1985 GMC S15 2.8v6

I needed a new water pump(dripping from hole) so I also replaced the thermostat and radiator cap and flushed the system. Now at hwy speeds it overheats but when I let off throttle the temp. goes down to normal. Also at idle there is no overheating, only at higher rpm’s. Even in very cold weather the overheating occurs at 60-70mph but if I let off throttle and coast the temp drops right back to normal in a few seconds. The temp goes right to red under load after the thermostat opens, and if I stop the temp goes to normal as it should always be. I’ve owned the truck 15 years and never had any cooling problems before. I also do all of my own repairs and feel confident in my installation process.  Could this be a fan clutch causing my problem? Any info would be appreciated.


1 thought on “1985 GMC S15 2.8v6”

  1. First check and recheck the radiator fluid level after the thermostat has opened. Could just be an air pocket. It is possible for it to be a fan clutch, more commonly would be the lack of a fan shroud.

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