I have a 1990 GMC Pickup, 5.7L V-8 with 230K miles.  Original engine well maintained, and while burns a little oil, always starts.  Thermostat repalced 2 months ago, but vehicle still running in High Temp Range. Runs around 210 when temp 75 or less, but around 220 when 80 degrees outside or higher.  When driven 75-80 MPH, temp is going up above 235, and I turn off A/C until lowers slightly.  Yesterday, going over 235 degree when driving  slow or idling, A/C on, but 220 when driving 55 mph.  In Florida, and 95 yesterday with High humidity.  Looking for advice for best option to find issue - Fan Clutch, Radiator ??  Any advice?

Rhett

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Start by having the radiator removed and cleaned (inside and out,
obviously) - high mileage truck, probably needs that.  Replacing the
radiator would probably be even wiser.

Have the block flushed as completely as possible.

Those would be my first two ideas on a problem like that on a high mileage
truck.

Next, if you haven't had the water pump replaced, you might have an
internally cavitated pump and/or eroded impeller.  Cavitation of the
reaction surface the impeller spins next to will cause reduced pump
efficiency.

 I've seen that.

It's probably a stack-up of several factors like the ones above.

When you get all that done, you should see a difference.   One way or
another, all that stuff needs to be done.

Let me know what happens.  The fan clutch wouldn't be something I'd consider
until everything else had been handled.

Richard

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Richard,

Due to the age of my truck, I decided to just replace the radiator as the first choice, per your recommendation.  That solved the problem!  The temp stays under 200 no matter the driving condition, or outside temp.  We cut open the old radiator, and it was full of sediment, scale, etc., especially on the lower half.

Thanks for the advice.

Rhett