My quick GTO runs hot

January 1, 2012 | By Richard Prince


I have a perplexing problem that has stumped several different local resources, so I thought I might see if you can shed some light on the subject.

I have a 1969 Pontiac GTO convertible and recently had the engine completely rebuilt by a local well-respected muscle car shop that specializes in Chevrolets. Between them and the machine shop they send work to, we rebuilt it with a .069 camshaft, roller rockers, Ram Air III heads, intake and HO exhaust manifolds. The car runs great and is super fast. I am totally happy with the rebuild and love driving it with the new engine.

The problem arose on the ride home and mimicked a classic case of overheating. The radiator was boiling over and spilling water all over the ground. Because my gauge cluster was inoperable, I couldn’t tell if the engine was getting hot or not, so the guys changed the thermostat to a 160-degree, changed the radiator cap from a 13 lb. cap to a 16 lb. one and installed a temperature gauge in my glove box that was direct-wired to the thermostat.

They also inspected the radiator to confirm that water was circulating and that it was 3/4 full, which it was. After these changes, the radiator still purged 1/3 to 1/2 of its contents, but the temperature gauge barely sniffed 200 degrees.

Here in the Sacramento Valley, the temperature is regularly over 100 degrees on summer afternoons and if I take her out on the freeway in those conditions, she will hit 210 at the most, but usually hovers around 180 to 200.

Timing, the improper seating of the water pump gasket, a “vapor lock” of water in the block and a clogged or blocked radiator flow all were suggested as possible culprits, but when investigated, none of these items seemed to make a difference.

After exhausting these possible solutions, the guys at the shop all shrugged their shoulders and we agreed that I really needed to keep my eye on the temperature gauge, which I do, so far without incident.

However, my gut tells me that it just isn’t “cool” to run the radiator half full, especially when, in afternoon rush hour stop-and-go traffic with the outside temperature at 100+, any muscle car engine can overheat and burn up.

The water in the radiator is a deep red/orange/brown because it’s ultra rusty. Yesterday I decided to fill the radiator and the water was almost thick with rust. I know that the rust isn’t “cool” either. I am going to flush the system and then use a rust remover/inhibitor as recommended by some of the GTO club members.


Engine coolant will come out of the radiator when the coolant pressure overcomes the resistance of the radiator cap’s seal or when there is something wrong with the sealing system. Excessive coolant pressure normally results from very hot coolant temperature but from your description it doesn’t seem as though the engine is running excessively hot.

Since a change of caps didn’t make a difference we can assume that the cap is not defective. But what about the radiator neck? Or more specifically, what about the area of the neck where the cap’s seal seats? Very carefully inspect this area for any dents, deformities or other defects that could prevent the cap from thoroughly sealing.

As far as the rust in the coolant goes, that is very surprising given that the engine was just completely rebuilt. The engine block and cylinder heads should have been completely cleaned inside and out and all loose rust inside the castings should have been removed. Was this done? If the old radiator was reused after the rebuild was it completely cleaned out? A great deal of rust in the coolant can certainly impair the function of the coolant and can clog water passages in the block, heads, water pump, valves, head gaskets, and so on. Extreme rust also can indicate a serious problem with the block or head castings. Very severe corrosion causes the casting walls to get thin and thin walls cause elevated operating temperatures and hot spots. In the most extreme cases severe corrosion of the castings can lead to coolant and oil mixing in the engine. All of the contamination in the coolant fluid can also lower the boiling point of the fluid, causing it to “boil out” of the radiator even if the engine is not running excessively hot. You should completely flush the car’s cooling system until all traces of the rust and muck are gone and then refill the entire system with a 50/50 mix of antifreeze and steam distilled water.