Radiator Overflow

[AW-100]

When I start the engine it starts building pressure in the coolant system. But it does not get high enough to push coolant into the overflow canister, unless I open the radiator lid before the pressure has gone down. I have a Brand new radiator lid, so I believe the pressure relief valve is correct. If I run the engine without the radiator lid, coolant is pushed out of the radiator through the open neck.

 

I've changed the cylinder head, and replaced the gasket. Thermostat is new. Engine temp and heater system works fine. No access smoke. Oil is fine, no oil residue in coolant. So I hope that it's not the compression entering the cooling system.

 

Now, this is a 1986 2,5gasoline. But in Norway it has been registered as a 1987. I see that there are different water pumps ->86 and 87-> Could it be that the previous owner fitted a -87 waterpump on the -86 engine, causing too much flow? ( Wrong size belt pulley for example? ) Maby in conjunction with a partly clogged radiator? ( Have not cleaned the system yet, waiting for a new radiator. )

[onlyinajeep726]

Forgive me if I'm wrong, but I don't think coolant is necessarily pushed into the overflow bottle after the system gets to operating temperature.  I believe the overflow bottle is there for when the system gets too hot (boil over), it will go there instead of all over the place.  It doubles also as a reserve tank in the event that the overall system has lost coolant (either by leak or not enough poured in).  That's at least my understanding of it based on my observations on my Cherokee.

[mvusse]

Coolant expands when it gets hot. This build up pressure, and once the correct maximum pressure has been reached (16 pounds per square inch), the excess coolant and pressure is bled off into the overflow tank. The coolant being under pressure is also what keep it from boiling until ~265 degrees fahrenheit.

 

But even when cold, there is supposed to be some coolant in the overflow tank.

[danbyrambler]

Sounds like a cracked head if you're building pressure immediately on cold start up - Like in 30-60 seconds - 

:hmm:

[AW-100]

Yeah, but it does not build enough pressure to get pushed out of the system... The water level is constant in the overflow tank, engine on or of, engine hot or cool. That's why I think maybe a clogged radiator causes the water pump to push coolant out through the neck in stead of down through the radiator with the lid off. Hence also builds pressure. Anyone know the difference btw <-86 and 87-> water pump?

[HOrnbrod]

The pumps definitely changed in 1987. From the parts manual:

1981-1986 w. 2.5, J8134321, Export

1987-1990 w. 2.5, 83502957, Export (w/o Serpentine Belt Drive)
1987-1990 w. 2.5, 83502938, Export (with Serpentine Belt Drive)

I suspect the impeller vanes are different, the pump is pushing rather than pulling. Or it's rotational. I don't remember when these engines switched the pump rotation.

 

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[danbyrambler]

If T stat is removed should be no immediate cold start pressure build up regardless of water pump pulling or pushing - - -  Lower radiator hose collapsing?

:hmm:

[AW-100]

Good point... No collapsing, all new hoses. But if they mounted wrong waterpump ( pushing in stead of pulling ) the waterpump will send coolant in the wrong direction... And with thermostat closed, coolant is pushed out if lid is off... Hmmm...

[AW-100]

Anyone know the direction of coolant on the pre-87? The 87-> goes water pump-thermostat-upper hose-radiator-lower hose-engine block.

[HOrnbrod]

Good point... No collapsing, all new hoses. But if they mounted wrong waterpump ( pushing in stead of pulling ) the waterpump will send coolant in the wrong direction... And with thermostat closed, coolant is pushed out if lid is off... Hmmm...

 

This thread will answer your questions. Check your impeller fins.

 

http://www.jeepforum.com/forum/f12/waterpump-tutorial-893428/