TDi 75 Reluctant starting when warm

Intercept

Intercept

A2OC Donor
The wife's 2005 TDi 75 (but actually 96 thanks to Stealth) is occasionally reluctant to start when warm. Not cold, not hot, just warm. Cold starting is always fine, and hot starting is always fine. A typical scenario would be restarting after a 1-2 mile trip after previously starting from cold, or restarting an hour or so after finishing a journey which properly warmed up the engine. It doesn't do this every time - perhaps 20%, but it takes 5-10 seconds of cranking to get it to fire up. After which, everything is fine. The car has no other faults (ignoring rattly trim etc) and is always OK once its running.

I recently changed the fuel filter in the hope that that would improve things but it hasn't made any difference. The old filter was filthy, but not an Audi part so I suspect that it has been changed at least once during its 105K miles.

Any thoughts?

Thanks in advance

John
 
Is the battery healthy when cranking? I think someone had a problem like this before. His was a relay and a dodgy wire going to the starter.
 
Intercept said:
The wife's 2005 TDi 75 (but actually 96 thanks to Stealth) is occasionally reluctant to start when warm. Not cold, not hot, just warm. Cold starting is always fine, and hot starting is always fine. A typical scenario would be restarting after a 1-2 mile trip after previously starting from cold, or restarting an hour or so after finishing a journey which properly warmed up the engine. It doesn't do this every time - perhaps 20%, but it takes 5-10 seconds of cranking to get it to fire up. After which, everything is fine. The car has no other faults (ignoring rattly trim etc) and is always OK once its running.

I recently changed the fuel filter in the hope that that would improve things but it hasn't made any difference. The old filter was filthy, but not an Audi part so I suspect that it has been changed at least once during its 105K miles.

Any thoughts?

Thanks in advance

John
Click to expand...

Does the temperature gauge read 90 when the car is hot, because a faulty temperature sender could be the cause, it could be thinking that the engine is still cold and therefore over-fueling it?

Probably not the cause but worth checking as it is so easy.

Steve B
 
Birchall said:
Does the temperature gauge read 90 when the car is hot, because a faulty temperature sender could be the cause, it could be thinking that the engine is still cold and therefore over-fueling it?

Probably not the cause but worth checking as it is so easy.

Steve B
Click to expand...

Not always the case since on the AMF the coolant temperature sensor has 2 bits in the same sensor. One sends the info to the dash and one sends the info to the ECU. So only 1 bit might of broke. The only real way to check is Using vagcom and comparing the 2 values.
 
Dblock said:
Not always the case since on the AMF the coolant temperature sensor has 2 bits in the same sensor. One sends the info to the dash and one sends the info to the ECU. So only 1 bit might of broke. The only real way to check is Using vagcom and comparing the 2 values.
Click to expand...
Totally agree but I have had this happen and so worth asking at least?

Yes, a scan should clearly indicate if this could be a possible cause.
I must admit that I can't think of much else that would give these symptoms.

A low battery or worn starter motor is more likely to result in poor COLD starting.

So the usual advice then. Find someone local with a VCDS and get a scan done.

StevebB
 
From the symptoms you describe it could be a faulty crankshaft position sensor. Happened to my A2 & cost £120+ to get replaced by a main Audi agent (6 years ago).
 
I would also agree with the crankshaft sensor, after having a similar issue, mine was removed and it was dusty, a good clean up and it went back on the car, has been find ever since.... how long it will last, who knows.

The part was available from TPS for £70.

Hope this helps.

bba
 
If the starting problem only happens with a warm engine I'd be tempted to go with Steve B's diagnosis of a faulty temp sensor. Both coolant and fuel temp sensors influence injection timing and fuel quantity so either one could be giving mid range spurious readings.
Anyone with VagCom could log sensor outputs from cold to see if temps rise at a sensible rate, in line with ambient conditions etc

I've not tried this but in theory it should be possible to unplug each sensor (only one at a time) then run the car and see if it improves warm starting. As a back-up the ECU usually interpolates signals from an alternative source in the event of a total sensor failure - so the coolant sensor should cover fuel and vice-versa.

Crankshaft sensors have no back-up so the engine stops if they fail. Its fairly common for this to happen as the engine gets hot but from your description it appears the car is only a reluctant starter (but always start eventually) when warm.

Cheers Spike
 
Dblock said:
Is the battery healthy when cranking? I think someone had a problem like this before. His was a relay and a dodgy wire going to the starter.
Click to expand...

The battery cranks the starter just fine regardless of temperature. Good strong crank with no RPM drop.

Thanks

John
 
Birchall said:
Does the temperature gauge read 90 when the car is hot, because a faulty temperature sender could be the cause, it could be thinking that the engine is still cold and therefore over-fueling it?
Click to expand...

The temperature gauge behaves normally and eventually rises to 90. It takes a good few miles to get there though - perhaps 8 miles of 50-60mph. I've always thought that this was a bit slow, but assumed that it was due to the engine efficiency.

Thanks

John
 
spike said:
If the starting problem only happens with a warm engine I'd be tempted to go with Steve B's diagnosis of a faulty temp sensor. Both coolant and fuel temp sensors influence injection timing and fuel quantity so either one could be giving mid range spurious readings.
Anyone with VagCom could log sensor outputs from cold to see if temps rise at a sensible rate, in line with ambient conditions etc

Crankshaft sensors have no back-up so the engine stops if they fail. Its fairly common for this to happen as the engine gets hot but from your description it appears the car is only a reluctant starter (but always start eventually) when warm.
Click to expand...

Thanks for this, I'll get the 2 sensors outputs compared.

John
 
bye_bye_alfa said:
I would also agree with the crankshaft sensor, after having a similar issue, mine was removed and it was dusty, a good clean up and it went back on the car, has been fine ever since
Click to expand...

Where is the crankshaft sensor located? I presume its a Hall-effect device?

Thanks

John
 
The problem was found to be the camshaft position sensor http://wiki.ross-tech.com/wiki/index.php/00515 Since changing that last week the car has started fine and not cut-out whilst driving. The temperature sensors were in agreement with each other. Faith restored, but still annoyed by resonance near the bulkhead side the driver's footwell, despite resecuring an unclipped brake pipe which looked to be a perfect candidate.

John
 
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