Hey Guys,
Ive got an 89 GTA which i recently (less than a month ago) put in a freshly rebuilt 350 motor edelbrock carb 600 cfm, bored .30 over, 194 heads and a comp cam with a 254 intake, 265 exhaust lift at a 50 degree duration stock hei gm distributor with vacuum advance. Well i went to get it registered and a day later it started overheating, but only on the highway. Stop and go traffic it stayed at/around 190, but on the highway it jumped way up (between 210-250). We took out the thermostat, took it for a drive and still same situation. Next thing we tried was replacing the water pump (a reverse rotation pump), still badly overheating. Went to a bone-yard, replaced the air dam, (the one we had was missing the two side pieces) and still nothing. Desperate to try anything, we took the radiator completely out, brought it to a car wash and power washed it to make sure all the gunk was out, re installed it, when we did, we realized the fan was rotating the wrong way, fixed that and took it for a drive. Now the worst part happened, took it about 10-15 minutes to Cumberland farms, got there and the car was smoking like crazy, untill this point wed had no smoke whatsoever. The antifreeze in the radiator and overflow bottle were bubbling and after sitting for about 20 minutes, the temp gauge still showed a solid 230. We finally got the car back home after 2 stops and waiting for it to cool down in between. Opened the radiator which was completely empty, and the overflow bottle was cracked and empty as well, besides the leaking from the cracked overflow, there was no visible leaks anywhere else. After looking at a few forums, we saw a few suggestions like checking the radiator cap, which we did, and making sure the direction of the water pump was correct. Now we sit here stumped, and even one of the best mechanics we know is stumped. Please help!

 

is the oil ok? are you sure you dont have a bad head gasket. or the gaskets that are blocking something off. i would put a pressure tester on it and start the car. see if the pressure rises.

 

the oil is fine, no milky mixture or anything. the motor was just rebuilt and it has less than 50 miles on it.

 

Did you replace the overflow bottle already? when the coolant expands, it flows into the overflow bottle but being that it has a leak, it goes to waste and now the car has less coolant every time it expands.

 

The electric fan was turning the wrong way? That shouldnt matter if your overheating while driving down the road. Down the road. Just because you powerwashed your radiator out does not mean it's okay. So therefore it's either a bad water pump, radiator, air dam and baffle! is the water circulating??

 

It could have internal blockage , If there is no air damn like undercover says, it will overheat going down the road

 

The radiator may need rodding out.

 

did you change the water pump? is it serpentine? or vbelt? was it overheating before all this happened. does it have all the same stuff on it.

and i dont care if it was rebuilt yesterday with 3 miles on it. if it has something wrong with it then it does.

so
1. did it overheat before the rebuild.
2. does it have the same cooling system as before
3. what was changed during the rebuild in the cooling system

 

Yes, the overflow is new, the radiator was replaced, as was the water pump, it's a serpentine setup. And were pretty stumped because we had a 383 stroker in before this motor, same internals as the 355 and it didn't overheat like this. It usually stayed around 210. We asked a race guy around here and he suggests going with a bigger radiator like a be cool. But its not like we have a crazy build and I'd hate to spend $700 on a radiator and itnot be that. And I also contacted the machine shop who did the heads and whatnot and they refuse to back their work and are saying it's not the build, yet if we take the heads off it voids the warantee. Catch 22 here

 

ok did you replace the water pump with a serpentine one that rotates counterclockwise.

 

Yep we took it out and double checked.. The first time pep boys gave us the wrong one, and then we replaced it with the correct one

 

sounds like the same problem i had after i put a new radiator in. to be honest I didn't fill the radiator properly, so I was told. radiator kept being out of fluid because I would just fill the radiator and go. it would get hot and turn to steam and leak out of the weakest part in the system. which ended up being a hose...then the overflow tank...then the heater core. I was stumped too. it would build so much pressure it was finally coming out of the intake by the thermostat. took out the thermostat and changed the gasket. left the thermostat out! started the car and let it run while adding fluid let it run with the cap off for 15 min or so and just kept adding fluid until it just set there with out over flowing. put the cap back on and havent had a problem.

 

Yeah I mean we did that, made sure all the air bubbles were out nd that kind of thing, we ran it with and without the thermostat and both ways it still overheat. Were gonna try using these water restrictors and see if that helps. I just don't know where else to go from here.

 

I forgot to mention that I also had an after market aluminum radiator and the new cap from autozone wasnt sealing. so it didnt take long to loose alot of fluid. so I replaced the new cap with one from Smileys and thats when all the pressure problems started lol
so everything is new...holding pressure and fluid. good luck!

 

did you change teh routing of the belt? if your spinning a reverse flow pump the wrong way your not pumping. with the cap off. goose the throttle does the coolant suck down in the radiator?

 

man im having the same problems. its weird, the other day it ran like a beast on the highway, drove from one side of the city to the other for approximately 30 min. soon as i got off the highway and came to a red light, she started sputtering and bogging out. temp rose up. then the overflow tank was bubbling hot and full. I think I read somewhere on here that if the lower hose is not in good shape and you have a good water pump (for example, ours are brand new) that it will suck the hose closed and cease waterflowage. i think i might try this next. since my water pump, rad cap, and thermo are brand new.

here is some food for thought also, i had a 305 in her before and it ran like a beauty. put this 350 tpi in and have had problems ever since around the summer time. i cant grasp the problem. ill log in things that i do too since it seems like were in the same boat, and no point of starting a whole other thread. not trying to hi jack a thread here...

 

im going to try this for good measure also. so with the cap off and hitting the throttle, if the water gets sucked down this is a good thing correct? just making sure.

ok we filled up the rad with the engine running, and i blipped the throttle a few times and the water sucked down a substantial amount then rose back up.

 

OP,
You're probably running lean or another "causes heat" issue.
No egr, excess timing or something like that.
Which isn't to say that the radiator doesn't need replaced with a new stocker.

 

bottom hose should have a large spring in it to prevent collapsing.