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Im looking for some help, the engine management light came on the car but was driving fine. I put Vagcom on but no faults came up? I put it into the garage I use and the only fault they could get was Random Engine Misfire. I hadn't experienced it but the mechanic said the engine was running very lumpy, if you left the engine running it would randomly rev up, after about 15 minutes the brake servo stopped working but came back to life again? He replaced the plugs and coil pack but this made no difference. He cleared all the faults took the car for a run but again after about 15 minutes brake servo failed, engine running very lumpy. He checked for fault codes again but none came up, anybody had the same problem or any idea what this could be?

Sounds like a mixture problem, possibly caused by a vac leak. Codes don't appear unless you hit the trim limits. Also, dodgy MAF readings can lead to incorrect torque calcs which cause the TCU to put the EML light on without leaving a fault code in the ECU. Have a look at the TCU DTCs and also the ABS/EPS fault codes.

Check group 032 (IIRC) to see what the trim is after 10 mins of driving. Note: If you reset the ECU fault codes it also reset the trim levels to zero can give you a false sense of security.

Log the MAF values when idling, driving normally and WOT.

Since your master cylinder is also involved, that strongly suggest a vac leak, possibly from the cylinder itself.

Sounds like you have a vacuum leak from the brake servo vacuum pipe valve which is under the inlet manifold, they are common for cracking. And the reason why it’s ok to begin with is as it gets hot it expands and looses vacuum due to the air leak hence no servo assistance . Follow the pipe from the servo through the bulk head to make sure it’s sealed there first, after that it’s manifold up to check the rest. Also ask your garage to carry out a smoke test of the intake system to check for leaks and it might be worth doing a compression test as well but I don’t think from what you’ve said that it will have a compression fault. Doing live data of the mafs may indicate an air leak as well.

The vacuum pipes were superceded many years ago, check if yours have the latest type on.
But as mentioned above, 99% you have a vacuum leak somewhere.

Thanks for your help guys, I have passed on the info to the mechanic so hopefully that'll cure the problem. I remember now when I first got the car many years ago that I had a similar problem and a lot of the tubing was replaced. Once again thanks for your help, its greatly appreciated

Gents'

I need your help again if possible? Given the age of the car I had intended to replace all the servo vacuum tubing and the bracket, but as usual Audi have nothing. Is there a good aftermarket place for items like this or somewhere that can make these pipes for the car so that I can replace them? Anyone got an idea or can think of a solution, way of getting these items would be greatly appreciated.

Thanks

I would go for the reinforced silicon type like the pic below: