Misfire....

[mrbeige 0]

It seems our OBD2 VR (Golf) is suffering from a rather bad misfire. It seems to be evident at idle and part throttle, but goes away under WOT. I've checked for DTCs with VAG-COM but only have one for a 02 sensor having no signal (00525 O2 Sensor). I was expecting at least to see a problem with the coilpack.

 

I've taken the top plastic covers off that sits over the Coilpack and had a look for any obvious cracking on the top and sides, but nothing obvious, but I would expect the misfire to be there under heavy load/WOT if it were a cracked coilpack and indeed to have the MIL illuminated on the dash.

 

Would a faulty 02 sensor really cause it to misfire? I would think it would just not use closed loop fuelling??

 

Help :(

[dirtytorque 0]

it does sound like an ignition prob but misfires can be caused by either ignition or fueling faults so.Don't know anything about these engines or their management systems but if one cyclinder is not getting fuel or spark or a poor mixture then... etc etc.

[mrbeige 0]

I was wondering if the faulty 02 sensor would mean that when in closed loop fuelling mode, which from experience is all but WOT, a modifier is applied to the fuel delivered. But I'd expect the closed loop fuelling to be abandoned if the 02 sensor signal was faulty.

 

I've just whipped the spark plugs out. Cyl 1,2 and 6 have a white deposit on the electrodes, the other 3 look ok.

[mrbeige 0]

Think I've found the problem (famous last words...) just need a new female terminal now....oh and a pin extraction tool...

[dirtytorque 0]

is that the lambda plug stu ?

[mrbeige 0]

is that the lambda plug stu ?

It is indeed!

[Critical_Mass 10]

This seems to be a very common problem with the Lambda wiring breaking... but how does it manage to break????

[mrbeige 0]

This seems to be a very common problem with the Lambda wiring breaking... but how does it manage to break????

Well, I also discovered that the slightly knackered gearbox mount, is actually completely detactched!! I suspect that the movement of the engine, combined with heat of the exhaust has caused that wire to be damaged.

[Yandards 0]

This seems to be a very common problem with the Lambda wiring breaking... but how does it manage to break????

 

Thermal cycles and vibration.

 

We (car owners) never consider just how hot lambda wiring gets with it's proximity to the exhaust, we also assume the engine is a static lump that does not move much. The combination of hot/cold cycles, engine movement, constant vibration of the wiring loom and the state of the roads will give you exactly what the Beigester has photographed above. It's also the reason it's good to use some rubber/silicon pipes in boost or coolant systems - the rubber absorbs the vibration.

 

As for the original missfire, it does make sense that a lambda problem goes away when on WOT given that is the only time (except for the warm-up phase) that it has no input into fuelling. I suspect the real reason you have a misfire on all but WOT is that as the Motronic ECU is continually adapting the base values for best running it is on the last set of adaptive values; which were derived during an unreliable signal from the lambda as the wiring was failing.

 

That's my 50p anyway and makes sense in my head right now :)

[Critical_Mass 10]

Ah thats explained then. Cheers :D