Why would Mark's oil light flash only when the AC is on?

Dear Car Talk

Dear Car Talk | Dec 01, 1992

Dear Tom and Ray:

In 1986 I was driving my 1984 VW GTI across the country. I was moving, so my car was packed. At the time my car had about 28K on it. I was driving on the interstate with the air conditioner running and a full load in mid summer. As I exited to get gas and slowed down, my oil light started to flicker and buzz. This happened a few times driving from Connecticut to Arizona. I asked a few mechanics along the way why this was happening, and they said it was probably just a momentary drop in oil pressure, and not to worry about it unless the oil light stayed on. In October of '86, my AC stopped working, and I didn't get it fixed until the summer of '92. I never had that oil light problem during that time. But no sooner did I get my AC fixed, than the oil light problem started again. It happens only in very hot weather, at freeway speeds, under load, with the AC on. When it started to happen again, I talked to my mechanic and he said if the light doesn't stay on, don't worry about it. I called VW, and they had no idea because the incidents happened six years apart, but suggested an oil pump. Now that the summer's over, the problem is gone again. The car has 130,000 miles. I have changed the oil every 3,000 miles. The car has never had any engine trouble, and never burns oil. Do you have any idea what causes the oil light to come on?

RAY: Yeah. Low oil pressure.

TOM: Pretty helpful, isn't he, Mark?

RAY: Seriously, it sounds pretty straight forward to me. Back in 1986, you were driving the car was under the worst possible conditions. The temperature was sweltering , you had 18 pieces of furniture and six crates of Zamfir "Pan Flute" albums in the car, you were probably driving on the highway for many hours AND you had the air conditioner on. This is about as heavy a load as you can put on this crummy little Rabbit...uh, sorry, I mean GTI. And the mechanics you asked were probably right. It probably was a temporary drop in oil pressure due to the excessive load on the engine and the thinning of oil due to the heat.

TOM: And now, the same thing is happening, although since the car is older and the oil pressure generally lower, this set of circumstances is reproduced more easily. It still requires a hot day, when the oil heats up and becomes less viscous. You still have to be using the air conditioner, which puts an extra load on the engine, slows down the oil pump, and makes the engine run hotter.

RAY: And if you're lucky, all you'll need is a new oil pump.

TOM: You have all winter to save up, Mark. Next April, have a new oil pump installed, and I bet this problem will disappear. And if you move back across the country, leave the Zamfir albums behind. They're all available on CD now, anyway.

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