Cherokee help!

You might be on to something.

Did you pull the thermostat when you flushed the coolant? If not, there is likely a lot of junk still in the system... might need to do it again (I like the simple method, disconnect the upper and lower hoses at the radiator... garden hose through the radiator at the top and into the upper hose to reverse flush the the heater core and block).

The melted plastic is interesting, although if it really got that hot, you would have really cracked the head. Keep checking the oil color and check under the valve cover for signs of chocolate. Oil pressure might not be a good indicator, mine didn't change noticeably.

Have you done a compression check yet?

Might want to take it somewhere that can do a power flush of the coolant system to get ALL the gunk out. Also to check what kind of pressure it can hold (thinking it will make anything weak that needs changing fail)

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Quote from Slimer

Might want to take it somewhere that can do a power flush of the coolant system to get ALL the gunk out.

Not to cloud the input from everyone, but if you do this I would recommend disconnecting the lines at the firewall where they go in and out of the heater core and loop them together. If you have that much crud in there, you don't want it getting all gummed up in the core. They aren't fun to change...

Dont forget to flush the heater core, they can collect a lot of crap too .

That could be a faulty or cruddy oil sending unit

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Quote from BetterInTheSand

looks like she's biting the dust more and more each day. worked on it today. changed thermostat and pump and sending unit. ran about 210. heat wouldn't get hot. seems like heater core is done and now the oil pressure is falling. goes up to 40then drops to 20 and right back up to 40. just keeps going back and forth.

sometimes its hard to get all the air out of a 4.0. Be sure to burp the heater core and lines.

Quote from deadfeat

sometimes its hard to get all the air out of a 4.0. Be sure to burp the heater core and lines.

Mind if I ask a simple explanation on how this is done?

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Open the upper heater core hose at the highest point. Add antifreeze. Can also add some via the hose.

I have flushing T's mounted in my heater core lines. I just crack open the caps slightly while running and any air should bleed out

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I would also check your temperature sending unit. On my 97 ZJ 4.0 it is located on the thermostat housing. As for burping the cooling system I used a no spill funnel like the one below. Once the engine gets to temp and the thermostat opens the air works it way out through the funnel which is maintained at about 1/2 full with antifreeze to prevent reintroduction of air to the system. You can borrow mine if you want to try it.