Recently purchased it off of a guy, it was his "project car" to say the least. I don't think a nut or bolt on the car hasn't been touched lol. It has a new crate engine with about 3000 miles. About a week after driving it I left the car idling when I went in to get some food and came back to the radiator fluid coming out of the the coolant hoses, turned out the radiator fan relays both went bad. Put new ones in and worked fine. Reason I say this is that now the car is overheating when driving it as well, but it only happens when going for 75ish + down the interstate for 5-10 minutes. The cars temperature will slowly creep up to 230ish and then keep rising from there. If I stick it in neutral and just coast it goes down from the outside air being pushed through. I've been driving it to work every week(15 minute drive) and have put a decent amount of miles on it and it hasn't overheated at all, except when going around the mph i said above. It will cool itself down to 190-200 I'll start driving again and it goes right back up. This has happened a few times now and have changed the thermostat, and blead the coolant. Have no clue why its overheating.

 

If the engine only overheats at speed and runs at normal temperature in slow traffic and at idle, this would suggest a clogged radiator. I suggest you remove it and give it a thorough flush internally. If very little dirt comes out, it may be clogged with lime scale if the PO used tap water. Therefore you could fill it up with diluted vinegar and leave it to soak overnight. The acetic acid in the vinegar should break up the lime scale and it might just save you the cost of buying a new replacement radiator.

 

I’ll try that and let you know what happens

 

you can also try replacing the coolant tank cap. it may be letting the system lose pressure at high temp. a good flush of the system like DDB said is also a good idea. how beat up are the condenser fins? those could also be restricting air flow across the radiatior. good luck