Audi 80 V6 Quattro not starting

[Halix]

Hi all. Newbie here.

Anyone had a situation where the car starts first turn of the key for weeks and then suddenly won't fire at all (cranking over perfectly but no firing). I leave the car alone for 2 or 3 hours and it starts up first time and works perfectly for another few weeks.

The car is K reg Audi 80 2.8 V6 Quattro but I assume the basic electronics are the same for all models of that era.

Before you say "seach the forums", I have and nobody seems to have come across this before.

Hope you can help before I take a hammer to it.:zx11:

[kuzz]

yes. i have this prob on my 2.8 v6. garage can't find the prob cus it always seems to work perfectly when it's there which makes it tricky to find a prob that dosn't exist. changed the crank sensor and temp sensor. still no better. anyone out there have an answer?

[Crasher]

ECU relay

[Halix]

yes. i have this prob on my 2.8 v6. garage can't find the prob cus it always seems to work perfectly when it's there which makes it tricky to find a prob that dosn't exist. changed the crank sensor and temp sensor. still no better. anyone out there have an answer?

Hi Andy

I resolved the problem early last year.
It was the camshaft sensor. Located on the back end of the right-hand camshaft looking from the front of the car.

One thing I don't understand is why Audi designed the damn computer to keep on cranking the engine but refuse to start when it gets an error signal from the bloody sensor. If there is something wrong with the camshaft, the last thing you want the engine to do is crank over on the key!:zx11:

Anyway, after shelling out an arm and a leg the little beauty has not gone wrong again - touch wood!

[kuzz]

are camshaft sensor and crank sensor different things?

[Halix]

are camshaft sensor and crank sensor different things?

Hi

Yes. There is a sensor on the flywheel that checks for TDC as it rotates and a sensor on the camshaft to check if the camshaft is in the right place at the same time. If there is anything wrong with the camshaft (like a snapped belt) then the computer won't let the engine start but will let it crank - crazy!
In my case there was nothing wrong with the camshaft at all - just the sensor periodically giving the wrong information to the computer. You leave it for a few hours and everything is ok again. It seemed to happen when I just started the car and drove round the corner to the shops - when I came back the damn thing cranked but wouldn't start. It didn't seem to happen after a long run unless I left it to cool down for an hour or two - then it wouldn't start again. Must have something to do with expansion within the unit at certain temperatures.

[kuzz]

sounds very familiar. I don't dare get fuel when it's busy, only have to drive around the corner but often end up sitting there waiting, looking well foolish because the dam thing won't start again. But with some patience and on the verge of a flat battery it'll always start in the end as if there was never anything wrong. I'll get it changed, fingers crossed it'll sort it out. thanx.

[Halix]

You might want to get a friendly VAG dealer to plug your car into their diagnostics computers before buying the part - I paid £20 to get a dealer in Hereford to check it.

[Crasher]

WHY would VAG build into the ECU a line 50 cut out relay output because of any stored fault code? The G40 sensor tells the ECU where number 1 piston is so as the ECU can initiate sequential injection for emissions reduction but if there is a fault with the G40 before starting, it will prevent injection and therefore no start. To diagnose this there is a fault code and fault code reading equipment, it would be pointless to prevent cranking, it is like saying if the fuel gauge reads zero, the starter should not operate. As far as I know no manufacturer has a fault code starter cut out system, it simply would not make sense to provide such a facility. The only function I can think of that comes close is Rover putting a fuel pump cut out on the oil pressure circuit of the SD1 and this malfunctioned so often that people simply bridged it out.