Jim Grant's Tech Tips
'86 Chevy Suburban, Oil in the coolant
Q: I replaced the heads on my 86 Chevy Suburban with a 5.7L engine. It started up and I made all the adjustments. It runs good, does not overheat, but when I rechecked the coolant I found oil in it! I checked the oil for water or coolant but did not find any. The heads were out of a good running engine. Any ideas?
A: Due to the configuration of the cylinder heads on your vehicle, Im inclined to believe that you have some other problems. There are no oil pressure passages in the cylinder head of your engine type. The oil is pumped up through the push rods for the rocker arm and then just drains back to the oil pan. The cooling system operates under pressure with about 12 to 14 psi. Even if there was a crack in the head it would be a coolant in the oil problem not the other way around. Does your Suburban have an engine oil cooler? Many of the larger GM vehicles have an engine oil cooler that is mounted inside one of the radiators cooling tanks. Engine oil pressure runs in the area of 40 to 50 psi in most cases. For oil to enter the cooling system, it has to be pumped in. Your engine was okay before the heads were replaced, correct? Now there is oil in the coolant. I cant help but wonder, if equipped with an oil cooler, that the engine oil cooler was damaged while the heads were being replaced. How can this happen? All you have to do is bump or lean on the oil cooler lines in the general area of the radiator. The oil cooler inside the radiator can develop a hairline crack that will allow oil to enter the cooling system under pressure. Engine oil coolers do fail on their own as well. If you dont have an engine oil cooler then youve got major engine problems.