SUDDEN ACCELERATION

my 1986 Audi still occasionally won't start when warm. The mechanics cannot figure it out, it takes five min to start and it seems to be rough for a second. Otherwise it runs perfectly, it is not the injectors. Either it is not getting gas or air or it is flooding but I have a clean exhaust. out of 3 mechanics no one can figure out the problem, there are no error codes. The computer says ok. I am using 91 octane fuel, new filter, all wires, cap, distributor and plugs have been replaced. Injectors are working fine, engine never stalls. 90 percent of the time it starts on the first turnover, even when it is hot. The cooling system and high power fans are working perfectly, the a/c works, the cooling fan comes on after the engine is shut down when needed. All the systems seem to be working properly....PLEASE HELP

Mister Bally

Three things required to start & run. Compression, which we know you have since the car runs fine most of the time. Fuel and Spark. When she won't start, determine if you still have spark by removing a plug wire and insert a small screwdriver and place it close to a ground point. Crank engine and look for spark. Or, insert another spark plug in the removed plug wire and place on a ground point and look for spark when cranking. No spark; fix ignition problem. Probably either a bad coil or Hall-Effect sensor. If you have spark, fix the fuel problem. Probably a bad fuel pump relay (they do go bad and behave intermittently before failing 100%. They are easily bypassed temporarily to limp you home with a jumper wire with two male blade terminals. Or a bad fuel pump or wiring between any of these points.

I have had the ground terminal on the top of the engine corrode at the crimp on the ring terminal causing a no-run, no-start condition as well. This causes a "no spark" condition too as I believe it affects the coil from firing.

SUDDEN ACCELERATION

What would cause it only to do it when it is warm or hot, never cold, also there is always sparks, I have replace the plugs twice. When the car runs it is smooth as silk. the tube across the top of the sparks plugs, warm air return has a crack in that pipe, I was told it was not important. does this car have a electric fuel pump or is it mechanical at the engine....thx

Kneale Brownson

You have a fuel pump in the fuel tank.

Is your car a turbo model?

Warm no start can be due to weak signal from the igtnition timing sensor. You need a good signal from that and the engine speed sensor to get spark.

Leaking spark plug wiring can also cause start difficulty on warm/moist days. Try spritzing the spark plug wires with water in the dark and see if there's a light show.

SUDDEN ACCELERATION

it is a Turbo model. I replaced all the wires, caps, distributor and rotor. They seem to be fine, I also replaced the o2 sensor. Could it be something to do with the catalytic converter on the car. The exhaust system has been previously replaced and there are absolutely no engine modifications

Kneale Brownson

Again, warm start issues can be the ignition timing sensor. It's an easy test described in the Bentley manual.

As mister bally notes, there also can be issues with the engine ground wires corroding.

Katman

No start when hot?
"Classic" fuel pressure not holding up when hot (and motor shut down), so you kind of get a form of 'vapor lock' that lots of cranking gets you around (eventually!)

Typically due to:
Leaking injectors, bad check valve in fuel pump, or accumulator not holding pressure..

SUDDEN ACCELERATION

The car has started well for two weeks even though I am scared to try every time I get in the car. I read the owners manual and of course you need to use only 91 octane, but they do not recommend ethanol blended fuels which may result in a vapor lock in warm to hot conditions or high altitude. You could also cause engine damage using less then 91 Octane or more then 10 percent ethanol. I believe, any ethanol is garbage in these 5 cylinder engines especially the turbo charged engines. Thx for the help too

SUDDEN ACCELERATION

Don't use ethanol blended fuels, it results in a vapor lock at warm to hot temperatures and high altitudes, found this in the owners manual. Just have to figure out why my break light warning still goes off, and then clears after some highway driving, all levels and pressure check out....