2004 Audi S4
4.2L V8, 6 sp manual, quattro
102k miles

Short version:
Engine turns over strong but will not fire when the engine is warm. No check engine light or codes shown. Starts fine when engine is cool, runs fine when started. Any ideas?

Long version:
This morning I drove to Autozone to pick up some brake pads for another car. The S4 started fine first thing in the morning and drove fine for the entire 20 mile trip. After about 5 minutes buying the brake pads I get back into the car and it won't start. It cranks over strong but never fires. There is no check engine light and nothing seems out of the ordinary other than it not starting up. Since I was already in the Autozone parking lot I go back in there an ask one of the employees if they wouldn't mind checking for codes, checking the battery, etc.

The battery tested good. The alternator could not be tested since the car would not start up and their scan tool did not show any codes.

I am no mechanic but since the car was turning over but not firing I pulled out one of the spark plugs to check if it was fouled or anything. The plugs were replaced last summer and the one I checked was still in good shape.

At this point I called my Dad and told him to bring the trailer so we could toe it home. After he gets there about 30 minutes later I "demonstrate" the strong turn-over but no start condition for my dad and it fires right up like there was never anything wrong. I am lucky the Autozone guy was standing right there otherwise my dad would have thought I was crazy.

So I drive it home without any issues. Pull into the garage, turn it off, try to turn it back on and once again there is no spark and it will not start. I then let it sit about 45 minutes, once the engine had cooled back down and it started without issue.

After a little searching I have heard that it could possibly be an ignition module (expensive), cam sensor or crank sensor.

Has anyone ever heard of a similar issue or have any suggestions of what it could be?

Please let me know if there is any more information that may be useful.

Thanks in advance for any help,
Tyler

 

I realize this thread is a little old, but I'm hoping you resolved the problem.

My '02 S6 4.2l just did this yesterday night. I drove about 3-4miles, ran into the store to get some mother's day cars for me and the kid, got in and it fired up ran for about 2-3 seconds and shut off like I turned the key off. Tried to refire and no dice. Gave it about 5-10 minutes to cool off and tried again still no luck.

I walked over to my friends place around the corner to see if we could tow it over from around the corner. Got all the stuff together and rode over in his truck about 45 minutes later. Jumped in to see if it would start and it fired up after an extended crank (5-6 seconds) and ran fine.

Checked it out at his place and had no codes, other than the one for the cat (which is under warranty,still got to get in to the dealer to get that fixed). Have not been able to cause the failure since then and it has me completely paranoid to drive it. My ride to work is all back roads with almost no where to pull over.

Checked the fuel pressure tonight and it is actually slightly higher than spec but only like 5psi or so. I left my gauge plumbed in so if it quits I might get an idea if it is fuel or not... Made the exact same run tonight and just stopped at my friends place instead of the store and let it hot soak for about the same amount of time I was in the store, started fine.

Let me know if you resolved your problem, I'm hoping in 2 months time you have or now you're completely insane from dealing with it.

 

Sorry to hear you are having issues. For me it ended up being the crankshaft position sensor which was the first thing I tried after a bit of searching on the internet. It seemed to be a fairly common problem and the part was cheap at around $120 so that's where I started.

You said it did start after cooling off but with a delayed crank. This is not consistent with what mine was doing. It either cranked indefinitely with no start when the engine was warm or fired right up.

Mine also did not pull any codes originally. A few days after this originally happened I had a scanner with me and scanned while the car would not start and was able to get code P0322. I forget the description of the code but it did support the crankshaft position sensor theorie.

I am no mechanic so for me it was fairly difficult to get at since the engine bay is quite cramped but I was able to have it swapped out in about an hour and a half. The car has ran fine since replacing the sensor.

I would recommend trying to pull codes while the car is not starting and see if you get anything. Hope this helps and good luck.

 

Thanks for the quick reply Sighbot.

It really was either no start, crank crank crank or start up for me. It just took a little more than normal when it started finally 45 min later. I cranked it for about 30 seconds at a crack when it was dead, nothing, not even a hint of life.

I saw all over the forums it was either Crank sensors or fuel pumps, with a few other things scattered about that I didn't necessarily believe... 20 years in the field of auto repair pretty much had it narrowed down to crank sensor or fuel pump for me from the get go.

Anyhow, my issue was fuel pump. It quit on the way home from work tonight, and of course, the worst possible spot it could, back road with almost no where to go. I had just enough to coast into a guys driveway that was wide enough to not block it completely with my car on one side of it.

Got it towed back to my buddy's shop where I keep my tools and had plenty of power to the pump, no fuel coming out of it. The pump and seal is around $375. Crank sensor would have been easier on my wallet.

At least it didn't string me along for weeks...

 

Sometimes a failing coolant temp sensor can cause these hot starting issues as well. Something to keep in mind and being a $20 part that's known to fail, it's not a bad idea to try it out as well.

 

Hi Sighbot,

Thanks for the post, it's a relief that I'm not the only one with this problem.

I have the exact same symptoms. You said you changed the crankshaft position sensor. Can you explain what is the mechanism that prevents the engine from starting? Has the problem reappeared after replacing the sensor?