S3 (2010) Intermittent Misfire *HELP*

[markmaciver]

Hello,

I've been plagued by a very intermittent misfire for a while now.

On first inspection cylinder 3 plug was very coked so replaced all the plugs and a new coil on cylinder 3. Not long after it seemed to return so the rest of coils were replaced..problem persisted. A new set of plugs after this seemed to show a dodgy plug in the set as the car ran fine after for probably 2 weeks.

Now I feel back to square one as the intermittency makes it very difficult to plot or show a pattern of when it happens. Inspected the plugs this week and they were fine, no excess coking or signs of anything untoward.

No lights on dash or faults showing on diagnostics. Car idles fine, holds boost and never misses on WOT.

Any help or suggestions will be much appreciated, thanks in advance.

(43k miles, Tiptronic)

 

[dave_beastttt]

Injectors dirty
Maf faulty
Carbon deposits
Wiring damaged
Valves sticking
Compression low

 

[adi3390]

As Dave says, could be a few things...

Had the same problem on mine.... tried the sparks, coils etc too, only to sit and watch the misfire count on VCDS still.

Had my injectors cleaned, and cleaned out the inlet/ carbon deposits.... not done it since..

Injectors had a good few mm of soot on the end when removed, and the inlet.... well, surprised it ever ran relativity smoothly.

 

[Retroman]

Low mileage car (average around 6000 pa). Does it do a lot of shorter journeys? I'd put a quality injector and system cleaner through the system with a tank of premium fuel. I'd try and coincide it with a longer journey. Have a look at the Forte range which is used a lot (and rated) by the trade.

 

[QuattroCalum]

Is it direct injection on these? I think it is, if so no amount of fuel system cleaner will help the coking in the inlet manifold and on the inlet valves themselves. The only solution is removal and clean manually. Lots of vids on YouTube regarding this.


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[adi3390]

As Calum says no amount of fuel cleaner will do much than clean the internal system.

Carbon will still be there... its a 'manual intervention' job. I did it in my garage at home, not the hardest, not the easiest; few fiddly bolts, there is a handy guide for doing it though.

Very satisfying when done however!

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[Retroman]

Audi use a fuel additive in the US for the problem Part G001770A2. BMW and Toyota offer similar in the UK. Exhaust gas (containing unburnt fuel with its additive) is recycled via the EGR system when the engine is not under load and so some of the fuel additive will also be recycled via the intake system.

Whilst I accept that "direct injection" causes intake carbon build-up, it is also exacerbated by EGR recycling of the wrong engine oil giving off a carbon-laden vapour and lack of sustained heat caused by regular, shorter journeys.

 

[QuattroCalum]

It's the crankcase gasses that cause it, egr is on diesels.


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[markmaciver]

Hi folks, thanks for all the replies.

I'm fairly happy to discount low compression and sticky valves, it's a bit too intermittent to be one of these and it runs almost perfect a lot of the time.

I'll pull the injectors and check over the next few days. IT would make sense as the plugs when they were changed initially were fairly coked so that coke will obviously have built up on the injectors too.

I do a fair amount of short journeys but try and keep them to a minimum and the car is always warmed up etc.

Thanks again.

 

[markmaciver]

Update for anyone that's interested:

Fitted a new diverter valve and my hesitation/misfire feel is gone (touch wood). Very easy job to do and takes no more than 10 mins. On comparing the two valves, the one I took out had a slight feel of resistance when initially pushed (opened) whereas the new one slid freely.

 

[QuattroCalum]

Glad to hear you have it fixed, there's nothing worse than when it's just not running 100%