Overrun issue..

[Lee Goodall]

Yet another problem has come to light!..

When I come off of the throttle (overrun) occasionally I get a judder that feels almost like you're blipping the throttle.

I assume this is probably an injector issue but I'm not seeing any fault codes. Injector sticking open?

Any ideas?

 

[Lee Goodall]

Anyone?

 

[Broken Byzan]

Mine does it, i think its the DV closing and it causes a slight surge. It doesnt do it all of the time, just at certain revs and throttle openings, i have learned to live with it for the mo

 

[aragorn]

does it happen at any particular rev point?

I would imagine that even if an injector was to stick the engine wouldnt actually ignite the fuel in overrun, as it doesnt expect there to be any fuel in the cylinders...

I know that at very high revs (perhaps >4500rpm) some management systems will inject fuel without igniting it to cool the pistons on overrun. I've no idea if the Motronic system on the A4 does this though.

Perhaps take a vagcom log of the throttle posistion and injector duty cycle to see if anything odd occurs when you get the juddering?

 

[Lee Goodall]

Mine does it, i think its the DV closing and it causes a slight surge. It doesnt do it all of the time, just at certain revs and throttle openings, i have learned to live with it for the mo

It seems to occur at lower revs.. usually below 3.5K. What sort of revs are you getting it at mate?

 

[Lee Goodall]

does it happen at any particular rev point?

I would imagine that even if an injector was to stick the engine wouldnt actually ignite the fuel in overrun, as it doesnt expect there to be any fuel in the cylinders...

I know that at very high revs (perhaps >4500rpm) some management systems will inject fuel without igniting it to cool the pistons on overrun. I've no idea if the Motronic system on the A4 does this though.

Perhaps take a vagcom log of the throttle posistion and injector duty cycle to see if anything odd occurs when you get the juddering?

Good point.. there wouldn't be a spark to ignite the extra fuel!

I think I might have to get vagcom on it to see what's what.

I have notice that the exhaust gases smell richer than usual and the plugs did seem to be slightly blacker too!

 

[aragorn]

perhaps a CTS or Lambda fault?

 

[Lee Goodall]

I've replaced the CTS fairly recently but the Lamda sensor is a second hand one so that is a possibility.

I hate chasing faults.. wish it'd throw up a code to give me an idea!

 

[aragorn]

the S4 certainly logs lambda correction, never looked on the 1.8T but on the S4 you can bring up the long term trims. If the trims hit a certain % then it will bring up a fault code too.

What happens if you remove the bleed valve?

 

[Lee Goodall]

Tried running with the valve closed but it's still the same. Just seems to me like it's over-fuelling.. Rich smell and coked plugs too although discounting the weird judder on overrun no other major running issues.

 

[Lee Goodall]

I still have a suspicion that there's something not right with one or more of the injectors.

 

[aragorn]

if an injector was sticking you'd see the difference in the plugs, either white if its lean or black if rich... If all the plugs are the same its unlikely to be an injector

if the coolant temp is normal then MAF or Lambda is where i'd be looking

 

[Lee Goodall]

Got a new MAF and CTS on so I'll swap the Lambda next.. spoke to my mate at Parkers and he can do me a good price.

Cheers mate.

 

[Lee Goodall]

Lambda will be with me tomorrow.