Running rough (possibly) and back fire

[White S3]

Hi All,
Hoping that the fonts of knowledge in the forum might have experienced this or have a solution. In getting car out of hibernation and readying for MoT my S3 seems to start fine but after a while it idles a bit lumpy possible on 5 but not sure. All plugs have had a clean and spark checked but exhaust smells very rich and possible too rich for MoT gassometer. Points are pitted a bit but although it could do with new cap soon I don't think is the problem (might be wrong?). After a while when the fuel mix is leaned automatically, is when it starts to run a little rough and every now and it back fires in the exhaust. Last look I do have a couple of small leaks from the unions either end of hockey sticks that I haven't sealed properly after changing engine mounts - could this be the problem not creating enough back pressure? It hasn't had a good run since Jul so might just need that(?) but the only run will be to MoT station where I'd rather it was running smoothly when I begin the run!

Any suggestions as to what next to check or sensor to disconnect as a trial to fool the ECU into leaning the mix? Otherwise it is off to MoT with fingers crossed.

[jimed]

I had an S3C and think your engine is the same except for some lambda stuff so it will have an idle control vale at front left of the head. This does need a good clean every now and then or it can stick so may be some/all of the problem. It comes off easily with 2 bolts and can be cleaned with petrol or carb cleaner and then refitted. 10 minute job which might sort it - worth a try anyway.
If you want you can make a new gasket from cereal packet and put a little smear of Vaseline on it before fitting, this seems to work OK.
Good luck!
Jim

[White S3]

Thanks Jim, will have a look and clean it anyway.

[Barkychoc]

There are no points it's an electrical sensor as it spins, so you can rule that out - could be worth investing in a distributor cap leads and possibly plugs if they haven't been changed in a while.

Also check the bottom of the brake servo hasn't rusted out.

[White S3]

Thanks, I hadn't even thought of the brake servo. It is corroded a bit but I don't think it has gone through - any easy way to check? Braking is still as responsive and good as ever along the driveway.

[mep12345]

Mark, you are up and about again? Nick H was asking yesterday if I'd heard from you, clearly the op went well if you are back messing with the beast again?

Mark

[White S3]

Yep up and about, 2 days before having the disks removed I was changing the engine mounts and a week after the op was running through as many MoT checks as I could - just not got under car to putty up the exhaust. Say to Nick that all is well please and I'll probably be in at some point when passing for physio when cleared to drive by she that must be obeyed! Meanwhile back to investigating why the beast of burden is not running as sweetly as it should be.

[tvrgit]

White S3 said:

Thanks, I hadn't even thought of the brake servo. It is corroded a bit but I don't think it has gone through - any easy way to check? Braking is still as responsive and good as ever along the driveway.

Take the servo hose off the back of the plenum (its held on with a jubilee clip) and plug the end of the plenum connector firmly (i.e. not with something that's going to get sucked into the engine!) If the engine then runs fine, then the servo or pipe is leaking.

While you have that connector off, it might be worth connecting a vacuum gauge to that same take-off, and check that it's nice and high - if it's too low (below about 18 inches Hg at idle) then you might have an inlet vacuum leak - I had very similar symptoms when my plenum gasket fell apart.

[White S3]

Thanks, further investigations to follow as haven't had the chance to get back out to the garage yet.

[mep12345]

Sure you've checked it Mark, but when I had a snag like this it took me ages to find the hairline crack in one of the spark plugs that was causing intermittent misfire. Work checking if you haven't already