bronco still puking on the freeway

[CRUZIN66]

i just dumped in a rebuilt 302. the entire cooling sysyem is new rad,pump, t-stat(180),cap (16lb),etc.. im running mechanical gauges. no problems in the city until i get on the freeway. the temp gauge will read 180 then suddenly it will rocket to 230 for a few seconds then drop back to 180. i can smell rad fluid when it rockets to 230. when i get off the freeway it will run about 200. when i rev the enging at a stop it will drop down to 180. when i checked the fluid return tank and its at a normal level.

ive burped the system, installed a new t stat, i dont see any air bubbles or carbon in the rad when i run it without the cap.( trying to rule out a head gasket). oh ya, and i rechecked the timeing. my engine builder swears the head gaskets are on right. ( in other words im going to have to fix it myself if thats the problem)

need help!!! thx,rob

 

[Broncobowsher]

Does your lower rad hose still have it's spring in it?
Is the lower hose new?

 

[4x4DRIVER]

Sounds like you may have a bad fan clutch. it may not be moving enough air along with the incoming air to cool the truck, but when truck is not moving it engages just enough.

 

[Dr. Awsome]

mechanical or electric fan?

 

[mp]

Running with shroud or without?

 

[CRUZIN66]

new lower hose w/ spring, mechanical fan w/shroud no clutch. im stumped. i know the gauge is good. it was working on my gto

 

[SteveL]

Could the stat be sticking closed? A friend of mine drills an 1/8" hole in the edge of all his stats to eliminate air pockets. When the 66 had issues with a new motor I was swapping parts trying to get it to stay cool at idle. I fixed it by using a reverse flush kit on it. My guess is it had gasket seal or something from the rebuild floating in the system.

 

[ransil]

At highway speeds fan & fan shroud do nothing for cooling, ram air is used.

Big gaps around the radiator should be sealed so the air is forced thru the rad and not around it, I think your problem is the lower rad hose sucking shut.