[jlpmghrs]

Car: 1996 GT Auto
Mods: underdrive pulleys, CAI (recently installed, MAF positioned at 12 o'clock), PI intake manifold

Scenarios:
1) A/C on and coming to a stop from moderate to high speed, car dies or RPMs drop dramatically.
2) A/C off at idle and then switched on, car dies or RPMs drop dramatically
3) A/C on at idle or driving in traffic, no problems.
4) Defroster off at idle and switched on, RPMs drop or car dies.
5) If defroster/heater is already on, no problems at any speed.

Notes: I don't suspect the pulleys cause I've had those awhile with no previous problems. A/C pulley spins freely. A/C is not short cycling. PCV clicks and sucks. I suspect the vacuum system behind center dash that changes the vent openings for the defroster problem, but I assume there's a different problem for A/C, since the A/C problem crops up even when no vent changes.

Regarding the sticky on idle problems, when the IAC is disconnected, car dies (even though it has the black vent; sticky says RPMs should only drop, but not die. Is this correct?)

I know I could just pick up an IAC for $50, but I hate changing parts and want to sell the car, so I don't want to dump too much money into it based on guessing (also don't want it stalling on a test drive, though).

 

[DJAlsip]

Scenarios:
1) A/C on and coming to a stop from moderate to high speed, car dies or RPMs drop dramatically.
2) A/C off at idle and then switched on, car dies or RPMs drop dramatically
3) A/C on at idle or driving in traffic, no problems.
4) Defroster off at idle and switched on, RPMs drop or car dies.
5) If defroster/heater is already on, no problems at any speed.

Do you mean the rear window defroster? If so, that is electric powered, have you had your alternator/battery checked out? If the alternator bearings are bad it could cause a parasitic drag that when combined with the added drag of the A/C will cause the engine to stall.

I know I could just pick up an IAC for $50, but I hate changing parts and want to sell the car, so I don't want to dump too much money into it based on guessing (also don't want it stalling on a test drive, though).

If you do pick up a new IAC, get an OEM one. The aftermarket ones from Autozone etc. are known for having a lot of problems, many will not even work the first time they are installed.

 

[jlpmghrs]

I just realized...

It's the front defroster, sorry I didn't specify.

I just remembered that the compressor kicks on when you turn on the defroster to dehumidify the air. So scenario 4) is basically the same as 2)! So it probably has nothing to do with the vacuum system that switches the vents.

So I think the question is: is the IAC sticking and not opening fast enough when the load increases on the engine (scenarios 2/4) or the engine power rapidly returns to idle (scenario 1)? Or is the IAC operating normally, but the throttle body set screw should be opened slightly to allow the engine get air during the instant it takes before the IAC solenoid opens up?

 

[DJAlsip]

It's the front defroster, sorry I didn't specify.

I just remembered that the compressor kicks on when you turn on the defroster to dehumidify the air. So scenario 4) is basically the same as 2)! So it probably has nothing to do with the vacuum system that switches the vents.

Alright, that makes more sense now.

So I think the question is: is the IAC sticking and not opening fast enough when the load increases on the engine (scenarios 2/4) or the engine power rapidly returns to idle (scenario 1)? Or is the IAC operating normally, but the throttle body set screw should be opened slightly to allow the engine get air during the instant it takes before the IAC solenoid opens up?

If I recall correctly the IAC has two settings; partially open, and fully open. Under normal driving conditions and idle it should be partially open, but when the oil temperature is too low it will open up to allow for a higher idle (between 1200-1500 rpms I believe) to get up the temperature faster.

It sounds like the IAC valve is either sticking, or the actual actuator might be going bad. Have you tried removing the IAC valve completely? If not, remove it and clean it with some q-tips and seafoam/intake cleaner.

 

[jlpmghrs]

I picked up some seafoam but I haven't had a chance to clean it yet. Actually the weather here's been pretty nice so I haven't had to use the A/C much. Also, I replaced the evap flow sensor (purge valve replaced last year). It had been showing p1443 so maybe that fixed it. I guess I'll see when I have to run the A/C again.

 

[jlpmghrs]

So here's an interesting solution to the problem I don't recall reading about. It turned out my alternator was dying.

I replaced the IAC valve, which made the car react quicker so it didn't always die if the new valve could pop open fast enough (thats why I hate just changing parts; didn't work). Then last weekend my dash went haywire and the alternator pulley was whining like mad, and then the car died.

New alternator now, no idle problems! When I switch on the A/C or defroster in traffic, the car just gently idles down about 200 rpm, then stabilizes! I'm not sure if this is cause I have underdrive pulleys, so the engine is more sensitive to the increased alternator demand or a possible solution to the problem even with stock pulleys.

 

[wmburns]

It's funny. As I was reading this thread I was thinking to check the battery and alternator. Scrolled down to post and found above.

Good work.