Renix with weird over heating at freeway speeds

[my2monkeys]

So I'm leaving the jobsite in Palm Desert for my 110 mile commute home. Doing freeway speeds the temp gauge rises to the last hash mark before the red. I kick the heater full blast which usually has some good affect, but not this time. It never went into the red (above 230) but wouldn't drop any until I started going downhill then cooled a bit.

Pulled into rest area popped hood ,saw no boiling over or leak. By then it cooled to 210 -220 with desert winds helping as I fast idled. I'm thinking stuck tstat or crud in there blocking flow intermittently.

The rest of the trip it would ebb and flow from pegging needle to 230 to 210 and i got it to cool way down on a long cool downhill as now I was close to the coastal breezes.

My mt 2500 was hooked up and validating the gauge temp. Any ideas would be appreciated. I'm going to pull t-stat asap and will report back. Thanks guys. BTW rad is newish still closed system operating with volvo cap.

 

[Fred]

Burp it, sounds like a bubble to me. If you really want to stop overheating, get a Hesco water pump. My 90(425k really, mine was blowing a quart of oil a week because of bad valve seals at 170k) running a 4.1, ported HO head with 11-1 compression, Hesco cam and water pump, bored T-body, Mopar headers into 2.5 inch exhaust pushing a stick shift heavy XJ on 35's at 6k feet only needs the electric fan in multiple stoplights at 90 or climbing in 4-lo at 11k feet. It runs cool, and beats kids in Audi's off the line.