B Class 180 CDI engine hunting at idle only when warm

[Nomoney]

Hello, I have a really annoying issue with my wife's B Class that recently developed. The background story relates to an injector issue and the car sounding like a box of nails on cold start up. Turns out it needed an replacement injector and this issue was resolved. However, now it has developed an issue at idle when warm only.

When warm the engine sort of hunts during idle. The rev needle does not move but you can hear the engine note changing and feel the vibration, almost like you are blipping the throttle just a little bit.

The really annoying bit is the car is not throwing any codes at all as checked by STAR. From my limited experience my thoughts are towards the following, MAF sensor, MAP sensor, EGR, O2 sensor or maybe a vacuum leak.

This is what I have checked so far, breather pipes, cleaned EGR, cleaned MAF sensor, cleaned MAP sensor and replaced fuel filter. Sadly non of this has cured the issue.

One thing I noticed recently is if you unplug the MAF sensor the idle returns to normal. So this was replaced with a new part and the problem is the same still. If the MAF is unplugged it returns to normal, I assume when unplugged the ECU defaults to other parameters for the fuelling and therefore whilst it may seem to be MAF sensor related its not really?

I must admit to be a little confused and frustrated by this issue. So before I start replacing expensive parts, I though I would ask if anyone has any ideas or pointers of which way to go now.

Thanks in advance for any help.

 

[Nomoney]

I thought this was a puzzling problem. Whilst hard at work today my mind drifted onto the b class issue yet again!

I'm thinking that if the Egr valve only operates at normal operating temperature this is more than likely to be the issue. I think the MAF takes signals from the egr so this might explain why the idle returns to normal when the MAF is unplugged.

My Independent told me he cleaned it out but I'm now wondering if it's been cleaned as well as I would have. So I will remove the egr and clean it again, unless anyone has a pointer for a different things to look at.

 

[mattkh]

.... So I will remove the egr and clean it .

Hi
Good luck.
Please put up some before and after pictures.
What is the mileage ?

 

[Nomoney]

Hi
Good luck.
Please put up some before and after pictures.
What is the mileage ?

Thanks, my experience tells me the EGR will either just pop out or be welded in situation with the years of gunk. For this I will employ my rubber hammer aka the final solution

The car has done 90k miles and I will try and take some pictures of the location and fitment.

My original plan was to clean it, however, having checked the online EPC as its a non DPF engine, I note a genuine replacement is just £75 So based on reading some posts from a chap called Television on this forum replacing is the way forward as the springs get weak and cause unintended leaking. This chap has something like 159,000 posts so he must be a complete Mercedes guru or a nutcase :wink: either way Im betting on the former and going to change the EGR instead.

 

[Nomoney]

It's been an arse today and SWMBO is not happy with me or the car as it's not starting to stall now. I hurriedly asked if it had a EML on, of course it didn't that would be too simple?

Any ideas people? Is this still pointing to Egr related?

 

[turbopete]

air in fuel?

 

[Cole@MBS]

Check the rubber elbow which joins onto the EGR at the rear of the airbag, they spilt!!!!!

 

[Nomoney]

Check the rubber elbow which joins onto the EGR at the rear of the airbag, they spilt!!!!!

Cole, thanks for the heads up, do you think I'm on the right track with the Egr considering the symptoms. Also will this elbow you mention come with a new replacement Egr?

 

[Cole@MBS]

Could be and no, very much doubt your need a new EGR.....

 

[Nomoney]

Thanks, one thing that crossed my mind was a blocked CAT? but would have thought I would experience other symptoms as well. Any other suggestions from your vast experience on working on these cars.

 

[Cole@MBS]

Nope, they don't have many probs at all.........

 

[Nomoney]

Thanks, what I thought, to be fair that's been our experience and the lack of any codes on STAR makes diagnosis hard. Even my indy has struggled to make sense of it.

 

[Nomoney]

Tackled the B class this weekend. On the agenda was cleaning intake area, replace egr and map sensor.

Intake area where the EGR sits was really gunked up, so whilst I don't think this was causing my issue I cleaned it anyway. Fitted the new EGR, took the car for a spin to get it up to temperature and the car goes into reduced power mode. No manual control over gears and little to no boost pressure.

Went home and checked codes and got a P0401 EGR valve circulation insufficient. Checked all vacuum lines but codes persists. In the end I put the old EGR valve back on and the code went away. The EGR is the vacuum type so not exactly complicated.

Im now suspecting the vacuum solenoid, my thinking is the old EGR is worn and the solenoid is producing enough vacuum to operate it but not enough for the new stiffer EGR valve. When the solenoid is unplugged the rough idle stops!

I have to laugh as this engine is going to become like Triggers broom from Only Fools!

 

[Nomoney]

All sorted now, thanks for all the reply's and help.

 

[JinxyTog]

Well done for getting it sorted. What was wrong? I know someone with the same problem.

Thanks
Jon

 

[Nomoney]

Very slight crack in a vacuum solenoid, a pain to diagnose but not costly. Would have had it sorted earlier if not for Mercedes supplying me with a defective brand new EGR valve. Ended up applying a vacuum manually and the piston would not move!

 

[icky]

Hello, I have a really annoying issue with my wife's B Class that recently developed. The background story relates to an injector issue and the car sounding like a box of nails on cold start up. Turns out it needed an replacement injector and this issue was resolved. However, now it has developed an issue at idle when warm only.

When warm the engine sort of hunts during idle. The rev needle does not move but you can hear the engine note changing and feel the vibration, almost like you are blipping the throttle just a little bit.

The really annoying bit is the car is not throwing any codes at all as checked by STAR. From my limited experience my thoughts are towards the following, MAF sensor, MAP sensor, EGR, O2 sensor or maybe a vacuum leak.

This is what I have checked so far, breather pipes, cleaned EGR, cleaned MAF sensor, cleaned MAP sensor and replaced fuel filter. Sadly non of this has cured the issue.

One thing I noticed recently is if you unplug the MAF sensor the idle returns to normal. So this was replaced with a new part and the problem is the same still. If the MAF is unplugged it returns to normal, I assume when unplugged the ECU defaults to other parameters for the fuelling and therefore whilst it may seem to be MAF sensor related its not really?

I must admit to be a little confused and frustrated by this issue. So before I start replacing expensive parts, I though I would ask if anyone has any ideas or pointers of which way to go now.

Thanks in advance for any help.

this might be the problem