Bucking/violently cutting power during low rev acceleration

[familycarguy]

Hi all,

Looking for some direction with a problem I've discovered with the C180 I've recently bought.

So far I've done a full service on it, including a transmission oil and filter change. I've since put a few hundred miles on it and noticed that under certain conditions, it violently cuts the power. Almost giving whiplash.

It tends to do it when cruising at a constant speed and rolling into the throttle. I'm not sure if it isn't kicking down when its meant to, or if its more engine related. It doesn't happen straight away, which makes me think its either the gearbox not kicking down properly and violently slamming into the lower gear, or something boost/fuel/timing related.

The car doesn't have any warning lights on, but I just scanned for codes and discovered a strange one that seems to be pending / archived. After clearing it, it doesn't come back, but then I went for a short drive and it did come back. I don't know if this is related to the bucking issue, but its made me even more confused since the check engine light hasn't come on (it does work though).

Anyone got any advice or experience before I give in and take it to a garage? Cheers.

2013 C180 estate 1.6L turbo petrol 7G auto

 

[familycarguy]

Right so there's an American on YouTube who describes this exact behaviour in a CLA, and he says replacing the crank position sensor fixed his car.

Video title is "Mercedes CLA -car bucking while accelerating -CEL P0339-Crankshaft Position Sensor fix! Cheap Fix!"

I guess I'll order one of those and keep you updated!

 

[ManofKent]

My E class did just like you described. The air intake had become detached after the turbo, and that was causing pretty unmanageable power changes.
Worth a look...

 

[DSM10000]

Right so there's an American on YouTube who describes this exact behaviour in a CLA, and he says replacing the crank position sensor fixed his car.

Video title is "Mercedes CLA -car bucking while accelerating -CEL P0339-Crankshaft Position Sensor fix! Cheap Fix!"

I guess I'll order one of those and keep you updated!

They are cheap to buy. Only buy from MB or find out who makes them.

Don't be tempted to buy very cheap ones from Amazon etc.

 

[familycarguy]

My E class did just like you described. The air intake had become detached after the turbo, and that was causing pretty unmanageable power changes.
Worth a look...

Thanks, good to know. I'll check that too. Is your E class a petrol? Did you have any fault codes?

They are cheap to buy. Only buy from MB or find out who makes them.

Don't be tempted to buy very cheap ones from Amazon etc.

Cheers, I've had mixed results with aftermarket on other cars and given that this particular sensor on this car is a pig to get to, I think I will go genuine or premium aftermarket to save having to do it twice. I think Bosch is available from a motor factors so might go with that, especially if the genuine one seems to be a common (ish?) failure point. Sometimes the aftermarket stuff actually lasts longer than genuine.

 

[DSM10000]

Thanks, good to know. I'll check that too. Is your E class a petrol? Did you have any fault codes?


Cheers, I've had mixed results with aftermarket on other cars and given that this particular sensor on this car is a pig to get to, I think I will go genuine or premium aftermarket to save having to do it twice. I think Bosch is available from a motor factors so might go with that, especially if the genuine one seems to be a common (ish?) failure point. Sometimes the aftermarket stuff actually lasts longer than genuine.

I have had two CPS failures.

One on a Saab 9-5 V6 petrol, the other on a Mercedes 320 V6 petrol!

Both were, apparently, a "challenge" to change.

 

[familycarguy]

I have had two CPS failures.

One on a Saab 9-5 V6 petrol, the other on a Mercedes 320 V6 petrol!

Both were, apparently, a "challenge" to change.

Oh, on some cars its an absolute doddle. On the M274 engine it seems to be nestled right in the crotch of the engine, behind a cover plate so must be accessed from under the car. I'm sure its doable on jack stands, but I don't think I'd have a go without a ramp... luckily I have access to hiring one for a few hours.

I come from 10+ years of Toyota ownership, never had any sensor go bad other than lambda sensors, which I always replaced with the absolute cheapest ones I could find on ebay... you know the ones that don't even have a connector so you have to splice/crimp/solder the wires into the existing... I never had one of those fail again.

 

[familycarguy]

Success! Updating this in case it helps anyone else.

I just fitted the new crankshaft position sensor (Lemark brand - cheap and cheerful from Parts In Motion, about £14) and the car is fixed! No more bucking, it kicks down once your roll into the throttle. I've done about 20 miles of mixed driving and no OBD codes have come back!

The genuine sensor from my main dealer is about £55, I think I'll get one of those too and keep it as a spare in the glovebox for just in case.

 

[Bellow]

A word of caution. Your CPS failure does not fit with the classic symptom of engine dying at idle and taking a few minutes of cooling before restarting. And, the fault code to me reads as a wiring fault. What possibly has happened here is that in replacing the CPS you have either eliminated the bad wiring (pigtail) or reconstituted the break - for now. What I'm saying is if the problem returns go straight to the CPS wiring. Hopefully the new CPS has permanently fixed it either because of the fault being in the pigtail or it actually was a CPS failure but if not, you aren't far from a permanent cure.

 

[familycarguy]

A word of caution. Your CPS failure does not fit with the classic symptom of engine dying at idle and taking a few minutes of cooling before restarting. And, the fault code to me reads as a wiring fault. What possibly has happened here is that in replacing the CPS you have either eliminated the bad wiring (pigtail) or reconstituted the break - for now. What I'm saying is if the problem returns go straight to the CPS wiring. Hopefully the new CPS has permanently fixed it either because of the fault being in the pigtail or it actually was a CPS failure but if not, you aren't far from a permanent cure.

I'm with you, I'm familiar with the normal way a crank sensor fails and I've never come across this. However this guy on YouTube had the exact same thing. Have a search. Video title is "Mercedes CLA -car bucking while accelerating -CEL P0339-Crankshaft Position Sensor fix! Cheap Fix!"

Of course I will keep an open mind and I did inspect the wiring, as much as was possible when I changed it.