Frenchie99

Hi Long time reader first time poster
My 2002 A4 B6 1.8t has been having a hot start problem
The car starts fine from cold and runs great. But if I leave the car after I have driven it for a while and come back to it.
The car idles really rough for about 10-15 seconds.
If I rev the car the problem goes away.
I have recently replaced the fuel pump for another unrelated issue (I hope)

Any ideas would be greatly appreciated.

gtiryan

I've seen many engine speed sensors have a problem, especially after a hot start. You didn't mention whether there was a MIL or any other warning lights.

Frenchie99

There were no other warning lights or alarms.
The car starts and runs fine from cold it's just when the engine is heat soaked after I have driven it for a while.

gtiryan

May want to scan it anyway and see if there are sporadic faults stored in the ECU memory. Not all faults trigger a warning immediately. Could be coils too.....

Albert_3.0

I had the same problem, until I changed my coolant temperature sensor. To make a long story short, it tells the ecu how much fuel to ignite in order to start the car. They're also known to go bad on our babies. Here's a DYI and you can get it for ~$30 on ECS. Good luck.

http://www.audizine.com/forum/showth...erature+sensor

ALLDATAdiy INFO

This, see if you have any hard codes stored in there.

Otago_Audi

Hi just wanted to add my experience here in case it is helpful to somebody else. Have an Audi A4 B6 (2004) 2.0 FSI 127,000km which I had bought second hand from Japan (I'm in New Zealand) which started suffering from a troublesome warm start problem. It would crank and start no problem when cold. It would crank and start no problem 2 hours after being driven extensively. But if the engine was fully warmed up and then left to cool for say 20 minutes to 60 minutes, it would crank and crank without starting. On the rare occasion that it would finally fire up it would idle very roughly, accelerate weakly, and have the yellow engine check light on.

Replaced both coolant temperature sensors to no effect. Damn.

Then the independent mechanic spotted an error code *when the car was experiencing the cranking but no start fault.* Sorry I can't remember the precise code but essentially it said Signal out of bounds crankshaft zero rpm.

Had the crankshaft position sensor replaced as others on various forums have also had problems with that.

FIXED. Car behaves reliably as it should now, starts first time every time.

Otago_Audi

I should also add that before the repair, the car would also crank and start no problems when the engine was hot. That is, when the engine was properly warmed up after a decent drive. Turning the engine off, and waiting for say 5-10 minutes, it would crank and start again no issue while the engine was essentially still hot.