Any chance a bad thermostat could cause my check-engine-light to come on?

Dear Car Talk

Dear Car Talk | Mar 01, 1995

Dear Tom and Ray:

I have a 1989 Ford Aerostar with 64,000 miles on it. I have two problems. Problem 1) When I put on the brakes, my oil light comes on and my oil needle drops all the way down like I don't have any oil (but I do have oil). When I let up on the brakes and accelerate, the oil light goes off and the oil needle goes back to normal. Problem 2) My heating is terrible. It never gets hot, even though there's plenty of antifreeze in the system. What do I need to do?

RAY: You need to put 100 bucks in your pocket, and go to your local gas station soon.

TOM: I think all you need is a new thermostat and an oil change. A bad thermostat would explain everything. If the thermostat is stuck open in the winter, the engine would never get up to operating temperature. That's why you don't have good heat.

RAY: And if the engine is not running at operating temperature, you would get incomplete combustion, which means that some of the gasoline getting into your cylinders is not getting burned.

TOM: So what happens to that left over gasoline? It works its way down past the piston rings and into the crankcase, where it dilutes (thins out) the oil. And that's why your oil light is coming on. Your oil light indicates that you have low oil PRESSURE, not low oil level.

RAY: If the oil isn't "thick" enough due to dilution, the oil pressure would be particularly low at idle. Oil pressure is always higher when you're revving the engine, so that explains why your oil light goes off when you step on the gas. I think it's safe to say that if you keep driving it like this, eventually, the oil light will stay on all the time.

TOM: And that will be your cue to stuff $2,000 in your pocket and go get yourself an engine rebuild. So take care of it now, Helen.

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