Help, S3 missfire and black smoke between 3000 rpm and 3750 rpm

[sportquattro]

Hi All

Bit of an odd one, 1999 S3 APY runs beautifully except when under load up hill between 3000 and 3750 rpm, once over 3750, car resumes normal service and goes back to being a pocket rocket. between 3000 and 3750 rpm the car seems to splutter and I get what looks like black smoke appear in rear view mirror, once revs have risen to 3750 everything seems fine again, no smoke no splutter. I think it does it on the flat as well bet less so, on a motorway hill in top its definately not right, below 3000 pulls like a train in all gears. and the same above 3750 rpm. I've not had chance to run VAG-COM as I can't get to my lead at present, I'm thinking the black smoke must be due to massive overfuelling, could coil-packs be the problem? Any thoughts or suggestions most welcome.

Thanks in advance
Nige

 

[<tuffty/>]

Firstly, get the car read for fault codes... APY's can run rich due to various reasons but need to check first as it may well tell you whats wrong... it could be the MAF, coil packs, plugs, FPR etc....

Rather than just replace everything, its worth making sure that all the basics are right, plugs in good nick and correctly gapped, faults codes checked etc then ideally log airflow under load to check the MAF is not got a flat spot...

Seen an APY S3 recently with a similar misfire but it was under boost and remained under acceleration unlike yours which disappears... if its consistently doing this at a particular RPM then it could also point to a boost leak...

<tuffty/>

 

[sportquattro]

Firstly, get the car read for fault codes... APY's can run rich due to various reasons but need to check first as it may well tell you whats wrong... it could be the MAF, coil packs, plugs, FPR etc....

Rather than just replace everything, its worth making sure that all the basics are right, plugs in good nick and correctly gapped, faults codes checked etc then ideally log airflow under load to check the MAF is not got a flat spot...

Seen an APY S3 recently with a similar misfire but it was under boost and remained under acceleration unlike yours which disappears... if its consistently doing this at a particular RPM then it could also point to a boost leak...

<tuffty/>

Thanks Tuffty, the car had a service less than 1000 miles ago; new plugs air filters etc, will run VAG-COM and report back, prior to the service I thought it was just shagged plugs and it didnt seem to be as bad as it is today, so think something definately breaking down under load, MAF is less than a year old and was from Audi as opposed to an eBay jobbie, so hopefully it's not the maf. my betting is coilpack as they are the origionals, but wont be changing anthing until I have run VAG-COM, any particular measuring blocks I shiuld focus on?

 

[<tuffty/>]

Thanks Tuffty, the car had a service less than 1000 miles ago; new plugs air filters etc, will run VAG-COM and report back, prior to the service I thought it was just shagged plugs and it didnt seem to be as bad as it is today, so think something definately breaking down under load, MAF is less than a year old and was from Audi as opposed to an eBay jobbie, so hopefully it's not the maf. my betting is coilpack as they are the origionals, but wont be changing anthing until I have run VAG-COM, any particular measuring blocks I shiuld focus on?

Errmm... 002 for airflow, 020 for timing pull and 115 for boost request vs actual would be a start... not gonna be able to check fuel without a wideband gauge of some description..

Check for pipe splits and loose clips etc too and once you have done some logging check block 032 while idling (without turning the engine off) and see where the fuel trims are sat... no need to log these as you just want to see where they are after the runs...

<tuffty/>

 

[sportquattro]

OK have run VAG-COM as per Tuffty recomendations, not entiirely sure what to look for, but any pointers welcome

 

[sportquattro]

Update, vac leak cured, the pipe under the inlet manifold had a 1cm spit in it, not sure if this is the cause of problem, as not had chance to test yet. But what a sod of a thing to change, just no access to it.

 

[<tuffty/>]

Update, vac leak cured, the pipe under the inlet manifold had a 1cm spit in it, not sure if this is the cause of problem, as not had chance to test yet. But what a sod of a thing to change, just no access to it.

Sounds about right tbh mate.... effectively a boost leak and with air escaping from there it will make the fuelling go rich as non wideband ECU's can't adjust...

They are a ****** to get too but removing the metal plate with the N249 etc valves on it helps with access

<tuffty/>

 

[sportquattro]

test drove the car for 80 miles today and all is happy with the world again, no coughing, no spluttering and no black smoke, a big big thank you to Tufftybloke for his help, I would never have thought a 1cm split in a hose could cause such grief.,