Help Please - Constantly Misfire at idle on Audi A3 1.6

[FJ0372]

I have an Audi A3 1.6 that's had a misfire problem for some time now. Going back a few months, when I first connected VAGCOM/VCDS to the car this is the (massive) list of faults that had accumulated on the car since the previous owner had never had the car scanned:

Since then, I have spent months messing about trying to get everything right. I changed the spark plugs, coil packs and both lambda sensors (one on the engine and one on the exhaust). I've cleaned and reconnected the MAF sensor and I've even gone to the extent of changing the head gasket. Having done all of this I've managed to clear every single fault on the above list but I still have this incredibly annoying misfire upon idle, which is the reason I've been scanning and changing parts on the car in the first place.

I've re-scanned the car numerous times with VCDS having put hundreds of miles on the car between each scan and it no longer ever finds any faults. The misfire keeps the car jumping between as low as 600 revs up to just under 1000 revs at idle. What seems strange to me is that this makes it much harder to pull away without stalling than it should, but once the car is actually rolling it appears to run perfectly. It's not as if it's only firing on three cylinders etc. As soon as I get to a stop the misfiring will start again.

Does anyone have any recommendations as to what I should try changing next? I'm really running out of ideas here...

 

[FJ0372]

*BUMP* Has no one got any ideas please?

 

[Rogue-24-84]

try the thermostat as it works together with the first lambda and if its stuck it will cause a mis reading on the lambda resulting in the wrong mixture hence misfire... and as there only about 6-7 quid it'd be silly not to rule it out,

or fuel pump, there a common fault yet dont show up on computer, hope this helps you mate

 

[Ashley g]

Have you done a compression check?

 

[FJ0372]

try the thermostat as it works together with the first lambda and if its stuck closed it will cause a mis reading on the lambda resulting in the wrong mixture hence misfire... and as there only about 6-7 quid it'd be silly not to rule it out,

or fuel pump, there a common fault yet dont show up on computer, hope this helps you mate

Thanks for the reply. I guess I could try the thermostat first but my car doesn't over-heat etc. and I've taken it on long drives before. If the thermostat was the fault would this not show up on VCDS? I could try the fuel pump too but again when actually moving the car runs perfectly fine, it's just this very odd misfire at idle that makes it hard to pull away and easy to stall, it tends to lead to slipping or dumping the clutch to pull away.

 

[greengunner4511]

I've had the same problem mate. Thermostat and fuel filter change fixed it right up, apparently this is quite a common fault on the 1.6 especially as they age. What sort of milage is it running on the clock?

 

[greengunner4511]

The guy who advised me on my fix also had this problem and said that once he had made the changes it might be worth looking at a re-map of the engine as this should then manage any of the slipping problems. So far I haven't had the re-map and am experiencing that same slip.

 

[Lewis583]

try the thermostat as it works together with the first lambda and if its stuck closed it will cause a mis reading on the lambda resulting in the wrong mixture hence misfire... and as there only about 6-7 quid it'd be silly not to rule it out,

or fuel pump, there a common fault yet dont show up on computer, hope this helps you mate

If the thermostat was stuck closed the engine would overheat. The only way thermostat would affect fuelling is if it were stuck open, thereby not letting the engine warm up, keeping it in open loop mode. Even then though the effects wouldn't be enough to cause hunting/missfire.

Looking at the list of error codes, someone has been unplugging random things in a bid to find the cause.
Sounds like you have a vac leak to me or a fault with the idle speed control valve. Vac leak could certainly cause the MAF error and unstable idle.

 

[Rogue-24-84]

Thanks for the reply. I guess I could try the thermostat first but my car doesn't over-heat etc. and I've taken it on long drives before. If the thermostat was the fault would this not show up on VCDS? I could try the fuel pump too but again when actually moving the car runs perfectly fine, it's just this very odd misfire at idle that makes it hard to pull away and easy to stall, it tends to lead to slipping or dumping the clutch to pull away.

no it wouldnt nessi overheat or be noticable atall, trust me these are a strange little thing they do!! and as for the fuel pump they run off pressure so can behave different under different loads the engine requires i.e it runs completely different when the car is under load as in driving along, as it does on idle, just a thought mate

 

[Rogue-24-84]

no it wouldnt nessi overheat or be noticable atall, trust me these are a strange little thing they do!! and as for the fuel pump they run off pressure so can behave different under different loads the engine requires i.e it runs completely different when the car is under load as in driving along, as it does on idle, just a thought mate

If the thermostat was stuck closed the engine would overheat. The only way thermostat would affect fuelling is if it were stuck open, thereby not letting the engine warm up, keeping it in open loop mode. Even then though the effects wouldn't be enough to cause hunting/missfire.

Looking at the list of error codes, someone has been unplugging random things in a bid to find the cause.
Sounds like you have a vac leak to me or a fault with the idle speed control valve. Vac leak could certainly cause the MAF error and unstable idle.

lambda 1 runs along with engine temp and maf and adjust's fuelling accordingly, and if the thermostat is gone then they DO misfire as the lamba doesnt read right and fuels all wrong.. vac leak poss yes tho, thought this would of been checked first off tho..

 

[FJ0372]

Have you done a compression check?

No I haven't done a compression check. Wouldn't lack of compression lead to an obvious lack of power though? the car seems to run really well when revved.

I've had the same problem mate. Thermostat and fuel filter change fixed it right up, apparently this is quite a common fault on the 1.6 especially as they age. What sort of milage is it running on the clock?

