[LEGACY_GN]

1972 Pontiac Grandville convertible 455 all original. Car only overheats when on highway above 60 mph and outside temp makes zero difference. Whether it's a 100 plus or 60 out, same result. I installed a 160 thermostat that made no difference. Has new water pump but installed by idiots so that could be the problem or just clogged radiator unable to get rid of heat?

[blubomber]

Looks like you've answered your own question.

Pull the rad and re-core it and re-install the water pump just to be sure.

[David Brown]

X2 Sounds like partially plugged radiator. The sediment falls to the bottom of the radiator, so it might appear to flow just fine.Back flushing rarely clears a plugged radiator. It either needs rodded (if anyone still does that), re-cored or replaced. A stock brass in my 70 Bonne 455 stays at 180-190 all the time in south Florida, including with the A/C on.

[LEGACY_GN]

I wasn't sure so I thought I'd get some input before dumping more money in the car.

[north]

Its probably a cooling system problem but also consider the possibility of an internal engine problem, a bad bearing or a lean condition on the primary circuit would also result in similar symptoms as heat rises once a certain rpm or engine load is reached.

A good mechanic can easily assess this kind of problem.

[LEGACY_GN]

It runs and drives perfectly. If it's not raining it gets driven about every day. I'm taking it to costco today. Around town it's perfect. Starts right up even after sitting in hot garage for a week due to rain. It does have an exhaust manifold leak that I am scared to replace due to risk of broken bolts but that shouldn't affect anything. I know on modern cars it will cause a lean condition due to 02 sensor. There's no noise and good oil pressure on road but idle hot pressure is only a few pounds which from what i've read is common on the 455. The only concern I had was maybe the water pump was cavitating at high rpm and not pumping water due to bad installation of that shield that directs the water.

[north]

What do you mean by "only a few pounds" for oil pressure? Stock warm idle pressure is usually 20 pounds, anything under 15 is a cause for concern.

Common cause for lean condition with an engine from that era is either the wrong carb with too lean jets on the primary circuit (not the idle circuit) or some fuel jetting issue like low float, dirt build up in the jets etc

[LEGACY_GN]

In gear idleing at 210 temp it's got about 5 pounds I'm guestimating because the factory gauge doesn't give specifics and could be wrong, but it's nothing worth worrying about. It's got 110k miles on it, if it needs rebuilt it needs rebuilt, going to happen eventually but when it overheats it's got 50 plus pounds on the highway. Right now I'm looking for a 200-4r out of a grand national for overdrive which will help immensely on the highway. Hopefully bolt that in within the next few months. Ironically I'm in the process of reinstalling the one on my GN right now.

[north]

I'd throw a mechanical gauge on it to find out, 5psi will kill an engine and could cause a catastrophic failure that would mean replacing rather than just overhauling the engine.

Often low oil pressure without a knocking noise means a bad cam bearing which will eventually seize (generating lots of heat before that day).