Hunting Idle

[unclebilly]

Hi Guys,

My idle hunts once my car warms up. Often it stalls at lights. When it does idle, it idles around 600 RPM.

My guess is the O2 sensor (it doesn't do it when cold in open loop mode) and it doesn't do it all the time - just 75% of the time.

Thoughts? I don't want to spend $150 on an O2 sensor if I don't need one.

Otherwise this car runs awesome and the plugs all looked awesome when I changed them recently.

Scott.

[unclebilly]

I thought I would add that this condition didn't change with new caps and rotors, new spark plugs, new air filter, and valve adjustment.

I do have a durametric - will this help me diagnose this problem?

[Don Plumley]

Have you cleaned the ISV?

I don't know enough about OBD1 and the Durametric tool, but in theory it should allow you to watch the O2 sensor to make sure it is okay.

[unclebilly]

I have not cleanedthe ISV - what is involved here?

[Don Plumley]

Not sure where it exactly is on a 964 motor, but this is a good place to start:

http://p-car.com/diy/isv/

[bigchillcar]

it's easy to clean. it's a silver canister on top of the motor held on by a cpl hose clamp and two hoses that run into either side as i recall, and there's one plug to disconnect. it's about 4 inches long and 3 inches wide, cylindrically-shapped, above the intake. anyway, you spray it out thoroughly with brake cleaner over and over until black gunk quits coming out. give it a final wipe inside as best you can reach with a paper towel and re-install. maybe a half hour job if you take your time.

[unclebilly]

This seemed to improve things considerably. Thanks for this suggestion.

[bigchillcar]

glad to hear it. you're welcome.

[drisin80]

you might also want to check to see that the idle control microswitch is being depressed fully. earlier this year i had low idle, and occasional stalling. it turned out that i had to adjust the throttle cable about 1 mm so that the microswitch depressed fully. ----tom

[yh_bg]

Tom has got it. That micro-switch issue is not that uncommon.
When the switch is not being depressed by the throttle linkage the computer believes that the gas pedal is being pressed so it does not control the idle.
This is the most likely culprit when the car lops at idle, but works perfectly whenever you are on the gas.

All the best.

BG

[911Urge]

+1 on the idle micro switch. One last thing to clean (since you did the ISV) is the Air Flow Meter (AFM).

[unclebilly]

I checked the microswitch last night and it seemed as though it was being fullydepressed.

There were no motronic codes at all.

Then I stsrted the car and fiddled with the throttle / mictoswitch and it threw an 02 sensor code. This is wierd because there we no 02 sensor codes stored in memory.

[bazar01]

My idle was also fluctuating (ltwo years ago when I got the car) between 850 and 1000 rpm.
Disconnected the microswitch connectors and check for continuity and was barely making.
Disconnected the battery, put some slack by adjusting the throttle cable so the idle switch makes, reconnected the batery, it's rock steady since then.

Not sure if the battery reset or the microswitch adjustment fixed it.

[bigchillcar]

any pics of where the microswitch is located? got me wanting to check my own..

[bazar01]

The connector is right beside the Idle air control valve.



Here is the pin assignments.



Idle microswitch should be closed or zero resistance when in idle.

[bigchillcar]

gotcha. what am i checking for exactly? i can disconnect it and reconnect. is that all you're saying to try? of course, i have a lightweight flywheel, which probably explains my funky idle...but i've got a new issue i posted on the 964/993 forum here last night, too..

thanks for taking the time to post those pics, bazar. i really appreciate that.

[bazar01]

gotcha. what am i checking for exactly?

You may want to hook up a multi-meter on the terminlas noted.
Meter set to continuity or resistance.
Put one probe on the idle contact terminal and another probe on the common terminal.

You should get continuity or almost zero ohms when throttle cable at idle position.
When you lift off idle, it should go to high resistance.

[bigchillcar]

thank you, bazar. i follow you now..and adjustments are made on the throttle cable.