Occasional High Idle Speed - $8D 383 fully modified TPI

[JeepsAndGuns]

Where is your o2? Is it in the stock location, is it the original one, do you have headers? I dont know anything about the chevy cars, is it a heated sensor? If you have headers and its not a heated sensor, that might be your problem with it dropping out of closed loop. The o2 is cooling off. You could try wiring up a heated o2 sensor.

[zaut]

I have Edelbrok headers designed to replace the stock manifold. I was thinking that the O2 was cooling off too. Is that common? It happened when the car was idling at normal speeds. Can you recommend a heated replacement? (year, model, etc) I was thinking about doing that anyways because I can get the car in closed loop quicker. I understand there different types of O2 sensors, one I have heard of is a wide band O2 sensor. Not knowing the difference, I have been hesitant to change it. Thanks.

[EagleMark]

I do have your pdf file and it is very nice work thank you! Just not sure where to put it right now since it covers Memcal type ECM/PCM...

Sure sounds like "IAC Command speed open loop offset" is an idle RPM setting for open loop. You can easily see if system is going Open Loop when the idle issue happens in data.

O2 sensors cooling off in headers is a common problem. Most narrow band O2 sesnors work the same whether heated or not, just the heating element is added. For GM a heated replacement can be from many vehicles, the only difference would be the connector. IIRC a 1995 Chevy truck and 1995 LT1 engine are the same and a good place to get one ordered from. Then add a rely and wiring for the heat circuit.

[zaut]

I did a little research an now know the difference between a wide band and narrow band O2 sensor. A wide band has a 0 - 5v output range, a narrow band has a 0 to 1 v output. My ECM uses a narrow band O2 sensor. What I need to know is what vehicles used a narrow band heated O2 sensor.

[zaut]

Thanks EagleMark. Do you know how many amps the heater circuit takes? If not I can bench test and measure the current.

The "IAC Command speed open loop offset" is set at 1000 RPM. I never changed it. I don't see where or what it affects. In open loop my idle (when working normal) follows the target idle rpm versus coolant temp table.

[RobertISaar]

I did a little research an now know the difference between a wide band and narrow band O2 sensor. A wide band has a 0 - 5v output range, a narrow band has a 0 to 1 v output. My ECM uses a narrow band O2 sensor. What I need to know is what vehicles used a narrow band heated O2 sensor.

actually, to get more in-depth the wideband controller outputs a 0-5 volt signal.... the wideband sensor itself is likely different.

your car uses a narrowband.

Thanks EagleMark. Do you know how many amps the heater circuit takes? If not I can bench test and measure the current.

most heater circuits i've seen peak at ~50 watts or so. at 12 volts, that's just over 4 amps. they quickly drop the amount of current used as it heats up though.

[EagleMark]

It may be for start up?

I don't know the amps of heated O2? For wiring I have used another fuel pump relay.

[zaut]

I think I found the root cause of my high idle. My Charcoal Canister Purge is on all the time. Unfortunately it appears that the ECM is at fault. The ECM turns the solenoid valve when it does not want to purge. The valve is normally open. I traced the circuits and I get 12 volts and continuity to pin F7 on the ECM. I never get a low resistance on that EMC circuit. It’s always about 12kohm. What my service manual says is that you will lose four outputs at a time. But that is for the original 85 ECM. This one is upgraded to a 1990 7730 ECM, which I do not have the service manual for. Does anyone know it I should have lost three other circuits and if this is repairable? Another thought I have is did GM change the logic of how the 90 canister purge worked? I was surprised to read that the valve was a normally open valve. Could by chance the 90 version use a normally closed valve and I am getting the purge at idle and not during driving? I will test this tomorrow.

[RobertISaar]

GM seemed to use normally open valves with setups that used ported vacuum, so that if the valve were stuck open, it wouldn't effect idle. when they transitioned to normally closed, they generally used a constant vacuum source. or at least this is what i've found to be true of most of the FWD GM stuff.

[zaut]

This is a thread I stated last year when I was having a high idle problem. I thought I got it worked out, but I thought wrong. When I start the car, it idles correctly. After driving it, it starts to idle very high. If I turn the car off and back on, it will idle correctly until it take it for a drive.. When it is idling correctly the IAC position will be in the high 30's-low 40's. When the engine is idling high the IAC will be in the 80's and 90's. Does anyone know why the EMC will not lower the IAC counts to slow the engine down?

[zaut]

Well, I think I finally got to the real root cause of my high idle. I noticed that when the engine was idling high that the TPS percent was not zero. After close examination of my TPS on the BBK throttle body, it turns out that the follower arm of the TPS was dragging against the throttle body. So the TPS was not returning to zero every time. This was then making the IAC follower function think the throttle was open and therefore opening the IAC motor. Amazing. 10 seconds on the bench grinder and the TPS now has plenty of clearance. One thing I don't understand is why the IAC motor does not move for a set period of time when the engine is cold started? Is that the IAC learn timer setting? And does anyone know what the maximum count the IAC go to? On cold start, the 160 counts is not enough to keep the engine running. And lastly, what does IAC open loop command offset do?