AC problem effect on Idle

[ltmark]

You folks, and this forum, have been absulturle great on my efforts to get my '85 running in top form.....I think I'm down to my last major problem. I've struggled with my idle, not high until warm, and although it does pretty well when warm, it still searches up and down a bit.

I have read on the forum and in my service manual, that the ECM uses AC readings (even when not turned on) to choose the proper idle. My AC is NOT currently charged, BUT the AC pressure on/off switch is "open", ( with the AC switch not even on!) as though the pressure in the system is too high and causing the switch to cycle open.

I've done the proper setting of the IAC, TPS and all other obvious idle related funstions over and over.

New IAC, TPS, EGR valve, all vacuum hoses replaced, coolant temp switch & sensor, rotor, cap, wires, plugs, etc. etc, etc.

Runs great when warm and idles pretty well, other than the searching.

Any of this cause any light bulbs to go on?

 

[OLDCHEV]

Do not see a reference to changing your O2 sensor in your post. Granted your base map program (limp home mode) is in effect until the O2 sensor kicks in, they do "wear out".

For less that $30.00 for a Bosch unit, it might be worth you time & money to change it.

 

[jmccloud]

A/C pressure switch

There are usually two A/C pressure switches. One near (or on) the compresser or somewhere along the smaller diameter pipe. This is the 'high' side switch that is normally closed but opens if the pressure goes too high. The other switch is usually near (or on) the accumulator or somewhere along the larger diameter pipe. This is the 'low' side switch that is normally closed with a normal charge. With a low charge (or no Freon) or operation during cooler (<50F) weather, this switch will be open. I don't know whether the ECM reads the status of this switch to choose the proper idle but it makes good sense that it would.
Hope this helps.

 

[WhalePirot]

Connect a vacuum gauge and, I suspect you will find it varying. It may also be lower than normal, which for a stock engine, should be about 19-20 psid.

You may just have a vacuum leak which is allowing unmetered air into the engine, which the ECM is trying to compensate for; hence the variable idle. Listening carefully and using WD-40 to spray suspect areas, while listening and monitoring the idle rpm can help pinpoint the leak. Also, disconnect and temporarily plug the brake booster line and any other vacuum lines to check for leaky components. Plastic Ts for vacuum junctions can crack with the years of heat cycles, too.

Often it is the simple, not the complex.

 

[tyrel]

Connect a vacuum gauge and, I suspect you will find it varying. It may also be lower than normal, which for a stock engine, should be about 19-20 psid.

You may just have a vacuum leak which is allowing unmetered air into the engine, which the ECM is trying to compensate for; hence the variable idle. Listening carefully and using WD-40 to spray suspect areas, while listening and monitoring the idle rpm can help pinpoint the leak. Also, disconnect and temporarily plug the brake booster line and any other vacuum lines to check for leaky components. Plastic Ts for vacuum junctions can crack with the years of heat cycles, too.

Often it is the simple, not the complex.

This is very good advice. Vacuum leaks are hard to trace, but they do contribute to rough idle. Somehow in my car's recent history, a mechanic somewhere broke the little vacuum nipple on the fuel pressure regulator, which allowed a small amount of air into the intake. I think this happened when I went to one of those quick-change oil places, they also forgot to replace my oil cap! Dunces! Anyway, after I changed out the regulator, the car idled smoothly again and I'm submitting a claim to the oil-change place (guess which national chain it is) for the cost of the new regulator. I'm the last one to patronize this chain, but I did not follow my own advice and stopped in for a quick oil change when I should have had it done at my regular mechanic.