Manfred

Feel like I'm opening a large can of worms here but here goes...

Normally my car runs great but in very hot weather and the AC on it will stall when idling. If I try to restart with the AC still on the engine will crank but not start. If I turn off the AC and try again it will start. The car will normally idle around 900rpms. Also, if I turn the AC on while cruising at highway speed there's a noticeable drop in rpms so that I have to accelerate or the car stutters a bit. My current fix has been to switch off the AC as I approach a stop sign or if I'm starting the car.



Possibly related issues:
1. I know I need to replace my throttle position sensor. I have maximally adjusted my throttle cable so that I achieve WOT but it only kicks in at full throttle, not at 3/4s.

2. I suspect I may have a coil issue. In very cold weather (I'm in the Carolinas so that means 10 degrees Fahrenheit or so) I have had three instances of the car going into "limp home mode". In all three cases I turned off the car and restarted and I was back to eight cylinders.

3. 91 model car. 71K miles. As far as I know I have the original O2 sensor. It's on the list of things to do.

Any help would be appreciated!

Dan.

Mrmerlin

have you had the computer rebuilt??
The brain will add RPMs when the AC is turned on

Manfred

I've only had the car for the last year (from 62k to 71k miles). As far as I know the computer has not been touched but then that's not saying much.

Formula94lt1

Don't suppose there is a clutch switch...?

Manfred

sorry, not sure what you mean. Are you saying I may have a cooling fan clutch problem?

WallyP

The idle on the '91 is supposed to be fully computer controlled - the ECU drives the Idle Speed Valve to control the idle speed. If the engine won't idle with the A/C on, either:
- the ECU isn't controlling the ISV (ECU problem).
- the ISV isn't functioning properly (ISV problem).
- the A/C is putting so much drag on the engine that the ECU/ISV can't overcome it with increased idle speed.

The comment that you have to add throttle to overcome the A/C compressor at cruise doesn't sound good...

Possibilities include a failing compressor and an over-charged A/C system.

BTW - "limp home mode" does NOT, repeat, NOT mean running on four cylinders. Running on four cylinders usually means that one ignition system has failed, or that the ignition monitoring system erroneously thinks that one ignition system has failed. The most common problem is a faulty coil wire on the left coil, but there are numerous other possibilities for four-cylinder mode.

sidehatch

I have this issue also,but the car doesnt stall,but its on the border of stalling,when i turn both front and rear on to the max, the rpms go down and the volts really drop big time and oil pressure, like its sucking all the power away,its beyond anything i know how to fix.

WallyP

The cooling fans, HVAC blower, and the A/C clutch pull quite a few amps. If your alternator isn't up to the job (especially at a lower idle speed), or the alternator belt isn't tight enough (it goes REALLY tight), you might be exactly right in your description. Check the voltage at the jump start terminal under these conditions.

The low oil pressure might be the low idle or might be low voltage on the dash.

Manfred

Thanks Wally,

For now I'm going to assume the ECU is OK.
I'll do a search and see what I can turn up on checking the ISV.
Are there some things I can check to run down these possibilities? I did some more testing today--I may have overstated the effect at cruising speed--I tried it today and turning on the AC had no effect. I could swear that I've had the car suddenly slow a little when the AC is turned on, even while cruising.

I have noticed that the voltmeter normally steadies right around 13amps; when the AC is turned on the amps will slowly fall to around 12amps and then stabilize there. Should I assume that this is normal? I also see a little drop (although not as drastic) when I turn on my headlights.

Manfred

OK--so anyone know where to find the vacuum supply to the ISV on a '91 GT? I've seen the trick where you spray WD40 into the vacuum line to help loosen the valve--thought I might try this just to see if it helps or not. A new ISV is $295 so I'd rather not use replacement as the method of testing.

John Speake

You say you have adjusted the throttle cable to get the WOT switch to work - are you sure that he closed throttle switch is still working correctly ? Can you hear the click when throttle starts to open, and again when the throttle is released ?

Closed throttle switch must operatre corrctly for the idle control system to work.

WallyP

John has a good point...

To check the idle switch:
- Get on the highway where you can maintain a speed such that the engine RPM is about 1800-2000.
- Set the digital dash for instant MPG.
- Lift your foot completely off the throttle and leave it there for at least for five seconds.
- Watch the MPG. If the MPG goes to "90" and stays there as long as your foot is off the throttle AND the RPM is over 1500, the idle switch is closing.
- Repeat the above procedure, but this time touch the throttle while the MPG is "90". The mileage should immediately drop to something in the forties or fifties. This will show that the switch is opening properly.

Manfred

Yeah, that's working fine--checked via the highway speed and then off the gas method (watching the MPGs) as well as at the connector checking the resistance. I can also hear the clicking of the switch. Also, this problem existed before I adjusted my throttle cable.

Rob Edwards

If you see this, Manfred, any followup to your situation?

After driving around for years without A/C, I'm (of course) discovering new niggling things. It's 100+ in Anaheim today, and I did a little thermal stress test on the '90 GT today driving up to the hospital and back. In a black/black car. And got stuck in traffic for ~15 minutes. I am having the same symptoms as Manfred - low idle and occasional stalling at traffic lights, I need to keep my foot in it a bit. Also, the voltage gauge drops to right at 12, maybe 11.5 during these periods, while for years it has always been in the 13-13.5V range.

The oil pressure, sitting in traffic in 106o heat (indicated) was 1-1.5 bars at 6-700 rpm. Time to move from Rotella 15W-40 to something a little heavier. No 'oil pressure warning', though.

Pertinent data:
-WOT and idle switches in the TPS both work.

-Alternator is original to the car AFAIK, have all the records from day 1.

-I do have an idle issue, it's right at 800 when cold, then creeps up slowly to 1100 as soon as the temp gauge nears 150 or so. Perhaps the ISV reaches the end of its ability to compensate for vacuum leak(s)? I need to pressure-test the intake (and stick a vacuum gauge on it to see whether I'm leaking somewhere- probably am.)

-Temp gauge never goes above an indicated 175 or so. I know that it's ~20o low vs. 'real' IR temps off the radiator hose, but there's no problem with overheating.


Thoughts/tests for an electro-retard to try?

Gdjsk

Hi,
did you find solution of this issue, I faced same issue, while AC on engine rpm goes down & engine stall .
Thankyou