BenKenobi

I have a 4.0TDi, starts first turn of the key cold, sometimes when hot but quite often following a drive it can be difficult to start, it always does but can take two or three attempts, this 'dodgy period' can last for up to an hour.

There are no faults logged (I have VCDS). The car is totally pampered new filters annually - all of them - but has this 'annoyance' and I'd like to get rid of the 'annoyance', I semi wish that it would report a fault, at least I'd have some place to start.

I know it could be temperature sensor but the values on it look OK. There are no reported crank / cam position sensor errors, no intermittent's, no immobiliser faults, nothing. This is a pretty complicated lump to be throwing new sensors at it in the hope of striking the jackpot.

Anyone got any ideas ??

mishar

No Audi Diesel experience but since it is common rail injection it must have crankshaft position sensor, probably just like gas engines. Failing when hot looks like it. Later it will not start at all.

BenKenobi

Wouldn't this throw an error code or something in VCDS or are these things that fickle ?

volvopentaman

Crank sensor is my vote too.

It WILL NOT give a fault code, unless totally broken/disconnected.

This is the only sensor, that the engine will not start without. So if it is dead, the engine just won't start.

And as said, it will throw no fault codes either, unless disconnected/totally electronically broken.

You can measure it's resistance, By unplugging the wire from the main engine harness, probably behind the engine.

The resistance should be around 100-200ohms, if much above that or close to 0, replace.

Other possible faults could be a failing fuel pump, would DEFINEATLY put a faultcode of low fuel pressure, same with fuel pressure sensor.

There are double ECU:s and air-mass meters, and double of almost everything on the engine, except the crank sensor.

You could test the sensor, By trying to get the fault "on", By taking a drive to get the engine hot, hookup VCDS, go to ECU, read engine speed, i think it's on "channel 1", and on several other "channels" too, and try to start the engine.

If the speed says 0, and no start = bad sensor

If the speed comes up after a while waiting, and the engine starts = bad sensor

If the speed comes up, and still no start = probably not the sensor.

The common rail engines are btw quite near gas engines to their management, just a bit faster injectors etc.

BenKenobi

Just out of interest where is the sensor located. I've had a quick look and you're doing nothing in a hurry behind the engine - there isn't a hope in hell of me getting my hands in there never mind tools, this better not be one of those engine out jobs because that's a leviathan task on these cars.

I know my old Passat W8 we had to take the engine out to do an O2 sensor.

mishar

Sensor is underneath the car in the gearbox close to the engine. There is another tread about that just on.

BenKenobi

If it's there that should be an easy enough change. My car didn't come with undertrays from the factory which is great for access (as long as you raise the suspension first )

haggisuk

If you have VCDS please check the various temp sensors on the engine, fuel temp etc both hot and cold.

I don't own a diesel so can't point you where to look in VCDS but I'm sure google will help you.

There are loads of Hot start vag diesel issues on google and a lot of them are dodgy temp sensors.

So if I were you I would start there.

BenKenobi

Biggest issue is that the 'fault' isn't occurring when laptop and VCDS cable are handy - need to get it hot and try to reproduce on the driveway.

Regrettably with the prices Audi charge indiscriminate sensor replacement in the hope of fixing an issue isn't a path I'm ready to go down, I'm not a fan of cheapo E-Bay lookalikes.

I've had crank sensor issues before although on a VW and that was a mare to track down because as has been indicated crank sensors don't throw errors when failing but cause anarchy when they do, I've ordered a new crank sensor because that can stop the car completely, so if it isn't the issue ultimately and I end up with a spare crank sensor so be it.

BenKenobi

Well the crank sensor didn't fix it, so time to start looking elsewhere for the fix.

For what it is worth the crank sensor is dead easy to do - 30 mins tops. It is underneath the car, just behind the autobox oil cooler. Easily visible, need to loosen the oil cooler bracket to give it enough room to come out but other than that it's easy.

There seems to be very little information to support diagnosis on the 4.0, I have Etka and to be honest it's pretty useless. There is no 'table' to identify which sensors are involved in the starting process, anyone know of such a thing?