So im posting this locally in the hopes that someone is just generally good with cars and can offer some advie. the local advantage is that you know what stores rent what tools/testers/where to go for service.

Basics
Car: 1989 Subaru Justy 4wd 5spd
Issue: car stays cool for about 20mins at 4K/70mph on the freeway then the temp gauge slowly starts to creep up. it never quite overheats but its getting wicked hot.

Parts replaced: water pump and t-stat with 15 degree lower temp, rad cap (stock pressure)
Parts known good: thermosensor and fan. i watched the fan run

questionable parts but might not matter: temp sending sensor for the gauge

Situation
when driving for extended periods of time the car gets warm...then hot but i never let it actually overheat. the needle just goes past horizontal on the temp gauge and i start running the heater, slowing down ect and it cools down somewhat.

this happens mostly on the highway at 4Krmp or about 70ish mph. it also happens very infreequently when city driving. please note i used to drive to oregon and back in this car and never had this problem before

Ive replaced the T-stat with a 180 degree t stat, and the water pump.

I know the fan is working as i watched it come on when the needle on the tep gauge was about horizontal (plus/minus a few degrees of angle)

I THINK its actually getting hot vs a bad gauge cause i ran it in my driveway till it got to where it does on the highway temp wise and saw a ton of cooland gurgle into the over flow tank and back into the rad.

I did a compression test and didnt coem up with any irregularities. also when i run the motor wiht the rad cap off, the cooland doesnt gurgle at all. which suggests i dont have any bad gaskets, cracked head ect.

what could it be? it just seemed to slowly start doing this. ive had the car a year now and i cant think of what else other than a radiator (but how would that "go bad" on me) or needing a coolant system clean out wiht some sort of chemical cleaner to get all the scale off the inside.

what are your thoughts on what else it could be?

 

Try testing it when the motors hot.

 

pressure testing it? how are you supposed to do that without opeining the cap and letting am immense amount of water, which you cant account for, durring the test?

 

your radiator is not rejecting enough heat, get it cleaned (internally) or get a new one

 

that was the last thing i was gonna try. i wonder if a "Super flush" from a bottle would work or if i need more than that.

 

for trouble-shooting, if the "Super Flush" makes it a little better, then you can buy the new radiator with confidence