Audi A6 4.2 Quattro Cold Start Problem

[inavogue]

Hi, this is my first post. My A6 4.2 (MY2000) has recently developed a cold start problem. The initial start is mostly ok, but if I turn the engine off after a few minutes i.e. not really warmed up, a restart is almost impossible. The starter cranks furiously, but the engine won't fire. If I turn everything off and try a restart it will eventually fire and run ok. This problem (may be coincidental), seemed to coincide with my brake pad replacement warning lamp coming on a few days ago. Any thoughts please?

[lonky]

sounds like a faulty temp. sensor. Does it smell rich when first started ?

Does temp guage stay solid at 90, or sit low ?

If you have these symptons it's cheap to buy, and shouldn't be too hard to change

[inavogue]

Hi Lonky, thanks for comments. Does smell rich when first started and continues to do so for say 2/3 minutes (when sun roof is open). Temp will eventually go to 90, but will drop back on downhill run. Would appreciate your suggestions on next step.

[Duffelcoat]

I believe that in normal use the water temp should sit at 90oC and not go below once warmed up, it might go higher in extreme circumstances.

My 2.5tdi is currently never getting to 90oC unless I am stuck in traffic for ages, it is getting a new thermostat very soon.

I don't know about your engine, but some are more difficult to replace than others, but the thermostat itself is less than £20 I think.

[inavogue]

Thanks for help on this one. I agree, the issue is probably related to the coolant temp sensor and/or the thermostat. I have contacted the local Audi service agent and they seem to agree. The car is going to them on Friday to confirm and fix (hopefully).

[lonky]

Would try temp sensor / sender first, they only cost about £15, often fail and are relativley easy to change if your hands are not to big. Not 100% sure of location on your engine, but would expect it to be at the upper rear of the engine on a water pipe, rather than the block.

The thermostat is a much bigger job. Technically requires cam belt to be removed, though can be done by sliding it 50% of the pulleys if you are brave.

[inavogue]

Thanks again Lonky. The coolant temp sensor is definitely my preferred 'fix' as I had the cam belt replaced a couple of years ago and would not like to incur that cost again, so soon after. Incidentally, the last two cold starts have been normal, without even a hint of the problem. I am starting to wonder if maybe a 'bad' tankful of fuel might have caused it. However, I have been around cars long enough now to know that a 'temporary' reprieve from a problem is probably just that! But, I am not too keen on the service agent spending a couple of expensive hours troubleshooting the problem. Bit of a dilemma really.

[lonky]

If you have Vag-com you can easily check the fault yourself, even with the freeware version.

with a cold engine look up the water temp sensor reading. It should be close to air temp.

Then run up the engine and check how it varies. It should get to 90 degrees in 5 - 10 minutes even if just left ticking over.

Last 2 times I have had this issue, sensor was reading about 10 degrees low. So when air temp was 15 degrees it thought it was 5, and gave plenty of choke.
At the other end of the scale it never fully got off choke, and always ran a little bit rich.

Nice thing when fixed was the fact that I got about an extra 20 miles per tank.

To be honest a shop should be able to confirm in under an hour if it is the sensor or the thermostat.


When you had the cam belt done, did you also have the water pump and thermostat replaced ? If so it is not likely to have failed again so soon.

[inavogue]

I have just come back from the workshop and after spending an hour on the car, they pretty much found nothing wrong, even though the problem occurred while they were working on it! They said their computer check revealed no faults and I think they probably ran the types of checks you described. Their workshop is glass fronted and I could see that they had their computer check equipment hooked up and were pretty busy around the car. Their workshop Manager was brought in to the process at one stage. They did clean the throttle body and did one or two other minor maintenance things that might be relevant. Basically, they said see how it goes. While I would have preferred that they find something like what you have described, it appears they didn't. The good news is they did not charge for their work
I did not have the water pump or the thermostat replaced while the cam belt was being done, but now wish that I had!
Thanks again for your very helpful posts.