A6 4.2 rough idle/hesitation/misfire on part throttle, OK on full throttle. Help!

[Paul D]

1. 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. 

[James]

I take it you've tried obvious stuff like injectors and fuel filter etc.

 

My knowledge of LPG is fairly limited I'm afraid!  :(

[Paul D]

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.

[James]

Fair enough, that make sense!

 

Well, I hope you get to the bottom of the problem - it's always so frustrating when you're willing to fix something and don't know what it is!  :(

[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?

[sonico3]

that engine needs more air ....simple

[Magnet]

Hello Paul,

If you think or know its running lean then its taking too much air in, and it might be worth spraying some WD40 or similar over the inlet manifold joins and anywhere else where air might get in. Tell tale will be smoke from the exhaust. If its not taking in air:-

Another possibility might be a suspect coolant temperature sensor, which senses the running temperature and signals the ECU to control the correct fuel/air mixture. as you probably know, the resistance on these varies with temperature and if they go faulty, usually default to a set resistance. You can obtain figures for the correct resistance values at different temperatures, so it would be possible to test it, but otherwise you just replace it, but i wouldn't use an aftermarket one.

Sorry but I cannot really suggest much more.

kind regards.

Gareth.         

[56Doc]

I ran an allroad V8 on lpg, used to ‘lean mixture’ and ‘misfire’ faults coming up fairly randomly, eventually worked out that these would show up after shorter runs where the engine didn’t reach full temp but it was warm enough to switch to lpg. Seems until the engine is at full normal temp, it can cause this sort of problem, especially if I floored it to overtake, seems the lpg and cooler engine caused a slight misfire which then triggers the fault codes.

letting the engine get fully warm and using a very, very slight slowing of throttle movement ( ie; no stamping on the throttle to get it to kick down earlier) stopped it. More of a driving style  and limitation of the lpg fuel. (It’s much higher octane than petrol, so ideally you need to advance the timing a lot for best lpg efficiency but most engine ecu’s  can’t accomodate that, nor can most lpg ecu’s.

Im assuming that it only does it when you run it on petrol and lpg, ie; it’s fine on just petrol?

Dave

[56Doc]

Hmm, schoolboy error, sorry, just re-read yr post, sounds like an injector not working properly, enough that it doesn’t show as faulty but not well enough to squirt enough petrol. Possibly more than one injector?

petrol can gum up injectors over time, which really messes up the spray and droplet patterns ( big drops don’t burn as well as small ones - less total surface area) and amount of fuel let thru on each injector cycle.

Hope this all helps but please Keep us updated won’t you.

 

 Cheers

Dave