[Tpower66]

Why would my stock 389 66 gto puke excessive amounts when the temp gauge(accurate) is below 200. I installed a half gallon overflow tank and in hot weather it will overfill it and the radiator gets too low on water. Much better in cooler weather. Engine is tuned properly and radiator is clean 4 core. It does this driving down the road at 190 on temp gauge. Correct cap.

[goatwgn]

Just some things to check. Make sure you have no air pockets in the system.Get a tight fitting funnel and fill it up with the engine running. Let it run for a few minutes with the cap off and let the air escape from the system. It may blow out some coolant as well. If you can get a funnel kit that has a radiator cap with a hole for the funnel in it this is best. Also, make sure there is no "gas" smell or any contamination in the coolant. this could indicate a bad head gasket.If you do wind up with the dreaded head gasket, make sure you have the heads checked when removed. I wouldnt recommend a "valve job" on an engine with some miles under it if it is running well. Ive almost always seen engines start to use a lot of oil if they were high mileage and had a "valve job". Also, pull the thermostat and make sure it isnt sticking (put in a pot of boiling water).I would replace with a high flowing unit anyway if you take it out. Im sure your pump, radiator, and fan is ok or the temperature would be rising. But air pockets in the system can make the guage read cool and the engine would have "hot spots"in it. The best of luck with your car. Just some ideas of thing to look for.

[wytnyt]

what pressure is your cap?
a perfectly good system with too low pressure cap can do this.
try going up a couple lbs on your cap
also the rad should have a couple inches at top not filled to allow for expansion

[Tpower66]

Thanks Goatwgn & Wyinyt for the replies. I haven't tried to find air pockets but can. Engine is in excellent condition as I rebuilt it a few years ago I thought with great success---uses no oil and runs great. But I guess a bad head gasket is possible. The cap is a standard GM 15 lb. Is it normal for the car to sit a few days and still make a "pop" when removing the radiator cap.? Always does it. My thermostat is gutted and from October to about April it does ok here in Texas. If it pukes out during cooler times it normally is not much and it will normally recover from the overflow tank. But something is not right because these cars didn't even have an overflow tank in 66.

[geeteeohguy]

I too would look at pressure testing the cap first.

[cptefd]

double check your radiator cap...have heard of these failing causing this very problem

[Tpower66]

I replaced the cap with an 18#. Haven't had a chance to take it on a longer drive that causes the overflow tank to fill up and lose coolant. Still no problem with short 5/10 mile trips in hot weather.