S3 lumpy idle, low boost, compression difference.

[Bez3]

Having a bit of an issue, first encountered last week when crawling through rush hour traffic. Car began to splutter and kept cutting out, unplugged the maf and it ran smooth but had gone in to limp mode.

Since got it vagcomd, showing a maf fault replaced maf but the problem is still there, not as bad but I'm running with the maf unplugged or idle is erratic.

Thinking it was just a boost or vac leak, I've had it smoke tested but no leaks are showing up. I've had a dry compression test done and the readings are; 200, 205, 170 & 175.

JBS believe it's a valve issue either sticking or carboned up as it happened suddenly and would explain the erratic idle and not meeting requested boost of 1.4 bar saying it was only reaching 0.9.

I'm stage 1 on an APY if that helps with all the mods for stage 2. Bit gutted really as I was literally about to go stage 2 and now this.

Not really keen to shell out £1600 to JBS to clean the valves when they can't guarantee it will fix it.

Any help would be much appreciated.

 

[badger5]

do a leakdown test to confirm if valves or not

 

[Bez3]

Thanks Bill really appreciate your feedback, I asked JBS if there was anything they could do to rule out piston rings or valves and they said not really. From what I understand a wet compression test would have ruled out pistons too.

I'll do both this week and let you know how I get on. If turns out to be valves is £1600 a bit steep for stripping and cleaning? I see a local vag specialist is offering it for £300...

 

[badger5]

We do a head rebuild for £595, which is cleaned, new guides, skim, rebuilt with your existing parts..
sometimes exhaust valves are pitted and in that event we swap them out for Supertech inconel items.. +£245 for those

I would get the leakdown test done personally before ripping it apart.
Some of what you describe & been told does'nt add up to me

 

[Bez3]

Thanks Bill! Good to know. I'll update you once I've done the leakdown test.

 

[Bez3]

Think I've just about got to the bottom of it. Intercooler pipe completely disconnected, vac pipe cut at both ends, wheel speed sensor snapped in 2. When carb cleaner was sprayed on the inlet manifold above cylinder 3 it momentarily aloud the engine to run completely smooth with maf plugged in on idle.

Ran out of time to source a new gasket but hopefully will get this sorted in the next couple of days.

I'm no rocket scientist or mechanic, but I've managed to get most of my boost back and I'm guessing if there was a mismatch in cylinder compression it can be attributed to some, if not all of the above.

You were definitely right about things not adding up Bill!

 

[Bez3]

Just to wrap this thread up, inlet manifold gasket fitted today. Idle is buttery smooth now with the MAF plugged in. Power seems to be back as it was before all of this. Never got as far a leakdown test, if I get time I'll get it done but I'm pretty sure this would be a pointless exercise, might be worth it for piece of mind though.

Thanks Bill for all your help, it means a lot. My dad and local trustworthy mechanic weren't convinced by the diagnosis either. All fixed including parts and labour for under £160. Less than a tenth of the quote which wouldn't have solved the problem. This isn't including the original £156 for false diagnosis though! I'll see what they have to say about this tomorrow.

 

[badger5]

glad it helped..

nice result

 

[scotty_24]

Perfect example of why these cars need expert diagnosis from the likes of Mr Brockbank. Guy deserves a medal for the help he gives us muppets.