Overheating advice (is it common for block to be vapor locked?)

f4u5t

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Jun 4, 2020
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Bristol, Va
so far, ive put a new thermostat, adn water pump on my 95 2.5 YJ.
it still overheated, instantly...

I just took off the heater core hoses, which to my dismay were bone dry, and I could blow through the core. Filled the radiator again, this time got very hot water out of the heater core tube...which leads me to believe the block was vapor locked...Is that common, I thought it would fill the block from the lower radiator tube
 

machoheadgames

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May 19, 2020
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Dallas, TX
so far, ive put a new thermostat, adn water pump on my 95 2.5 YJ.
it still overheated, instantly...

I just took off the heater core hoses, which to my dismay were bone dry, and I could blow through the core. Filled the radiator again, this time got very hot water out of the heater core tube...which leads me to believe the block was vapor locked...Is that common, I thought it would fill the block from the lower radiator tube
You gotta get the system as full as you can, squeeze the hoses some to burp a little bit of air out, and then you need to do warm up cool down cycles.

Since the system is sealed, it naturally works the air out by itself. When warming up, the heat causes the coolant to pressurize.This pushes some air into the reservoir, which obviously just bubbles out in the reservoir. Upon cool down, the volume of air that was originally pushed out during warm up is replaced with coolant. The system is sealed so it has to fill that space with something, which will be coolant as long as the overflow hose is below the surface of coolant in the tank.

It is very common to overheat right after servicing the system because you can’t fill it up all by yourself, you need to do a warm up/cool down once or twice to work the air out. Then it should be fine.
 
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Flyer58

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Apr 12, 2020
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Magnolia, Texas
I drove my Jeep up on ramps to get the front end higher, filled it and let it set for an hour or so. I believe the proper thermostat has a very small hole in the plate with a loose rivet that allows air to pass through it. Without that hole air can be trapped until the t-stat opens up. Be sure the heater selector is set for full heat when filling it.

This is a somewhat common problem if rushed or not done right.
 

machoheadgames

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May 19, 2020
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Be sure the heater selector is set for full heat when filling it.
Doesn’t affect it at all. There is no vacuum heater valve that stops or allows coolant flow to the heater core depending on the heater being on or off. It certainly won’t hurt anything to run the heater while burping the system, but there is no benefit whatsoever to doing so. Heater core has hot coolant circulating through it 100% of the time.
 

Flyer58

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Apr 12, 2020
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Location
Magnolia, Texas
Doesn’t affect it at all. There is no vacuum heater valve that stops or allows coolant flow to the heater core depending on the heater being on or off. It certainly won’t hurt anything to run the heater while burping the system, but there is no benefit whatsoever to doing so. Heater core has hot coolant circulating through it 100% of the time.

Ah yes I stand corrected. Looking at the FSM it says to fill the radiator and overflow tank with a 50/50 mix and run/cool the engine. The next paragraph refers to the XJ models which have the valve but the YJ does not so it doesn't apply to us.
 
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