Overheats on highway only

This one is boggling my mind.

it only overheats after a highway drive.

I've been attemtping to take care of the cooling system to factory specs. approved coolant, etc. since i have bad luck with vehicles overheating anyways...


have changed temp sender, thermostat(192*), fan clutch, upper and lower radiator hoses, heater hose,drain petcock & seal, 2 coolant flushes, filled with 50/50 coolant for japanese vehciles with alum heads and distilled water, no more leaks, bled air out several times...

currently, the truck will only overheat after a highway drive.
I can drive it around town stop and go, on and off for an hour of errands with no issues.
about once a week i drive out to destin, which is like a 20 min drive, with approx 7 min highway time. it had happened before when i was starting on the cooling system, now it seems a constant. as soon as im done with that short stretch of highway (60mph) the needle is going up and i have to pull over to cool down.

it will stay at level in a drive thru, it stayed at level idling for 20 min trying to help someone charge their batt since their alternator was dead...
it doesnt get hot in normal circumstances. only at/after highway speeds.
on last overheat, i parked, shut it off for 5 min, started it up when temp needle was back at level, and watched the temp go down more as i just sat there idling...

only things havent been changed yet are radiator itself, and water pump. will be doing water pump within next month when i do timing belt anyways. radiator has a few bent fins, about average for any older car...

just curious why it only wants to overheat on the highway. doesnt make any sense to me. the opposite of what it should do with overheating issues...

My bet is a faulty waterpump, it might be loose on the axel and therefor not give full flow when on the highway, but when you just cruise it can give enough.

this weekend, did cam/crank seals, timing belt, and water pump. filled coolant, bled air out... (gates timing belt and water pump.)

now it overheats constantly. like i make it a few blocks, then have to shut it off to cool down.

so far, have:
flushed coolant 2x
new coolant/distilled water 3x
installed thermostat
fan clutch
90% of radiator hoses have been replaced (just found the one under the intake, that, and one of the thick heater hoses are all thats left)
new radiator cap
bent fins back straight on radiator
new water pump with gaskets for pipe on exhaust side of block.

only things i can think left are radiator itself, or heater core (though it will put off heat enough to burn feet, with no leaks in cab...

no visible leaks anywhere, when i get one, at full system pressure, it shows me real quick where it is...

2 parts left, and it somehow got worse with a brand new water pump...
i put "waterpump gasket" silicone around the whole water pump gasket both sides... i did notice theres a flat circle without a bolt hole on the gasket. wondering if i maybe blocked something off with the gasket?
anyone have a coolant flow diagram for within the block/water pump?

any ideas or explanations?

drop down to a 180 tsat...try that

got worse with new pump, people suggested new water pump now pushes coolant harder, and makes blockages more obvious...

felt some cold spots on radiator, so got one ordered.

also, noticed the hose from the very bottom of the intake to the center of the pipe, felt shit crumbling inside it when i touched it.

my guess is the 2 flushes that cleared shit out of the heater core, let shit loose to get caught in the radiator. that, and the hose that turned to poo internally probably sent a rubber mess to clog there too.

replaced that hose, and new radiator, warms up to middle, then runs slightly below middle when thermostat opens. no more issues.

thanks.

weird side note. if you drain your radiator and get Xgal out. if you cannot pour all X gallons right back in, theres blockages inside your radiator...

sorry, I just came across this post. A clogged radiator would cause the symptoms that you are experiencing.

not 100% sure about this but im thinking the coolant travels through the heater core before returning to the radiator, try by-passing the heater core.

also could be a fan clutch issue

i had same issue and i fixed it by chancing radiator, water pump, thermostat, fan, all new hoses, and it runs like a champ now.

some of the coolant goes through the heater core, however there is a valve on the heater core that blocks flow when the heater itself is off. bypassing the heater core won't do anything for radiator flow.