Howdy, Guys.

My truck is having a "running warmer than normal" issue that has me stumped.

I have over 200k miles on it, and the coolant has been flushed twice... once around 100k miles and once around 190k miles. The truck still has the original radiator hoses, thermostat, and expansion tank pressure cap.

I get plenty of heat in the cab. There are no visible signs of coolant leakage. The upper radiator hose gets hot at normal operating temperatures to indicate proper thermostat operation. The lower radiator hose gets warm to indicate proper coolant flow. And, the coolant in the expansion tank is at the halfway mark.

My truck has always operated at a temperature that is marked as roughly halfway (50%) on the temperature gauge. This has always occurred at any time of year, regardless of outside ambient temps, regardless if I am running the A/C, and regardless of city or highway driving.

However, during the past few months something strange has happened. My drive from home to work is 25 miles. During the morning drive (65mph, no A/C, ambient outside temps 70-80 degrees) my truck runs at the usual temperature mark on the gauge (roughly halfway).

However, on the drive home (65mph, running A/C, ambient outside temps 90-100 degrees) my truck temperature gauge will read normal (halfway mark) for the first 20 miles of the drive home. After that it will creep up to about the 3/4 mark (75%) on the temp gauge. This occurs only on the highway.

Once I exit off the highway, for the last 1 mile to my house, the temperature gauge immediately drops back to the normal range. The temp drop doesn't occur gradually over the last mile or so to my house. The temp drop occurs in the first couple hundred yards when dropping from highway speed (65mph) to service road speed (45mph).

The truck never gets to the high end of the temp gauge reading (over 75%). It never hits the red zone on the temp gauge. It doesn't overheat. The "warmer than normal" condition doesn't occur on the morning drive, it doesn't occur in city driving during hot outside ambient temps and with the A/C running, and it doesn't occur during idling.

I'm thinking the 16psi expansion tank pressure cap is causing this issue.


Thoughts?

 

My guess is the radiator is getting plugged and losing cooling capacity.
Easy to replace the cap first so try it.
Good luck.