Audi S4 Avant B6 hot starting problems

[stobswood]

Since the weather got warmer I'm having problems with my 2004 B6 V8 S4. It turns over fine, but will not start when hot until I lift the bonnet and leave it for 20 minutes or so. It only happens when the engine is hot, the car starts fine from cold or if it stands for a couple of hours before restart.
I thought it may be fuel vapourisation or the fuel pump on its way out. A suggestion was it was an under bonnet temperature sensor but a friendly VW specialist checked the fault codes on the car and other than an alarm trigger a while ago, there are no faults coming up.

Has anyone else had a similar experience and/or suggestions please? I've had the car since 2005 and it has been fine up to now.

[ivaylo]

It's most probably your coolant temperature sensor. It doesn't throw a fault code, it just reads low.

[stobswood]

Thanks, I'll give that a go, they only appear to be about £6. I appreciate your quick response, I'll post how I get on.

[Dr G]

They're notoriously crap at that vintage and as you say, cheap! Does the temperature gauge work as it should? Can be difficult to tell as they're programmed to read 90 through quite a wide range of temperatures to stop people worrying about slight fluctuations.

The other starting issues S4s can suffer from relate to the solenoid (part of the starter in this case) and the ignition switch. If the car is turning over fine but not firing it's unlikely the solenoid but a slim chance it's the switch. Haven't fitted one in ages but from memory they're cheap too.

Let us know how you get on

[ivaylo]

The sensor actually combines two sensors in one housing. One is used for the gauge (how clever!) and the other for the ECU. You will need VCDS to see the value seen by the ECU.

[Dr G]

I didn't know that, useful info

[stobswood]

Sorry about the delay in getting back to you about my S4, family illness took over.
I have bought a basic VGate fault scanner and there are these faults coming up:
16706 - Engine speed sensor (G28): No signal:PO322 sporadic 004-No
17981 - Left Engine Mount Solenoid Valve (N144):Open Circuit:P1573
16946 - System Voltage Too Low:P0562 static - No define
00668 - Supply Voltage Terminal 30 sporadic - Power supply too small

Do these help at all in terms of suggestions please?

Thanks

PS the temp gauge worked normally, gradually going up as the engine got warmer settling at middle once it was up to temp (or appeared to be!).

Edited by stobswood on Saturday 6th June 16:55