Engine light - misfire

[Mik]

I have a 2008 S3, which has had an engine light on for a while now, it does go out but then comes back on again. It doesn't effect the running in any way, that I notice, but with the new MOT regulations I'm concerned I might have a big bill coming my way to get it passed. I took it into Audi to diagnose and they came back with fault codes 0300 - multiple misfire and 0304 cylinder 4 misfire. They said it due to worn exhaust valves and quoted £1800.
Could any body give advice or shed some light?
Many thanks

 

[Daveyonthemove]

The first thing to do with any warning light is check it out ASAP. Hoping it goes away normally leads to much bigger problems and bills further down the line, as you appear to be finding out.

Personally, I would look at the spark plugs and check their condition. If they haven't been replaced at service intervals then they could be past their best.
If the misfire continues after changing them, move the coil packs around to see if the misfire follows a particular coil pack, this would indicate that one is knackered and needs replacing.
If none of that helps, then you probably have to take Audi's word for it and get that job done by someone you trust and can afford.

 

[Mik]

The first thing to do with any warning light is check it out ASAP. Hoping it goes away normally leads to much bigger problems and bills further down the line, as you appear to be finding out.

Personally, I would look at the spark plugs and check their condition. If they haven't been replaced at service intervals then they could be past their best.
If the misfire continues after changing them, move the coil packs around to see if the misfire follows a particular coil pack, this would indicate that one is knackered and needs replacing.
If none of that helps, then you probably have to take Audi's word for it and get that job done by someone you trust and can afford.

Many thanks, I've changed both those and still has the same problem. Might just have to bite the bullet!!!

 

[ScottyP45]

Any mechanic can do a compression test for a few quid, or just buy a gauge and do it yourself. If the compression is bad or wildly different between cylinders then your valves could be worn but without testing you won't know. A gauge is probably £25 So worth a go. How does it actually drive? Heavy misfire, light misfire, worse cold or hot etc

 

[leshkin]

I'm not sure how they could tell that valves are the cause of the misfire without a compression test haha

Also, "Worn exhaust valves"? How did they arrive to that diagnosis? Bear in mind that Audi techs are very good at following predefined troubleshooting workflows and know very little about how to deal with older cars.

It could absolutely be that, but can easily be injectors, fuel supply or multiple other things that can cause random misfire.

I'd agree with others, get it booked into a reputable VAG specialist indie and get it checked out properly.

 

[Mik]

Any mechanic can do a compression test for a few quid, or just buy a gauge and do it yourself. If the compression is bad or wildly different between cylinders then your valves could be worn but without testing you won't know. A gauge is probably £25 So worth a go. How does it actually drive? Heavy misfire, light misfire, worse cold or hot etc

Many thanks. It seems to drive fine, no hesitation, flat spots, idle needle is always stable, regardless of temp. However, I might just be used to it, it might be running a bit rough and I can't tell the difference. That's a good idea, I might look at doing a test, thanks.

 

[Mik]

I'm not sure how they could tell that valves are the cause of the misfire without a compression test haha

Also, "Worn exhaust valves"? How did they arrive to that diagnosis? Bear in mind that Audi techs are very good at following predefined troubleshooting workflows and know very little about how to deal with older cars.

It could absolutely be that, but can easily be injectors, fuel supply or multiple other things that can cause random misfire.

I'd agree with others, get it booked into a reputable VAG specialist indie and get it checked out properly.

Many thanks, the kept the my car for two days and it had to be seen by the 'master technician' who came to the conclusion it was worn exhaust valve springs that had gone too soft, hence the random misfire across all 4 cylinders and misfire on cylinder 4. There is one down the road from me, so I'll book it in with them to have a look at. I just assumed, wrongly maybe, that Audi would be able to get to the bottom of things straight away!!

 

[ScottyP45]

Thing is, on an old conventional engine this behaviour on opening the throttle would have been sticking advance weights in the distributor or a burst diaphragm on the carb fuel advance since to suddenly accelerate needed a shot of extra fuel, as fuel moves slower than air, and a temporary lean condition existed without it so you could just be looking at an un-metered air leak on the intake path, if the misfire only happens on opening the throttle then disappears. The technology has moved on but the engine needs the same things at cylinder level. Where the bang actually happens, it's still 1850!

An easy way to find an intake leak is to get a plumbing store cap that fits your intake pipe from the maf onward and drill it to take a tyre valve, fit the cap to the pipework securely and pump it to 14psi or 1 bar MAX with a foot or bike pump then listen and watch to see if it holds. If not, you should hear the leak. Water in a spray bottle with a touch of fairy liquid will blow big bubbles where the leak is.

 

[leshkin]

Many thanks, the kept the my car for two days and it had to be seen by the 'master technician' who came to the conclusion it was worn exhaust valve springs that had gone too soft, hence the random misfire across all 4 cylinders and misfire on cylinder 4. There is one down the road from me, so I'll book it in with them to have a look at. I just assumed, wrongly maybe, that Audi would be able to get to the bottom of things straight away!!

It could easily be carbon build-up on the intake valves that's causing these issues, so not sure on how they concluded that it's the exhaust valves "going soft". I've never heard that sort of conclusion if I'm honest though.

I don't know where you are located, but there is no shortage of reputable indies about that deal with these engines on a daily basis. RTech keeps springing to mind as their healthcheck process and experience with the 2.0TFSI lumps is second to none. If there is a problem, they will find it and in many cases fix it. At the very least, going to an indie, you'll have a second opinion before splashing out a large sum of money.