86 S4 NA - cold start idle issue
 

Just rebuilt the engine, prior to the rebuild it didnt have the issue... I'm not sure where to look. First order of business, I can start the car and sometimes it'll idle up to 3000 RPM, other times it wont. how do I resolve that?

Second, on a cold start regardless if it idles up or not, once it tries to drop down it sputters and stalls. I can take a video if it would help. I hear no vacuum leaks indicating thats the issue. The car holds a wonderful 7-800 RPM idle after about 5-10 seconds of tapping the throttle lightly.

3000rpm start up is normal when cold. I believe it won't do it in (EDIT) *GEAR*. It's pard of the AWS (accelerated warmup system). The ecu applies a sidnal to a relay to the air bypass valve. (Not the same as the BAC valve, which maintains idle as load varies on idle)

See if ur AFM is plugged in. If it is, jumper the yellow fuel check connector nearby and see if it now stays running.

That and if you just rebuilt it it may not be broke in enough yet to hold an idle


And don't rule out a vacuum leak cause you can't "hear" it, you can have one and never hear it

Some will set the idle higher than normal on a rebuild until the engine builds enough compression.

The 3000 rpm cold start is based also on what the temp switch at the bottom of the radiator reads. If the coolant is below a certain temp then the high idle rev won't initiate, if above it will.

bump, fixed link to the video.

Annnnnnyone?

As previously suggested, a rebuilt engine can take a period of time before it can build up proper compression. Some will artificially set the idle speed higher than normal until the engine seals seat themselves and help improve the overall performance of the engine.

I have tested with my rotary diagnostics test and pulled 95 psi on the front rotor and 100 psi on the rear.

I had a recommendation to check the coolant sensor behind the waterpump. If that has a poor connection it defaults to the engine reading "normal temp" and in turn means less fuel is being injected.

I did come across something the other day saying idle issues tend to be more pronounced with lightweight flywheels. I do have a lightweight flywheel, whereas I had a stock one before being rebuilt.

about 4000 miles on the engine. still starts and dies. sometimes on warm starts the idle hunts like the TPS is off (probably is since I adjusted it a tiny bit) but that shouldnt cause it to start and die like in the video....

Anyone else have any ideas? Coolant temp sensor maybe? I had someone mention if the sensor is bad or not making a good connection it'll operate as if its always warmed up - and in this case that would kinda make sense since its lean on cold starts.

The water thermosensor ECU pin is 2I and the wire is Green/White. A cold engine should read 2 to 3 volts while a fully warmed engine reads .5 volts w/key to the on position.

The thermowax on the throttle body is supposed to hold the throttle open a little bit until it warms up. Then retract as the coolant warms up and the throttle stop holds the idle. It's purely thermo-mechanical, so no sensors or ecu involved.
Do you still have it installed and adjusted correctly?

My car does exactly the same thing as you describe above. But that is because the thermowax puked all over itself. FYI, the wax is pink.