Fault but high MPG

[ChrisEdu]

My W211 E320CDi is having an intermittent fault, whereby it loses power under acceleration. It's not going into limp mode, but feels like there's no kickdown or turbo boost.

Today, I took the car for a run to Portsmouth and had the problem on the way there. However, coming back the car was fine and pulling like it should.

The weird thing is that going there, the car gave the highest mpg I've ever seen, by a long shot, whereas coming back it dropped back to something I'm more familiar with!

So, almighty Mercedes sages, what is going on?!

 

[Bobby Dazzler]

I suspect the MAF sensor (mass air flow), assuming it has one.

 

[ChrisEdu]

I suspect the MAF sensor (mass air flow), assuming it has one.

Would that throw a code on the OBD?

 

[Bobby Dazzler]

Would that throw a code on the OBD?

Not sure, but the symptoms sounds a lot like my experience if MAF failures. Last one I bought was around £55 from the main dealer - or maybe cheaper if you can buy a Bosch or equivalent part - so a relatively inexpensive punt.

 

[DSM10000]

Not sure, but the symptoms sounds a lot like my experience if MAF failures. Last one I bought was around £55 from the main dealer - or maybe cheaper if you can buy a Bosch or equivalent part - so a relatively inexpensive punt.

The Batwing assembly with the two MAF sensors for the OM642 was several hundred pounds from MB. When one of the sensors failed on my car it would go into Limp mode, turning it off and on again would reset the problem for awhile.

A faulty turbo actuator could give symptoms such as lack of boost that can be intermittent.

Either should leave a fault code

 

[ChrisEdu]

The Batwing assembly with the two MAF sensors for the OM642 was several hundred pounds from MB. When one of the sensors failed on my car it would go into Limp mode, turning it off and on again would reset the problem for awhile.

A faulty turbo actuator could give symptoms such as lack of boost that can be intermittent.

Either should leave a fault code

As mentioned, I'm not getting what I know as limp mode, in as much as the car can still accelerate, albeit more slowly and is able to rev beyond 2000rpm.

Previously, I did find a stored code for an exhaust back pressure too high, so did wonder whether the sensor might be at fault and whether that would cause what I'm experiencing?

Either way, it seemed really strange to achieve circa 45mpg with the issue present. Weird that a problem could improve the mpg!

 

[DSM10000]

As mentioned, I'm not getting what I know as limp mode, in as much as the car can still accelerate, albeit more slowly and is able to rev beyond 2000rpm.

Previously, I did find a stored code for an exhaust back pressure too high, so did wonder whether the sensor might be at fault and whether that would cause what I'm experiencing?

Either way, it seemed really strange to achieve circa 45mpg with the issue present. Weird that a problem could improve the mpg!

You averaged 40 mph. That propably accounts for the mpg.

 

[ChrisEdu]

You averaged 40 mph. That propably accounts for the mpg.

I'm sure I've had similar average speeds before. I have never seen more than about 38mpg, so to get to 45 is a big difference.

 

[poormansporsche]

Exhaust back pressure too high can often be egr related. I had the same on my van and I cleaned the egr and the pipe to the exhaust pressure sensor and all was good (not a mb but same principle)

 

[PaulMason]

Similar problem to my S320, I cleaned the EGR, changed the turbo seal (plus the ones on the intercooler pipe recently) replaced some injector seals and the intermittence got much better. I haven't yet cleaned/replaced the turbo actuator as since injector seals it's done its a lot less frequent and I've just learned to drive around it (using the paddles to change down if needed). Not surprised on the MPG as it's like you are driving on a super light throttle (though I haven't ever measured it scientifically). I also do need to clean the exhaust pressure sensor as that can also be a cause, but again, not quite got round to that job yet

 

[W1ghty]

As mentioned, I'm not getting what I know as limp mode, in as much as the car can still accelerate, albeit more slowly and is able to rev beyond 2000rpm.

Previously, I did find a stored code for an exhaust back pressure too high, so did wonder whether the sensor might be at fault and whether that would cause what I'm experiencing?

Either way, it seemed really strange to achieve circa 45mpg with the issue present. Weird that a problem could improve the mpg!

Limp mode is 3000 limit . Yes there probably will be a fault code
45mpg is normal for a long slow run on your (our) engine .

 

[ChrisEdu]

Limp mode is 3000 limit . Yes there probably will be a fault code
45mpg is normal for a long slow run on your (our) engine .

Not for me it isn't! In all the years I've had the car, I've never seen anything over 39mpg.

 

[DSM10000]

Not for me it isn't! In all the years I've had the car, I've never seen anything over 39mpg.

Have you reset the counter?

 

[BlackC55]

That's limp mode. You should have said and I would have happily plugged it in today. I'm on the A3M.

It could be a whole manner of things. I have done a sticky in the engine section to help.

 

[ChrisEdu]

That's limp mode. You should have said and I would have happily plugged it in today. I'm on the A3M.

It could be a whole manner of things. I have done a sticky in the engine section to help.

I've been to see you before - you guys did the V-belt and service a while back.

As for limp mode, it really doesn't feel like my previous experiences of limp mode.

 

[ChrisEdu]

Have you reset the counter?

I often reset before longer journeys to see what I get.