1976 620 Cooling System

[LowCastle]

Hello everyone.  I am new to these forums but I am hoping to get some insight into a unique problem.

The truck has been overheating since I got it last month.  First, I replaced the thermostat, radiator, and radiator cap.  Next, I replaced the water pump even though it seemed to be working fine.

It is strange that the truck still overheats (yes, the thermostat is opening), but I am even more puzzled by something else.  The cooling system seems to build tremendous pressure long before the engine becomes warm.  I can fill the radiator in the morning, start the engine, and watch as water spurts from the overflow tube.  If I start the engine with the radiator cap removed, water sprays from the opening six feet in the air.  I have never seen this before . . . so much water flowing into the radiator before the thermostat opens.  And I have never seen a cold engine build up so much pressure in the cooling system that it blows water out the overflow tube.  Has anyone seen this before?

A friend of mine suggested that I need to add an overflow reservoir -- the overflow tube currently just runs down the side of the radiator.  But it is my understanding that Datsun did not add this to the 620 until 1978, so my pickup should not need it.  Can anyone verify that?

My only guess at this point is restricted water flow somewhere in the engine block.  Is there anything else that could be causing this?  I appreciate any help.  Thanks!

[LowCastle]

Some Internet seraching has revealed that I MAY have a blown head gasket, which is forcing exhaust gases into the cooling system.

I will do some testing and report what I find out.

[djmoreron]

thats what i figued, air in the cooling system

good luck

[LowCastle]

A compression test showed that all four cylinders have approx 150psi.  Should be closer to 170.  But the test proved there is a breach somewhere.

While testing #1 cylinder, water sprayed from the #2 cylinder.  I either have a bad head gasket, a damaged head, or both.

[lancekilgore]

well take the head off and have it pressure checked at a machine shop heck since it is there have the head redone if it is good you get all the stuff when you buy the headgasket set.  then put it back together.

Lance