2.8quattro starting problem.

[kuzz]

Hello people. first post here. Have an a4 2.8v6 avant. done 100k now so getting on a bit. It's developed an intermittent starting problem I'm struggling to get to the bottom of, so any advice I'll be grateful for. If it's been left alone for more than 2hrs it'll start 1st time 100% of the time. anything under that it gets a bit dodgy. nightmare getting fuel, you never know if it's gonna start again. strange one because even after an hr or so when it has gone completely cold it'll keep turning over but won't fire. 5-25 Min's of trying to start it, and it'll always go in the end as if there was never a problem, it'll just suddenly work. already changed crank shaft position sensor as advised on another forum. and coolant temp sensor as the car was repeatedly logging a fault code for it.

[spellmaster]

Annoying problem.

Im assuming the car has been serviced with the correct parts (i.e spark plugs etc). may be worth checking if unsure.

Has the coolent temp problem been solved now with the new sensor. a defective coolent reading could cause the ecu to inject too much fuel (thinking that the engine is cold).

The problem needs to be devided in to three areas. Fuel, Air and Spark. Two are easy the thrid is not quite so.

1. Fuel, over fueling on cracking could cause this, as mentioned previosly. pulling the plugs out and finding them wet, would indicate this. Also a smell of fuel.
The Fuel pressure regulator may be at fault here, check its condition and also check the vacume hose has not sprung a leek.
Injectors may be worth checking, whip them out and check there spray pattern, should be nice and misty not uneven. Some injector cleaner may be worth a go. personaly iv not had problems with injectors on any of my a4s and theyve all done over 100k.

2.Air, excess air could cause an issue but it is likely to be more of a consistant problem, resulting in masses of black smoke from the exhaust when flooring it. Usualy this is a simple hose problem split or not tight.
Not enough air could be a throttle- throttle body issue. Try starting with the throttle wide open and see if it helps. Some A4s have semi or fully automatic electronic throttles too.

3. spark. Harder to diagnose as you need to know that the spark is comming at the right time, not just that your getting one. again this would be more consistent problem. check wireing for signs of repair, when getting sender faults through vag.com ensure that its not the wires that are causing the problem. Wire often goes where it meets the plug and can be missed.

hope that helps, with out seeing the car my money would be on a fuel issue.

[kuzz]

Thanks for taking the time to reply. never sure if there was a problem with the coolant temp, as there were fault codes and the sensors are cheap i just had it changed anyway. i have noticed that the external temperature that shows up on the dash is always much hotter than it really is, it was icy outside the other day and the car was telling me it was 12 degrees. I've always assumed this is irrelevant and unrelated, am i right?
Someone I've spoken to with the same car and the same symptoms on this forum told me they sorted there's out by changing the camshaft position sensor, sound like a lightly cause?

[spellmaster]

External temp sensor is irrelevent as its a sensor just behind the bumper nr the rad. Ecu works of water temp as well as other sensors (like the MAF).
It would be good to ensure that all sensors are working correctly, using vag.com or similar, modern sensors should generate fault codes if they are reading out of specified tollerences.

Sensors often get the blame, even by vag mechs only to find a more fundemental problem is the cause.

But remember its a v6 so you will have two position sensors G163 is bank 1 (right hand) G40 is bank 2 (left hand) as you look at it.