Oscillating Idle

[Bowlerwell]

I replaced the battery 3 days ago, and yesterday noticed that the engine idle is now quite unstable. (probably completely unrelated events...?)
It oscillates from about 500-900rpm. After a run it is a bit better.
Am I right to initially suspect the ISV and TPS?
What else should I suspect/what else can I do to debug?

Thanks,
JB

[A80Avant]

Possibly ISV first

sent using Android

[Bowlerwell]

Could this be an 'ECU reset' problem?
Do I need to follow certain steps to get the ECU to callibrate properly?

(I read on an A3/S3 forum that one should :disconnect -ve battery terminal for 20 mins, re-connect, turn on ignition for 5mins, then go for a drive in order to reset the ECU...)

[prj]

You don't need to do anything with the ECU apart from pulling fault codes.
If there are none, start guessing.

[mcandmar]

Its probably just a side effect of the ECU reset, they do learn adaptation values for Idle valve and a few other variables, should sort itself out. For these cars (and almost all cars) i recommend you let them sit at idle for a few minutes until the cooling fan kicks in, after that drive as normal. For some cars its very important you do not move the throttle, especially for newer drive by wire cars.

[prj]

Its probably just a side effect of the ECU reset

No. Adaptation will never ever cause a 400rpm oscillation. It is 5-10%.

[RS2junkie]

I had this and it turned out to be a poor connection on the ISV electrical terminals. It is worth checking this and also the MAF connection and also the Lambda terminals.. Might be a coincidence about the battery !

P

[UrS4boy]

N71 Idle Speed Stabilization Valve (ISV or ISS Valve) Info

This might help some:

http://forums.quattroworld.com/s4s6/msgs/21853.phtml

[twoqu]

A quick test is to pull back the rubber from the connector and probe contacts 4 and 6.
They are the idle switch contacts. Should be Continuity shut, infinity open. Could also temporarily bridge them with a bit of wire to similate closed throttle.
Only do this stationary as its just to fool the ecu into thinking the throttle is shut. The ecu should then operate the idle control. So if car idles correctly when you bridge the terminals, then you are looking at the tps/throttle perhaps!
If not, put your hand in your pocket and buy some diagnostic capability for the car for Xmas! Not only will it read codes but you can also view certain measuring blocks which will greatly assist in diagnosing potential problems more accurately. Lets face it, the car is worth it!

[UrS4boy]

Throttle position switch/potentiometer information

This might help (or not):

http://forums.quattroworld.com/s4s6/msgs/20405.phtml

[Bowlerwell]

Thanks chaps. I'll check the connectors, and see about cleaning the ISV. And I think I can to a TPS test in VAG_COM?

Dave, your posts are always helpful. And interesting!

[twoqu]

Thanks chaps. I'll check the connectors, and see about cleaning the ISV. And I think I can to a TPS test in VAG_COM?

Dave, your posts are always helpful. And interesting!

If you have vag com then go to the measuring blocks and post the numbers. Audi put a lot of effort into the idle!
Then group 4. I think its the 4th field that relates to idle switch. The 1&0 should change when you operate the throttle indicating the idle/part load change over.

[johne]

I'd check the ISV and see if it's clogged with carbon and give it a good clean with carb cleaner. It's worked for me on the two occassions I've had an erratic idle.

John

[newsh]

I'd check the ISV and see if it's clogged with carbon and give it a good clean with carb cleaner. It's worked for me on the two occassions I've had an erratic idle.

John

+1 ^^