Car stalls when at temp and ac is turned on.

[Dhennick99]

It'll start and run fine until warm. It'll become sluggish off of stops and if I turn the ac on, it'll stall. Sometimes it'll just stall when the fans kick on. I installed a brand new alternator today and the issue still persists.

It idles at 13.9, when I brake it'll dip down to 13.7 and when I turn the ac on it'll drop to 13.4 and stall.

I'm not dropping voltage so severely that it causes the stall. It'll happen above 13v. Still, I can't get above 14v on a topped off battery and new alternator.

This only happens when the car is at temp. Once it stalls it becomes difficult to start and will stall even without ac engaged. Any ideas?

-Dustin
'81 automatic

[82DMC12]

It'll start and run fine until warm. It'll become sluggish off of stops and if I turn the ac on, it'll stall. Sometimes it'll just stall when the fans kick on. I installed a brand new alternator today and the issue still persists.

It idles at 13.9, when I brake it'll dip down to 13.7 and when I turn the ac on it'll drop to 13.4 and stall.

I'm not dropping voltage so severely that it causes the stall. It'll happen above 13v. Still, I can't get above 14v on a topped off battery and new alternator.

This only happens when the car is at temp. Once it stalls it becomes difficult to start and will stall even without ac engaged. Any ideas?

-Dustin
'81 automatic

Fuel pressure test? Is the pump crapping out when it warms up? Collapsing suction fuel hose in the gas tank as the tank heats up? Have you confirmed no ignition advance at idle when hot OR cold?

[Michael]

The first thing I thought of is insufficient voltage at the pump. Recreate the problem with the cover off and take a reading at the pump and the RPM relay.

[Rich]

Fuel pressure is worth a look.

Consider checking the idle speed control circuit, the idle speed motor and that system's connections/switches. And the idle throttle stop.

Does the engine ever idle smoothly at about 800RPM? If not then also perform a smoke test to see if there are vacuum leaks in the air intake system. Idle speed spec is 775-825 RPM.

The voltage levels? All of the numbers you shared are high enough to run the fuel pump and other engine systems well. The variations in voltage you report are probably symptoms of the root cause problem, not the root cause itself. As you seem to have proven by replacing what was probably a good alternator.

[Dhennick99]

The alternator I replaced was good. I had it tested. The car idles fine. It sits at 775. Once the ac stalls the car, it's difficult to get it started again. Once I do it runs sluggish, regardless whether the ac is on or not.

If left running it will stall without ac, once the first stall occurs. It's not until the car cools down completely, that it will start and run normal without ac.

[Rich]

Correcting a spec mistake I made here.

Idle speed spec is actually 725-825 RPM.

Good to see your idle speed is in spec and it normally idles smoothly.



Does the engine ever idle smoothly at about 800RPM? If not then also perform a smoke test to see if there are vacuum leaks in the air intake system. Idle speed spec is 775-825 RPM.

[dn010]

Have you tried a different coil?

[Dhennick99]

Have you tried a different coil?

I have an extra on hand. It's worth a shot.


https://youtube.com/shorts/pByWABai26A

I'm also experience strange mixture behavior on the dwell. Could this be the root cause of the engine stall with ac?