Inliners International Forums General Discussion Engines Radiator overheating

I am trying to get my old 56 gmc with 235 road worthy, but the radiator keeps periodically blowing the anti-freeze out the cap. I had the radiator re-cored last year, and put in new thermostat 2 years ago. The truck seems to run fine, and warms up okay,when I start it up in the driveway. But then once in a while, boom, the pressure blows through the cap, emptying half the water. Could this be an intermitent thermostat issue, or something else.

Try a new cap.

HI 56 GMC: Check out the very detailed piece on " Solving Overheating " by Henry Olsen at : www.automotiveu.com . I think you will find it very interesting --- FLEET

Hi
I just started the truck again and it did the same thing. It warms up to running temperature of 180 degrees, and then the thermostat opens, letting the 180 degree water enter the radiator, causing the water to blow out of the top of the radiator. The rad shop said I should have a cap rated at about 4 pounds of pressure?
After the erruption, I top up the water, and it seems to work fine. It is just at the point when the thermstat opens, that I have the problem

Ya know it almost sounds like you have a air bubble in the system. Drill a 1/16 hole in the thermosat to let the air bleed out when you fill the system. Do you have a heater? Sometimes they will trap air also. A 7Lb cap would be OK also .Another thought some older radiators used a 1 inch deep neck most newer stuff is 3/4s inch deep, make sure you have the correct cap. Some of the older style radiators did not like too high of pressure ,Fords were famious for blowing out the tank seams in the 40s-50s if too much pressure was used.

I think John has the answer.
The system sucks in air through the overflow as it cools down. If you will drill a 1/16" hole in the flange around the thermostat. It will act as an air hole and the air can escape from the system without pushing the out the water.

Joe

well I am sure that you probably know this but when water heats up it wants to expand, and I use a standard cap to keep it under preasure, like 12-16lbs and my engine runs great, not to meantion that if you only have four lbs the second that hot water comes out it want to escape due to the increased preasure and if the cap isnt rated high enough it will keep doing that, not to mention that at four lbs the boiling temp is much lower, if you increase the rating on your cap you'll be a lot happier, it wont leak and your car will run cooler and better because it wont let the water expand which keeps it cooler and you boiling temp higher, I tried this a ways back with like a 6lbs cap and it did the same thing and I just wanted to shoot myself, so finally I just fixed my rad leak and put a standard 16lb cap on it and have loved it ever since, give it a shot and see what you get.

oh and make sure to burp it when you do a rad fill up, and make sure to leave a little bit of room from the cap to the water in the rad to give it a little room to expand, but just a little like an eighth of an inch from the water to the top of the tank, good luck yo

I have used a 16# cap and closed system on my '40 radiator for over 30 years.
Try a higher # cap.
Drill a hole in the thermastat.

The 4 # is not enough to stop the sudden pressure as already mentioned by others.

Thanks to all for your suggestions. I drilled some holes in the thermostat and upgraded the cap from 4# to 7# as a start, and it seems to have cured the problem. One other question. The temperature gage when cold, sits at 220 degrees, and previously with a 195 degree thermostat, it would go down to 180 degrees on the gage when warmed up. Now I have put in a 180 degree thermo, and it goes down to 195 degrees when warmed up. Shouldn't the gage rest at the low end of the scale, and go up from there. How to correct?

Check the bezel of your gauge and see if it has a sticker.
Sounds like you have a gauge that has a bezel or sticker from another gauge that starts reading from the other side...

You can test temp gauges and thermostats by boiling water on the stove and a needle type cooking thermometer.
Boiling point of water at sea level is 212.
A 180 thermostat begins to open at 160 and is fully open at 180.

Sounds like everything is working and you just have to flip the numbers written on the gauge.
Never top off the overflow tank on radiators. Just cover the tubes and a little over.

yeah it should start at a theoretical 0 on the scale and go UP to temperature, not start hot and go DOWN to temperature, either your gauge is funky, or maybe you got some wires crossed on the back of your dash? Another possibilty is that maybe your temp sensor on the head is funky?

Your gauge should read ambient temperature or it's lowest value when cold. As the egine heats up, normally a temp reading a bit higher than your thermostat value will be indicated for a brief time. Soon after that, once the thermostat opens the temp reading should drop to the value of the thermostat and stay there +/- a few degrees. I would try a different temp gauge or at least check the wiring to/from the temp sensor.

Do you have an electric gauge?

I know it's a pain, but try a mechanical, I had an electric temp gauge that was reading wrong (50 degrees high :o) and it scared the daylights out of me (new engine build). I tried a cheapie mechanical (Exxis IIRC basically store brand from B&C / Carquest) and it read almost dead on (checking with an infrared temp gun, the casting of the intake where the temp sender was was within 8 degrees of what the gauge said).