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Matt
Matt, Engineer
Category: Audi
Satisfied Customers: 20591
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Audi A4 Avant: My audi a4 tdi has missfire when it warms up

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My audi a4 tdi has missfire when it warms up after about 65degrees . It also doesn't start as well and sometimes when driving cuts out and won't start . It will start if towed in 3rd gear. I suspect intermittent lift pump problem.

Sounds great when cold.?
Hello

I'd agree that the lift pump is a prime culprit here , it should run for a few seconds then switch off when you first turn the ignition on. If the connections to this are corroded or damaged then the pump could stop at any time.
Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V

Also check the connections to the crankshaft sensor, unplug it and check for corroded contacts and broken wiring, and consider fitting a new sensor - do the same for the cam position sensor

If both of these are OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
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