Can an oil cap really, truly, shoot off your engine? Find out.

Dear Car Talk

Dear Car Talk | Nov 01, 2007

Dear Tom and Ray:

I have a 2002 Daewoo wagon. (I'll pause here for your laughter.) I have owned it since it was new and have adequately maintained it, and it has just under 50,000 miles on it. Recently we took a trip, and because of an ongoing oil drip, my husband checked the oil, added a half-quart and then we headed home. After stopping and dropping off him and the kids, I headed out to my brother's house (10 minutes away) to pick up our dog. On the way there, I could see white smoke coming from the back of the car. Once I got into his driveway, the car was smoking profusely. I opened the hood to find the oil cap missing and oil coating the inside of the engine compartment and consequently dripping on my brother's driveway. When I went to open the back of the car to get paper towels to wipe up the puddle forming underneath, I found a spray of oil on the back window and hatch door (like someone sprayed it with Pam). I can't imagine what the car that was tailgating me looks like. I immediately called my husband (who, I might add, I trust with my life and KNOW BEYOND A SHADOW OF A DOUBT THAT HE PUT THE CAP BACK ON PROPERLY). He tells me that the oil cap shot off (reminding me that this happened once before). My question: What happened? When I brought it in to the service station (the same guys who have fixed and serviced it for years), they told me that oil caps are not under any pressure and will not "shoot off." Service Guy told me in not so many words that the cap was NOT put on properly. What do you think? -- Leslie

TOM: Well, Leslie, we know beyond a shadow of a doubt that your husband did not put the cap back on properly.

RAY: There's really no other way for it to come off. There is a small amount of positive pressure in the crankcase, but not enough to "blow off" an oil cap that was properly tightened.

TOM: I suppose it's possible that the cap was stripped or broken, or that it's particularly hard to screw on in this vehicle (we don't know, because we don't see many 2002 Daewoos in our garage). But if not, the only explanation is that it wasn't tightened all the way.

RAY: The oil cap is just not the weakest link in that system. If pressure were the problem, other things would blow off first.

TOM: You can try it yourself. Put the cap on correctly, and then try to pull it straight off. You won't be able to.

RAY: And the film on your back window was just oil, dripping down underneath the car and getting swept up onto the back window. We see that all the time when differentials leak.

TOM: Fortunately, you probably didn't do any permanent damage. If you were down only a quart or two when you took it in, the engine is fine -- or at least no worse than it was before this incident.

RAY: To reduce the likelihood of this happening in the future, I would suggest applying some Ginkgo biloba -- to your husband, not your oil cap. Good luck, Leslie.

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