poor warm starting AEL 2.5

[MGaz]

I have a problem with my Audi A6 (c4) quattro.

The car has the AEL 2.5L 5 cylinder engine.

Ever since I've had the car for the last two years, its never liked starting when warm. But I've basically put up with it but now its really beginning to annoy me, with the car often running the battery flat trying to start the silly thing

When the car's cold, it starts after a couple of turns, but if I come back from work.. leave it for couple hours and then try to start it again it just refuses to fire.. and produces a lot of white un burnt diesel smoke out the back till it drains the battery completely flat

any ideas?

Would really like to get this fixed as its driving me up the wall and has left me stranded a couple of times.

thanks in advance

[K7ERG]

Any fault codes?
Check pump timing.
Starter

[5cylinderman]

I ran one of these for years and I'd suggest the following:

Remove the connector on the temp sender at the front of the head between cyl 1 and 2. This will make the ecu think you're in the North Pole and will advance your timing. If it makes a difference in hot starting you need to get your pump timing checked.
Put an extra earth lead between the block and the body shell. The OE one is marginal and every rpm whilst cranking is vital.
If you're getting signs of a new Pope from your exhaust whilst cranking you're getting "some " fuel. Will it fire on a bump start when hot?
After 120,000 miles The plunger will wear in the injection pump and can make hot starting a nightmare. Run the fuel tank as low as you dare, and pour 2 litres of cooking oil into it. Yes, cooking oil, I'm not joking. Take the car for a drive for 20 mins to let the oil circulate and try your hot start. The increased viscosity of the veg oil helps to take up the wear in the pumps plunger, albeit temporarily. If it starts perfectly on the oil, it's time for a reconditioned pump. Not cheap.
Leave your fuel cap loose and see if it improves. I had to replace my fuel cap as it wasn't venting.
Seeing as it starts fine from cold It sounds like the needle lift sensor is ok, which is another usual suspect for bad starting hot and cold.

[diesel des]

Yea injection timing and maybe a hammer mod on the pump to adjust the iq.

[MGaz]

thanks for all the advice so far... I've also noticed that it doesn't turn on the glow plugs after only a couple of hours of sitting around. But it does turn them on when stone cold. Is that normal?

[diesel des]

Yes I think the setpoint in the ecu foe heatwrs is 5 deg c. So it is hardest to start just above that point.

[campbellWS]

Easy start ?

[MGaz]

I ran one of these for years and I'd suggest the following:

Remove the connector on the temp sender at the front of the head between cyl 1 and 2. This will make the ecu think you're in the North Pole and will advance your timing. If it makes a difference in hot starting you need to get your pump timing checked.
Put an extra earth lead between the block and the body shell. The OE one is marginal and every rpm whilst cranking is vital.
If you're getting signs of a new Pope from your exhaust whilst cranking you're getting "some " fuel. Will it fire on a bump start when hot?
After 120,000 miles The plunger will wear in the injection pump and can make hot starting a nightmare. Run the fuel tank as low as you dare, and pour 2 litres of cooking oil into it. Yes, cooking oil, I'm not joking. Take the car for a drive for 20 mins to let the oil circulate and try your hot start. The increased viscosity of the veg oil helps to take up the wear in the pumps plunger, albeit temporarily. If it starts perfectly on the oil, it's time for a reconditioned pump. Not cheap.
Leave your fuel cap loose and see if it improves. I had to replace my fuel cap as it wasn't venting.
Seeing as it starts fine from cold It sounds like the needle lift sensor is ok, which is another usual suspect for bad starting hot and cold.

Starts on a 'bump start' when warm absolutely fine.

Also, I've tried pulling the plug off the temp sensor and this causes the glow plugs to come on, and the car starts a hell of a lot easier now. Could this mean I need a new temp sensor?

[AndyP]

Starts on a 'bump start' when warm absolutely fine.

Also, I've tried pulling the plug off the temp sensor and this causes the glow plugs to come on, and the car starts a hell of a lot easier now. Could this mean I need a new temp sensor?

I don't think so, as said above the glow plugs do not come on above 5°C ( Although I thought it was 9°C ) so they shouldn't bo coming on at all at the moment, I know mine aren't.

Have you checked the pump timing?

[5cylinderman]

Your temp sensor might be screwed but do a resistance check on it first.
Have a look on Audifans website and search for posts by a member called Bladerunner. John (RIP) was a guru with the 5cyl tdi's and he wrote plenty of advice for these motors.
Disconnecting the temp sensor not only kicks the glow plugs into extra duration, it also makes the ECU advance the fuel timing, so at least you have narrowed your problem down to two areas.
I can dig out specs for the temp sensor if you need them, but I'm away from home at the moment.

[twoqu]

Bit of a wild stab but mine suffered with poor warm weather starting, good as gold when cold due to the glow plugs. I noticed that the grey oil pressure switch was seeping which I suspect was preventing the switch going closed contact by bleeding off the residual pressure. I replaced that with an OE one and its started reliabily ever since.
Peel the grey boot back and see whether there is any oil present.

[MGaz]

Is the temp sensor also connected to the internal temp gauge? Because the gauge on the dash doesn't seem to work either..

& Where about's is the Oil Pressure sensor on this engine? i'll have a look for it tomorrow.

[5cylinderman]

The temp gauge sensor is located underneath the top hose connection to the cyl head, with a black boot over it. They give their own problems, but the ecu sensor is the other one I described previously.

[twoqu]

The grey oil pressure switch is on the passenger side of the engine. Just follow the wiring on the side of the block you should come to a grey coloured boot that covers the switch. If the boot is full of oil, its leaking and probably a good idea to change it as they can fail emptying the oil system!

[MGaz]

The grey oil pressure switch is on the passenger side of the engine. Just follow the wiring on the side of the block you should come to a grey coloured boot that covers the switch. If the boot is full of oil, its leaking and probably a good idea to change it as they can fail emptying the oil system!

Oil pressure switch looks fine.. ie no oily leaks or anything like that.