Paul Bakker

Hi guys and girls.
This is a simplified topic of a problem I posted recently.

When starting my engine in the morning the engine idle speed is correct and stable at around 700RPM.
When the engine warms up and temp. gets around 50 degrees the idle speed goes bad.
The RPM's go up/down as the computer seeks a stable RPM speed. Probably because the engine is relatively cold the engine directly stalls in this cold engine scenario.
When I shut down the engine and start it right up again problems are gone for around 75 seconds (cold and warm engine).

Many have sugested to examine the idle sensor for stickyness but does the cold/warm issue make any sense with my trouble shooting diagnostic ?
Is there an other sensor that kicks in when temp. rises and could have an effect on the idle speed ? This sensor does nothing for the first 75 seconds after start ?

Problems are gone when slightly pushing the gas to > 1000 RPM. Also when driving no problems or loss of cylinders. Power=OK.

Thanks.
Paul
'90 S4

John Speake

Sounds like your ISV is working fine, that's why the RPM cycle up and down.

I assume your car has cats ? Might be a prolem with the O2 loop. Unplug the O2 sensor, do a battery ground disconnect for a couple of secs, reconnect and try the car again.

Paul Bakker

Thanks for the advise.
First I'm not sure if I have a catalist convertor. I have no LED's at relay board nearby passenger side. Does this implicate I don't have it (them?) ?
If I DO have it where can I find this O2 sensor ? Any pics would be welcome.

By unplugging do you mean removal of the whole unit or only the electrical wireing ?
Is this sensor to be cleaned ? With what ?

Sorry for these dumb questions but these are my first steps in automotive diagnostics.....

Thanks a lot.
Paul

Leon Speed

The O2 sensor is screwed into the catalic convertors. You can unplug them inside the car. The wire runs on the floor under the electric panel in the passenger footwell. To find it you need to remove the wooden board that covers the electric panel, also the small wooden board on the bottom. If you are lucky the cable runs in front of the electric panel. If not, then it is behind the electric panel, for which you need to loosen the electric panel (two large screws on the top side). The electric panel then can be tilted forward. It is a thick black cable, it runs through a hole in the floor on the left side of the electric panel. The cable can be unplugged in the middle.

Paul Bakker

Hi Aryan (hallo Aryan!),
thanks for the advise. I'll try it within an hour or so.
Can you tell me why it could be possible the problems go away after disconnecting the O2 sensor and getting the minus of the battery for a couple of seconds ????
Does something reset then ??

Thanks.
Paul
ps check your personal messages, I dropped a messeage for you.

John Speake

Look under the car at the exhaust, about the position of the driver's seat. If you see rusty pipes with shallow, thick fins, those are cats. If a smooth surface exhaust box, then non- cat.

Let us know. If you don't have cats then no point looking foir the O2 sensor.

I think your car will have O2 sensor/cats.

If your car is digi-dash, then it will have the ignition protection relay with the LEDs inside the relay, but visible through the trasparent cover of the relay.

Ayran has given you the info for the O2 sensor (thanks)

I assume you know where to find the battery ground strap, just under the edge of the tool tray ?

VehiGAZ

What about the MAF? Doesn't the engine run for ~1 minute not using the MAF input, then switches over to running using the MAF?

Paul Bakker

This WAS the solution to my quest!!!!
Did exactly as you said and took the car for a test run. No problems!

Can you tell me WHY a ground and O2 disconnect/reconnect solved this problem ??? Is the O2 sensor always powered by the battery and was it in error (like my former windows XP computers always did ) ?
Please inform because I want to know if this problem can arise again and I'm just very curious!

THANKS A LOT FOR ALL YOU INPUT!!!

davek9

By disconnecting the Battery you cleared the stored info in the computer, the O2 sensor is powered when ever the fuel pump is (same fuse).
You may want to think about replacing the O2 if has over 60 K miles or has been damaged by a Rich running condition.

Good job.. !

fraggle

Let us know if the problem re-develops. I can pull the ground strap and mine runs great - for awhile - then develops the same symptoms. Sometimes days, sometimes weeks. I'm not sure what kicks it into "broken".

Paul Bakker

I'll do!

Paul Bakker

Hi,
you asked me a couple of months ago about an update. Well, I replaced the MAF with another one and the problem is still there. Sometimes the car stalls at idle and after that I start the car and it runs perfectly for ~ 2 minutes. Then the stalling problem occurs again. The solution is to disconnect my battery for a couple of minutes and after reconnecting I can drive for weeks with no problems. Looks like the computer is building up false data and at some point it "crashes".
I was curious if you had an update because your problem looks very much like mine.
When the weather is better over here I'll disconnect the O2 sensor wireing (passenger side foot region) to see if this sensors input to the computer has anything to do with the problem.

Greetz,
Paul

fraggle

Ok, I'll update mine, too!
I chased a ton of stuff with the o2 sensor and temp sensors, only to flop the 928 diag tool on it and find I had no idle or full throttle signals.

My problem: throttle position switch was unplugged. Plugged in now, idles great and everything is fine. Had to pull the intake and add 3" to the wiring harness to do it, but it works fantastic. I figure I miss routed something when I pulled the motor and the tension on the harness unplugged it when re-installed.

Good luck!!

Paul Bakker

Okay, I´ll take a close look at this. I have no idea where to start, can you give me a small hint?

Thanks!

Paul

fraggle

There's a way to test these signals with a dvm but I can't remember which pins to test. Someone should pipe in soon.