So after buying the 1991 YJ, we realized there was a bad head gasket, purchased the kit and replaced everything. Found out the cylinder head was seriously warped and had a maxnaflux test done on it to check for cracks (no cracks, yeah!) and then had it Resurfaced. Needed a new water pump and also the front passenger side bracket under the engine mount was broken and had to be special ordered and replaced. All new fluids, etc. Jeep is running, sounds better, runs better, still slowing down a bit on inclines... but the big problem is the thermostat... it seems to be working, but goes up and down and almost over to the red...
Does anyone have any suggestions? Do you think the thermostat is bad? How can we check to see if its functioning properly?
Put the t-stat in a pot of water, and put it on the stove. Let it go to a boil. It should open just before it boils if it's a 195*F t-stat.
Also make sure there is a tiny pin hole that air can bleed through with. Mine didn't have one, and it was causing an air bubble to be trapped behind the t-stat, keeping it from opening. Position the pinhole ontop so the coolant will touch the t-stat.
Its easier to go buy a 12$ t stat then it is co check it.
Last time I changed my water pump, i had issues purging the air out of the system, So instead of being lazy I used my "Air-Evac Radiator Purge & Test System" cooling system filler tool and got it right.
Easiest way I've found to fill the 4.0 is to warm it up enough to open the stat, jack up the front axle till the front end is on a good incline, and hold the upper radiator hose down so it's lower than the filler neck. That "hump" in the upper hose traps an air pocket. Pull it down enough to be on a downward slope from the radiator tank, but not so much you're kinking it off, and the coolant will flow right down into the block and the air will burp out.
Fill the radiator till the upper tank is full right to the brim of the filler, pull the hose down and watch the coolant level drop, you can feel the air burping up through the hose, and keep repeating until the upper tank stays full.
you might want to check the Radiator if it is plugged up from having a head gasket fail. easy to do let the engine get to operating temp then feel the Rad (On the Front not by the fan) up and down and right and left. if you have a IR temp gun you can check it that way too.
1 Thermostat test at about 200 deg the thermostat will open ,the top hose will become hot to top radiator inlet.
2 check if radiator is plugged,Remove radiator fill with water from lower hose inlet garden hose and rag wraped around hose.Water will spew out of radiator about 1 ft and empty radiator in about 3 seconds if clean.
3 If clutch fan is bad will over heat in slow city driving.Also check if radiator core is plugged with bugs.Car wash time.
4 Have block test done on the cooling system.Cheap insurance