BenKenobi

I have hot start problems, I'm going to replace things but I need to know hpw many. I have VW docs but they aren't very clear, I can find only one coolant temperature sensor - the gauge in the instrument cluster works fine so I'm assuming that there must be more than one - assuming that a failing sensor is causing this hot start problem. I don't think it's cam position sensor because it runs fine, doesn't miss a beat and pulls like a train - but I'll change it (them) out anyhow since they're only pennies.

Just how many camshaft position sensors does the A8 4.0TDi have. I can only find one in the VAG docs but this is illogical -

And how many coolant temperature sensors - i.e. G62 -

I know G62 is a tart requiring removal of the AC compressor to get at - murphys law says that if anything fails it will be the cheapest part but also the most awkward to fix.

mishar

Yesterday:

Today:

So did you change rpm sensor or not?

BenKenobi

If the 'rpm' sensor is the crank sensor yes, it has a brand new one.

The temperatures all look good too, as does fuel pressure (303bar at tickover, fires up at around 240bar) but the sensors I have values for I don't have labels for.

VCDS Measuring Group 7 shows 4 temperatures that look similar to this

30.7 87.3 34.4 90.1

I'm guessing which is which but I think the second and fourth are the engine and radiator, the others are fuel temperatures ? - parts book shows one in the bottom hose but I can't see it. The other one requires removal of the AC compressor which isn't a 5 minute task - but the values seem OK.


I have just reproduced the error - I've been soaking the car by running it much of the afternoon - when the car doesn't start the fuel pressure never gets above 48 bar - way too low - so what can cause this ??

The car started second turn of the key and the pressure was OK - but where the hell to start ....

mishar

Again, no Audi Diesel experience, but all those fuel pressures seems to low for a common rail Diesel. 48 bar wouldn't start old Leyland engine, though it would be happy with 200 bars. There must be fuel filter, pressure regulator and of course pump. Start there. Also try some European Audi forums. They must have much more experience with Diesels.

BenKenobi

There are two pumps located in the tank. In playing around today I have noticed a 'bubbling' going on when you put the ignition on - like there is air in there.

This bubbling exists on our Passat since the low pressure pumps in the tank feed the filter, but it isn't as pronounced.

The pressure regulator should theoretically produce error codes but there are none - doesn't mean it's good though, just less likely.

The problem with the pressures is that Elsawin (VAG workshop data) doesn't identify what the pressure should be - it really is rubbish, I don't know how VW go on using it - I'm guessing that they rely on 'online data' ... to which we have no access here in the EU - the US legislation forces them to behave differently I believe - as in closed shop/ restrictive practices are not allowed.

The car's down at 80 Deg C now and starts first turn of the key ...

I've tried a few EU forums but there don't seem to be too many D3 A8 DIY'ers about with 4.0TDi's anywhere, most seem to chicken out the minute they realise how complex these 'new' Audi's really are.