davidt0526

I've had by B5 S4 for about 4 years now. No major issues, I've really just had to keep up with routine maintenance and upgrades. About a week ago, I walked out of a store and tried to start my car. Turns over fine, simply will not fire. Can't check for a CEL w/o engine running, but it does NOT flash while attempting to start. After poking around in the engine bay, checking fuses, trying to start it about 10-15 times, she finally starts right up without a hitch. I might add that the drive to the store was completely routine, car ran 100% normal. After the car started post the no-start condition, the car continued to run 100% normal. Scratching my head as I closed the hood, I drove home. Checked for codes at home, nothing. Next few days were completely issue free, even making a ~250 mile road trip. Fast-forward to 4 days after the first incident, same issue. Since the first incident, this has happened about 6 times now. Each time the car is warm- say I make the drive home from work, stop at a store for ~30 minutes, and the car will not start. My routine was originally pulling fuses, and checking for broken wires. I've tried narrowing down the symptoms, but nothing seems consistent. The only consistent things are it occurs when the car is warm, and it always eventually starts. The car runs and drives 100% normal, when it will actually start. No scary freeway shutdowns here...

Things I am not sure of:
Fuel - Not sure if the engine is getting fuel. I do not believe it is, because there is no smell of unburnt fuel, and it doesn't even try to fire.
Spark - haven't tried checking for spark during the no-start condition. Only one of the ~5 times have I actually been home when this occured.

Engine Mods if it helps:

APR ECU Installed ~1 year ago
B&B Exhaust - Installed 4 weeks ago - car ran fine for first 2 weeks.
Downpipes > 1 year old

Because this only occurs when the engine is warm/hot, one of my first thoughts was the coolant temp sensor. This was replaced ~3 years ago with the new "green" sensor during timing belt maintenance. The timing belt has since been replaced again (about a year ago).

Thoughts of mine of possible causes:
Coolant Temp Sensor - Relatively new sensor, could this be it?
Fuel pump relay - but why does the car eventually start?
Fuel pump - but why does the car run fine otherwise?
Fuel filter - again, should have other issues...
EGT sensors - but would this cause a no-start condition?
Broken wire/s - again, why does the car run 100% normal after it finally fires?
APR ECU - has 4 programs... I've tried switching programs during the no-start condition, it doesn't help.

Again, I have no codes. This is definitely not a battery/alternator/starter issue. When cranking the car, she sounds strong. After ~10 minutes of cranking on and off for a few seconds per minute, she finally fires right up.

Any thoughts would be greatly appreciated.

EuroXS4

In order to narrow things down you would need to make sure your getting spark and fuel.Thats the only way.Only thing I can think of is either no fuel or no spark,2 things i can think of.one would be the fuel pump relay and the second would be the power supply relay for ecu.Because the power supply relay turns on the ecu when ignition is turned on and the ecu sends out the voltage to turn the fuel pump on and also tells the coils to fire.I would check relays to be honest.But first things first.When this occurs check for spark and fuel if you dont have one or the other or both chances are you will have a bad relay that doesnt turn on.I have seen this in warm conditions only.

973boost

Crank sensor possibly,But with no codes its hard to tell.

EuroXS4

could be a crank sensor.But normally the ecu will throw a fault for engine speed sensor.

davidt0526

Thanks for the replies.

I seem to have narrowed down the issue some more. Durring the no-start condition, the air intake temperature measures > 150 degrees. This was measured after the car was sitting for roughly 30 minutes. Each time I try to start the car, the air intake temp comes down. Once the air intake temp gets below ~140 degrees, the car starts just fine.

Now I'm looking into whether the car is really getting that heatsoaked, or if the sensor is bad. I'd find it really hard to believe it's really getting this heatsoaked; we are just coming out of the hottest summer of all time here in Texas, and she was fine up until ~3 weeks ago.

Thinking I'll just replace the sensor, and see if the issue goes away. I can always take temperature measurements before and after, and the sensor is only ~$25.

Thanks again for the replies.

Lee Booker

I'm having the same problem on my 1.8 b5 A4 which sensor did you change and has it solved the problem?
Many thanks Lee