250 inline running hot at highway speed

[MagmaJct]

Is it just my imagination, or has this thread gone from the realm of likely causes of the OP's problem, into the realm of insanity?

Actually the thermostat keeps the coolant in the engine until it warms up, then it opens and flows to the radiator.

Just so...got me...the resistance theory still serves though no? I don't know. Today we troubleshoot

My bet is internally clogged radiator. Also outside. Bugs and crap will block the air flow through the fins. With trucks this old there's no telling what you will find.

The gob of gasket maker I blew out of there supports that logic. Thanks Boog

Thanks guys...hope to diagnose today...

You can buy a flush kit that has a fitting that you splice into the heater feed hose, i used to drain my coolant and fill with water, then hook my hot water tank up with a hose and fire up the truck and let it flush.

Bought a flex fan too which knocked off about 5 degrees Still 210ish when wrapped out at 70 mph

Put an air dam right under your rad and watch the temp drop.

Oooooo...I like that certainty....like a flat alum sheet in front of the crossmember ?

To cycle air in or block the bottom off?

Cooling is finicky work it seems. The things that work are oft counter intuitive.

To create a low pressure area below the motor so the air will flow thru the rad.

Somehow, the thread went from diagnosing a problem, to throwing crap at it. Is this what we learned in school and texts on proper troubleshooting? A stock configuration needs only stock solutions. What can cause an engine to overheat? Why is removing the thermostat so commonly the prescribed solution, and why doesn't it work?

Well, think about it for a moment. Thermostats are designed to open at a certain temperature. 160F, 185F, 190F, 195F, and so on. If the coolant leaving the engine is over 200F, here's what we can surmise. The 160F thermostat is fully open. The 185F thermostat is fully open. The 190F thermostat is fully open, and so on and so forth. There is no real value in installing a cooler thermostat, or removing the thermostat!

The argument is often made that the thermostat restricts flow, therefore, removing it will increase flow, and bring the temps down. This logic is flawed in a HUGE way! This logic assumes the thermostat is the cause of the restriction! Perhaps in some high HP engine, this might be the case. But I suspect the OP has a near stock setup.

In which case, the odds are great something is broken. Be it a clogged radiator, clogged engine cooling passage, or a bad pump. Perhaps a collapsed hose. The lower radiator hose has a spring in it to keep it from collapsing. Perhaps it rusted out. A temporary fix will not solve the problem if the problem is poor coolant flow.