appears to be made of unobtanium. The US didn't get OM606 cars, I don't think, which doesn't make it any easier, and the W210 has a different version. There was a G wagen used the same kind of EGR I believe...

No, I don't know why my edit made a new reply.

 

[91dm]

Have you a picture of the bulkhead sensor, I think you're looking at the MAP (manifold air pressure) sensor. Best best is to blank off the egr and disable it as per the guides on here, hopefully the wiring is the same as a e 210. I know on our 202 C250TD I pulled the vac pipe off and fitting a blanking plate.

 

[Llewelyn]

From what I've read the EGR is the difference between the 606.961 (W140) and 606.962 (W210) versions - the W210 uses a MAF sensor to detect EGR function, whereas the W140 (and, incidentally, 300 turbodiesel G300) use a sensor mounted on the EGR activator. The word is you can't easily disable the EGR on the W140 without it going into limp mode, although you can of course blank the pipe so it doesn't *actually* recirculate - might do that sometime.

Yeah it could be MAP sensor. I'll take a pic in a bit.

Yesterday evening I swapped the vacuum modulators between the EGR and the wastegate - they are identical. Today, it hasn't misbehaved. The basic idea was to try and spot if one of 'em was defective, but a secondary possibility occurred to me today, that maybe the wastegate has a different kind of duty cycle and thus if the modulator was slightly iffy it would be less likely to cause a problem if it's driving the wastegate. It might also be that a misbehaving wastegate doesn't trigger limp mode, too.

The other thing I want to check is the vacuum pump - i'm not sure the system has as much vacuum as it should - pressing the brake pedal twice in quick succession feels a bit like the servo vacuum isn't enough on the second hit although it's fine on initial braking, which could point to a semi-defunct vacuum pump; and the vacuum operated headlamp adjustment is a bit slow to respond too. Inadequate vacuum in the system as a whole might cause all the symptoms...

 

[Llewelyn]

This is the thing on the bulkhead, has a single hose connected to the body of the EGR valve Dropbox - IMG_20190620_203832.jpg - Simplify your life

A quick search indicates it is indeed a MAP sensor.