Hi, I have a 68 F250 360 and the radiator is giving me back pressure within 3 mins of it starting. Cold water. I put a new thermostat in and back flushed the block and radiator. Could the water be my problem? Thanks

 

What do you mean "back flushed the radiator and block"?

 

There are two reasons for pressure in the cooling system.
1. Thermal expansion. Normal. Usually the system will burp out the excess and you will see some space after you open the radiator cap on a cool engine.
2. Exhaust gases are being pushed into the water jacket.

If it is exhaust gases, there are three potential sources in order of likelihood:
1. Head gasket.
2. Cracked head.
3. Cracked cylinder.

Since your's does it quickly, I'm going with exhaust gases.

1. From cold, remove the radiator cap.
2. Fill if necessary to cover the tubes by an inch or so.
3. Start the truck.
4. If it percolates (you see bubbles coming up) then you have exhaust gases.

Then you can hunt out from where. From a cold engine, start removing spark plugs and inspecting. When you find the one(s) that look brand new because they have been steam cleaned by the coolant, you have found the offending cylinder(s). The 360 uses a metal shim gasket, so they are not as prone to failure as the regular type. If someone loosened or removed a head and did not replace that gasket, then that's the source of the failure. Replace with the shim gasket. Do not use the regular type or you will lose compression on an already low compression motor. Depending on the miles, you might just opt for a top end job, but let's take this one step at a time.

 

Thanks for the info I will inspect all cylinders and let you know. I hope it's just a head gasket. If it is I will do a top end on it. Thanks

 

I just ran the truck for 8 mins. Filled the radiator and waited for bubbles. I noticed there was no movement in the water. The water was not flowing. At the 8 min make the engine was warm and the radiator gave me one big burp of water and I turned it off....

 

The thermostat opened up. It is closed until the engne gets to temperature, then it opens allowing coolant to flow. Do it again, but let it run after the first burp. If it keeps spewing, you have exhaust gases in the water jacket. A system with no issues will just have 'flow' in the coolant (not standing still) but will not be trying to escape the radiator.

 

I'm hoping that there was just a lot of air that hasn't bled out yet.

 

Yesterday as the truck was warming up the water level was rising slowly (exspanding) and then it burp up about half a gallon of water. I hoe it's the thermostat opening and creating pressure in the system. What about a bad cap? Here is the run down. I bought the truck last year it was sitting for about 10 years I got it running and since I have had it the truck hasn't run for more than 15 min at a time. No street legal yet. If it's a cracked head or cracked cylinders I am going to have to sell it as is I can't afford a new motor...

 

When the thermostat opens, the only change you would see in a properly operating system that had no air pockets trapped in the jacket, is the coolant will start 'running' like a stream, where it used to be still while the thermostat was closed.
The cap only relieves pressure once it gets to 15psi. Leaving it off and still having coolant burping out removes it from the equation. Have you opened the heater valve (run the heat) while the engine is running? Could be some air trapped there.
And the cracked anything issue is pretty rare. You will have to investigate the head gasket first. Did you remove the intake or loosen any of the head bolts while you have been working on it? It was parked for 10 years, why was it parked?

 

It was an old farm truck, they no longer needed it. I have not touch the top end of the truck other than a carb kit and cable cover gaskets. I will turn on the heat when I get home to see if that makes a difference.

 

 

Another note, the back pressure didn't start until I replaced the valve cover gaskets and alternator. Last week.

 

You say it didn't start till you changed gaskets and alternator, is that when you changed thermostats and flushed. If it is you probably have air trapped in the block. Some new thermostats hold tight enough the air can't bleed out.

 

I put the valve cover gaskets in and alternator in at the same time last weekend and then notices water coming out of the over flow tube, so I put a thermostat in thinking that is was bad and in the closed position.

 

No offense meant, but are you positive that the thermostat is not backwards?