Interesting. I will try and change the both of these tomorrow and see where that leads me. My car has just over 85,000 miles on it. I meant I was slipping the clutch to get it to pull away, it doesn't slip the clutch by itself.

Looking at the list of error codes, someone has been unplugging random things in a bid to find the cause.
Sounds like you have a vac leak to me or a fault with the idle speed control valve. Vac leak could certainly cause the MAF error and unstable idle.

How many vacuum pipes are there on these A3s? I should probably mention that I've changed the largest breather pipe with a brand new one, that goes from the rocker cover into the air system as the original was battered. I also opened and cleaned the air flow meter.

 

[Lewis583]

lambda 1 runs along with engine temp and maf and adjust's fuelling accordingly, and if the thermostat is gone then they DO misfire as the lamba doesnt read right and fuels all wrong.. vac leak poss yes tho, thought this would of been checked first off tho..

If you look at your post you stated "stuck closed". If the stat was stuck closed it WILL overheat unless you run the interior blowers constantly at full temp.
The only time engine temperature affects fuel mixture is when the engine is COLD (cold start, open loop)

 

[Rogue-24-84]

If you look at your post you stated "stuck closed". If the stat was stuck closed it WILL overheat unless you run the interior blowers constantly at full temp.
The only time engine temperature affects fuel mixture is when the engine is COLD (cold start, open loop)

i ment stuck in general i just wrote it out wrong, obviously if it wasnt opening it would be boiling up anyone knows this, ive changed the post for future reference so ppl dont get the wrong idea, this does cause problems as greengunner4511 had the same prob and it was down to this!! its common on these engines, i was only giving my opinion and knowledge and experience i have.. i'l keep my mouth shut in future as you obviously know everything... qualified nvq3 motor engineer with 12 yrs experience means nothing lol

 

[Lewis583]

I'm a mechanic too:icon_thumright:

 

[Rogue-24-84]

I'm a mechanic too:icon_thumright:

didnt mean to sound rude btw mate, just so many ppl on here are forum mechanics and just pass on advice they've read somewhere else not knowing what they are even talking about and just talking chinese whispers.... but glad to hear your a fellow spanner jockey tho and have experience it whats what and what your on about PEACE lol

 

[Ashley g]

I'm also a mechanic!

We had a golf in not so long ago with a similar problem. A vac leak had caused 1 cylinder to run lean, and that had burned a valve, which is why I suggested a compression check.
Worth a go and doesn't take long

 

[FJ0372]

Hey guys, thanks for the responses. I've been working on the car closely with a mechanic friend (everyone's a mechanic here too apparently haha) and basically, he dismissed fuel filter as he said it would affect the car when revved (i.e lack of power/cut-out) which this never does.

Instead we tried changing all of the injectors as he had a spare set laying in the garage. This was surprisingly hard to do as the manifold had to come off. The outcome was less than pleasing as after replacing all the injectors I still have the exact same problem! following on from this we opened and cleaned the throttle housing innards with a brush and some petrol. Once we put it back together it actually idled well for the first minute or so, which seemed really odd. The conclusion we came to was the petrol we used to clean the housing was being burned as well as that which was being pumped into the engine and both together were letting it idle well. A few minutes later the old problem came straight back.

Then my friend noticed and showed me something really odd. When revving the car very lightly the revs actually dip slightly at a certain point just before going up. So not only does the car idle poorly but it's also much like the car has a little flat spot. This would definitely explain why it's harder than it should be to pull away without stalling. My friend thinks the next step should then be to remove and replace the throttle housing with the sensor that is on it as it's possible the sensor is faulty. No errors on VCDS. What are your thoughts on this? does it seem possible to you mechanics and enthusiasts that there is a fault here.

 

[FJ0372]

*BUMP* any opinions on the above diagnosis guys? this problem has me pulling my hair out! which is a shame because I really love the car other than this stupid issue.

 

[Ashley g]

Pull the vaccum hose off the brake servo and blank off the pipe, then restart the engine. We had an Astra at work last week with a similar problem, turned out to be a faulty brake servo!

 

[JKostas]

Hey. Did you find out what was wrong with it?

 

[Allan Macdonald]

If you did find the answer please let us know, I have the same problem, even if you didn't fix it have you tried anything else?
Al

 

[JKostas]

My was spark plug leads... Fixed ot for a while but after a month it apeard again

 

[piehole1983]

Bit of a thread revival here but did anyone get to the bottom of this? I've recently picked up a 1.6 with the BGU engine. Drives fine but I have a pretty noticeable and random "miss" at idle. I serviced the engine as soon as I got it but did not change the plugs. I did fit a new water pump and thermostat. Do these cars have a dual mass flywheel?

 

[<tuffty/>]

Bit of a thread revival here but did anyone get to the bottom of this? I've recently picked up a 1.6 with the BGU engine. Drives fine but I have a pretty noticeable and random "miss" at idle. I serviced the engine as soon as I got it but did not change the plugs. I did fit a new water pump and thermostat. Do these cars have a dual mass flywheel?

What year is your car?

<tuffty/>

 

[piehole1983]

Sorry it's a 2005 Sportback

 

[Tj 0785]

Wrong section you want the 8p section

 

[piehole1983]

Didn't even notice that, just seen A3 1.6 in the title!

 

[Motodenta]

have you ever try to measure the vacuum at idle ?
not just the number but also movement of gauge needle is important.
e.g. you can have a sticky valve or...
how about your exhaust note ?