Paul D

Yes, you're correct James - very frustrating indeed! I've just got to start ruling things out I guess, but I can see that it could start getting very time consuming and expensive pretty quickly. I thought possibly a faulty PCV valve, as I've read these can give similar symptoms - but I've had a look at that, and it appears to be OK. I also ran the engine with it disconnected (with the pipe leading to the inlet manifold blocked off) and the idle was still very lumpy, so I guess that rules it out - and it doesn't appear to be blocked or sludged up, just oily inside, which I assume is to be expected as it vents oil fumes from the crankcase. Anyone got any ideas where to look next?

Thanks for the response James. No, I haven't changed filter or injectors yet, primarily because if it was either of those things, then I'd expect it to be worse under high load/high RPM conditions, rather than the situation I have, which is that it's worse at idle and low load/low RPM. However, I'm no expert on modern fuel systems (I work on classic cars for a living, so am more used to good old carbs and distributors!), so if someone can tell me more about it, I'd appreciate it.

Hi everyone - hope someone familiar with this model can help. Car is a '99 Quattro with the 4.2 V8 (ARS engine code). 130K miles. Car is also dual fuel, so runs on LPG or petrol - converted by myself around three years ago. Fault appeared a few months ago - started with a lumpy idle and slight hesitation. Snap-on code reader showed no faults, but fault was consistent with a faulty MAF (which had happened previously, so I knew the symptoms). So, MAF duly replaced with a Bosch reconditioned item. This seemed to cure the fault, but a few weeks later it returned, same symptoms as before. This time had it checked with a VAGCOM and this showed faults related to running lean, but didn't specify why. VAGCOM operator said this was possibly consistent with a faulty MAF. Being a bit suspect of the quality of these 'reconditioned' units, I went out and bought a genuine new Bosch item. This cured the fault once again, but this time, only for a couple of days before the same symptoms returned. Now I'm pulling my hair out! The genuine Bosch MAF was £200, so I can't keep replacing them every few days! Why would the fault disappear for such a short time before reappearing? If it's not the MAF, then how could the act of replacing it cure the fault for a short time? And if it is the MAF, then why are they failing so quickly? I have driven it a short distance with the MAF unplugged, and it does seem better that way, but still not right obviously. I should add that it seems fine on full throttle - maybe not 100%, but not far off. The fault is most noticeable on idle and part throttle. Idle being particularly lumpy. Strangely, on intial start up the idle is smooth for about the first 30 - 40 seconds, then very suddenly it dips and becomes very lumpy, shaking the car. I know it runs in closed loop on initial start up, but not sure if that tells us anything about the likely cause of the fault. One final piece of relevant info - the fault was never quite as bad when running on LPG. And since the fault has appeared, I've accessed the gas ECU and effectively richened the mixture across the range, which has almost eradicated the fault when running on gas, although I do still feel a little hesitation now and then - but way better than when on petrol. To my mind, this rules out anything ignition or timing related, as the same systems are in use when on gas. Also, as a general rule, any ignition related faults are usually worse on gas due to the gas being slightly harder to ignite than petrol. However, the fact that richening up the gas system made it run better does rather prove that it's running too lean on petrol. Any suggestions greatly appreciated.