Dear Tom and Ray:
I was selling an old car that was starting to burn oil. One of the guys who looked at the car pulled the oil filler cap, and the engine died immediately. He said a mechanic buddy told him that this is a test for bad compression. We tried it on his car, and it didn't miss a beat. I had never heard of this before. Is this a true test, and why does it work? -- Rick
TOM: Hmm. Sounds good. But it's baloney, Rick.
RAY: There's no relationship between the oil cap and the engine conking out. Unless the car in question is a Saab.
TOM: For some reason, we've found that on many Saabs, when you remove the oil cap, the engine WILL stall. But that's just due to an engineering quirk that produces a massive vacuum leak when the oil cap is off. I have no idea if it was intentional, on Saab's part. But Saab's the only car I know of that does it.
RAY: There might be other cars that do it, but we're not in the habit of removing the oil caps from cars that are running. For one thing, there's never any need to. And for another, it always sends oil splashing all over my newly cleaned and pressed coveralls.
TOM: However, what you CAN tell by removing the oil cap is whether the car has excessive blowby. Blowby is created when the rings wear out, and lots of combustion gasses sneak by the rings and into the crankcase, where the oil is stored.
RAY: If there are more combustion gasses in the crankcase than the ventilation system can expel, pressure builds up in there. When you remove the oil cap, you might see smoke come out.
TOM: That'd be a sure sign that you'd want to avoid buying that car. Maybe that's what this guy had in mind.