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Overheating 1998 F-150

256,000 miles. Started overheating as soon as the truck started up. Replaced the radiator and thermostat, and now overheats when driven over 40mph. (Does not overheat in idle or under 40). Everyone I’ve talked to says I’ve got a blown head gasket, but i DO NOT have water in my oil or white smoke coming out of the tailpipe. Any other ideas? Or is everyone right? Thanks.

Run a compression test and see what the results are. Then run a leakdown test. See what happens, and we’ll go from there.

Actually a blown head gasket does not mean you will immediately see plumes of vapor from the tailpipe or that you will see signs of water in the oil. This is a lot miles on any car. Given that at this age ANY overheating occurred, I have to vote for head gasket damage. In which case, read the previous answer.