Mudshack

So my 99 XJ is overheating. It will warm up and stabilize at about 215-220 (Which I know is HOT). And within 1-2min it jumps to 250. Then 30 sec later it pegs out at 260.

I replaced the thermostat. I even replace the climate control water outlet and a new gasket. I do have the fan shroud on, both fans are running. It doesn't seem to be drawing from the reservoir as that coolant level stays the same when cold and overheating. What else could it be?

peligro113

Have you verified that the temps are actually that high? Could be a bad sensor or bad gauge.

Mudshack

I have not. But once I open the hood you can hear the water/coolant in the radiator hissing.

gat

What was the condition of the coolant when you changed the thermostat? How did the block look?

You can drain the system from the lower radiator hose into a large bucket or two. Pour water thru the radiator's throat. How long does it take to run thru the radiator? What is the color at the end of the lower radiator hose?

Do you also have your heater core open with the heat on high and the fan on high to ensure that no air remains in those parts of the system?

You could have a broken impeller or other part of the water pump. If you are leaking fluid or gas (hiss), then you shouldn't expect for the volume in the reservoir to increase when hot nor decrease when cold because you don't have vacuum to create those flows.

dave1123

Broken waterpump impeller is my guess.

Mudshack

Color is good. I didn’t turn heater on so I will try that next. My next thought was the 20 year old water pump. But if it was the pump not working, you would think the hose lines and radiator wouldn’t get hot. They are.

gat

Is the lower radiator hose getting hot very quickly? I'd assume that the upper would get hot once thermostat opens. Eventually a whole system without movement will still get hot.
I don't imagine that it is anything other than a clogged radiator or a broken pump.

You might be able to pressure test the system as is to ensure that your hiss isn't a leak, though. You can try to listen for bad noise coming from the bearing of the water pump, but if only the plastic of the impeller is junked, then you won't hear anything.

Dave51

It might be working a little but only enough to move 260 degree fluid into the radiator. You have to pull the pump and check the integrity of the radiator. You're gonna end up cracking that head even if it is a 630.

Mudshack

How would you pressure test the system? I have to admit I was a little surprise there wasn’t any coolant coming out of the block or the hoses when I put the new thermostat in.

Dave51

Words escape me...

Dave51

Is the thermostat upside down?

Mudshack

Hence my thoughts to water pump.

gat

I don't think that they fit backwards.

gat

You can "rent" a pressure tester for free from most parts stores. You put an adaptor/cap on the radiator and pump the pump to pressurize the system while the engine is cool and off. If something is gushing, then you have a problem.

Coolant should spill from the block where the thermostat goes when the system is full. Your system must not be full. Is your radiator full?

Mudshack

Will have to take radiator cap off when I get back in front of it. Would make total sense if my daughter ran it low/dry it wouldn’t be able to vacuum the coolant from the reservoir. I know after it is overheating and I shut it down to cool there is bubbling in the reservoir but not much fluid level change.