How do you wind up with oil in your coolant? Find out.

Dear Car Talk

Dear Car Talk | Aug 01, 2009

Dear Tom and Ray:

I have a 1991 Chevrolet S-10 with the four-cylinder mill. I bought it because I had to park my Ford F-350 Dually when diesel fuel went above $4 per gallon. I have had the Chevy less than a year and have had very little trouble with it -- until I struck oil! I noticed that I had to add oil frequently. I wasn't seeing any smoke or leaks. Then one day I took off the radiator cap and discovered: OIL. My question is, How does oil get into the cooling system without getting water in the oil? The truck has a five-speed manual transmission, so there is no transmission oil cooler. The oil definitely is coming from the engine. What could be the source of the oil coming into the cooling system when no water is getting into the oil? Help!

-- Allen

TOM: Well, unless in a drunken stupor, you added a quart of oil to your radiator one night, you have a blown head gasket or a cracked head. Or worse.

RAY: The reason the oil is going only one way probably has to do with the location of the crack. If the crack is near the oil pump, the oil passing by that crack is under as much as 100 pounds of pressure. So it's being pushed into the water jacket, where the coolant is pressurized only to about 15 psi.

TOM: Eventually, you'll get so much oil in there that all that extra volume and pressure will blow a cooling hose, or even blow the radiator. So, unlike Jed Clampett's backyard bubbler, this oil discovery doesn't bode well for you, Allen.

RAY: Here's what I'd suggest. If the truck is otherwise in good condition, have someone pull off the cylinder head. If you're really lucky, you'll find a bad head gasket, you'll replace it and you'll put the head back on. End of story.

TOM: If it's not the head gasket, then your mechanic can examine the head for cracks. If your head is OK (which has never been true in my brother's case), then you have to assume you have a cracked block, and you'll need to find a used motor for this little gem.

RAY: But if the truck's in good shape, and diesel fuel stays up near $5 a gallon, it may be worth it, Allen. We wish you luck!

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