I have an old thread or two about my cooling system but I felt it was easier to make sense with a new thread.

98 4.0 xj 4x4. Dual electric fans working well. Low temp t-stat

Ok so my jeep never over heats on flat bottom or downhill. A few times when I have stopped on a hill it jumps from 210 to RED in 1 second (not literally but damn near instantly) If I turn the truck and turn the key to AUX and let the fans run, it cools back down to 210 in 30 seconds (literally). OR if I just do a 180 and face downhill (just an about face and no driving) it cools off in 30 seconds.

It's not the typical "your truck works harder going up hill and if you stop, the airflow is reduced..." because if I run my truck hot and stop downhill it doesn't over heat.

I flushed the rad, replaced the t-stat and housing.

No signs of head gasket like loss of coolant, milky coolant or oil, white smoke etc...

 

I'd say do the water pump and throw in some new hoses while you're at it

 

I was leaning twards water pump too thinking that maybe it's harder to pump the water up hill? and the top hose IS kinda soft so maybe it's getting pinched when the water pumps uphill?

 

More than likely it's just rusted and doesn't spin like it's supposed to. The hoses are supposed to be soft, you're fine there. If you're overheating while in motion, the main components you should be looking at are thermostat, radiator and water pump. And since you have done maintenance already on the thermostat and radiator, the water pump is next. I say to do the hoses because you need to disconnect them anyways to fully drain the system and it's just good reassurance knowing that you have fresh hoses that won't crack, break and leak like old brittle hoses will.

 

Well I've only gotten hot once in motion and that was doing 70 up the mountains to San Diego (today). The other times were when stopped on a hill.

 

Do the water pump. More than likely that's where your problem lies. You could have also gotten a defective thermostat (heard of that happening many times), where did you get it from by the way?

 

T-stat? Autozone.

 

Hmmm Duralast? Your chances of getting a defective one just went up. I'd recommend doing the water pump and at the same time, getting a new thermostat from the dealer and throwing that in too. That way, if it turns out it isn't your water pump that's causing you to overheat, you won't have to drain and refill coolant again in order to redo the thermostat. Rather, you can knock it all out at the same time. Just what I would do though

 

Quick question. This is the part that's always confused me and made me not sure if it was the pump or not. Why would it cool off so fast when I turn around and stop downhill. Does it have to do with the direction the water is pumped? Is it "pulled" from the radiator or "pushed" in to it? If its pulled, would it benefit it to be up hill with gravity pulling it?

I'm gonna pull it off Sunday.

 

Almost sounds like an electrical connection/ground is giving a false temp gauge reading. Coolant temp rising from 210 into the red in a few seconds doesn't seem possible but a bad electrical connection/loose wire could cause a false gauge reading. If the motor's off, key at AUX, and the fans are running, they are cooling the radiator, not the motor, and the temp sensor is in the head of the motor, not the radiator. Electrical connection can make/break depending on a loose wire changing position depending on nose up or nose down.

Seems like the efans should be wired to only run in the ON key position, and not the AUX position.

The water pump pushes coolant thru the top rad hose and pulls coolant thru the bottom rad hose when the t-stat is open. If the t-stat is not open, coolant is pushed/pulled thru the motor/head via the bypass loop. Coolant is constantly circulated thru the motor and only circulated thru the rad when the stat starts to open.

 

im running two stock electric xj fans and when its above 70 out it is not enuff i have a 1994 xj with brand new radiator,new 195 thermostat,new water pump, new upper and lower and heater hoses it just don't get the job done im goin back to the stock hp sucking fan that works

 

It's not a false read because the overflow boils over

 

I'm goin to start with the pump because that was my next on my list. Then if that doesn't do it it's in tithe radiator.

A radiator is only what like $150? And a pump is $32 so it's cheap enough that I might as well anyway.

Let's say I replace those. That would be a new radiator, pump, t-stat and housing, temp sensor, cap, and hoses. Fans are working fine so if none of that corrects it and the head gasket is in order, what else could it be?

 

air pocket in the system will do that

 

Replace the pump, radiator and new radiator cap all at the same time. Flushing a radiator does not guarantee good flow.