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Misfire When Climate Control On

There is 112K miles and the car has been well maintained. A recent problem occurs when I first start the car and turn on the AC – the car misfires when the brake is depressed. Sitting at a light with the brake on you can feel the misfire; turn off the the AC and it’s fine. Once I’m on the highway it’s fine. At the end of trip when I’m at a light, it will happen again but not as much and it has not stalled at all. The car had a vacuum leak at 55K miles, new battery 9/2006, spark plugs replaced.

It is fine while idling in PARK? What year is the Sable. How do you know it is a misfire?

It is a 2003 and not it’s not fine in PARK if the AC is on the brake depressed. I was calling this a hesitation but someone (who knows more about cars than I do) said it’s a misfire. It’s hard to describe be it sort of coughs/pulses and I can almost feel it my foot. But I don’t KNOW that is a misfire.


It certainly could be a misfire, what was the mileage and date the spark plugs were replaced? The AC compressor may put enough drag on the motor at idle to cause the misfire and make it easier for you to feel it.

You could need new plugs again, perhaps some ignition wires and distributor cap as well. Some carbon on the intake manifold and clogged fuel injectors could also cause similar symptoms. In fact, there are any number of things this could be, so the best place to start is providing some more specific information on the car’s history and symptoms.

Thanks, I will provide the data after work east coast time.


Check your battery voltage when misfiring. Should be above 12.5v

Could be the alternator isn’t putting out enough current at idle.

The sparkplugs were replaced 11/08 and the mileage was 103,470 – not that long ago.

It could be the Idle Air Control valve acting up. The engine computer controls the Idle speed through the IAC valve and I read your post as the engine having a stumble rather than a misfire.
The A/C will apply a pretty substantial load to the engine which in turn will have a tendency to lower the idle speed. The ECM (computer) will sense the A/C being turned on and will compensate for this load by upping the idle speed through the IAC valve.

This keeps everything on the straight and narrow; if it’s working as designed.

IAC problems are not rare and it’s possible that removing the IAC and cleaning it could help. In some cases, it must be replaced. They can be contrary sometimes.

Here’s an update. I went for a road trip with the mechanic that services this car; he was not sold on my AC story. It behaved just as I described in the parkling lot but off we went nonetheless. He drove around for awhile unable to get the car to do what he wanted and finally headed off to a long not-to-steep hill. He let the traffic get ahead of him and the accelerated to the bottom of hill then slowed the climb (AC off) – sure enough the misfire occurred a couple of times. While he still does not know exactly what it is, he has ruled out the AC. He mentioned exactly what Uncle Turbo above suspected although it’s probably not the plugs. If anyone is interested, I’ll report back after a trip to the shop next Friday.

Thanks for the help as I went armed with some ideas to the shop today.

A failing ignition coil would also be a good suspect.

Check the vacuum hoses esp for the brake booster if applicable.

Final Solution!

For those of you interested, the mechanics identified the problem after some extensive trouble shooting. It their own words:

“Inspected spark plug holes and found water in #2 plug hole. Cleaned water from hole and removed all spark plugs for inspection. Found #2 spark plug is arcing at base. Inspected spark plug boots and found vent on #2 boot was plugged and not able to vent moisture. Cleaned 5 spark plugs and reset gap. Installed 1 new spark plug in #2 cylinder. Cleaned all plug holes and dried as needed. Cleaned vent holes in all spark plug boots. Blockage looked like a spider web and cocoon. Installed ignition wires and installed coil pack and all related parts”.

This did solve the probem although it took awhile for them to indentify.