Thermostat issues

I've checked around on this one and done a few searches of the site, but haven't seen anyone answer for a jeep older than a '91. I have an '89 XJ and had some issues overheating on a long trip to SC last week. It was probably close to 95 outside, driving about 75/80 mph, and my temp. gauge ran about 230/235 :wow: Thermostat is pretty much shot, as well as the water pump. I was looking at replacing the water pump with a high performance pump and the thermostat with a 180 degree one.
I have heard that this may affect my mpg (which is already bad) but that the effects are on the computer in the jeep. Is this applicable for me, considering my jeep's age? Also, I have been told that the computer goes into closed loop at 1000 ohms. At 180 the sensor is at 1100-1200 ohms reading.

I'd appreciate any help y'all could give me on this one and TIA for the answers! :wave1:

Assuming all the parts are pretty much original except maybe the thermostat and maybe the water pump I would do an R&R of the cooling system.
Drain it out and refill with water, add the 7 hour flush and go drive it.
Drain the system, pull the rad, water pump, hoses, tstat,fan clutch, pressure bottle and pressure cap and replace them all, personally I would use factory OEM for everything but the rad, for that I'd use a Modine. The reason for the 7 hour flush is to clean the engine itself out and the heater core before putting in the new stuff. Then refill with 50/50 mix of coolant and distilled water. You might also want to consider a new mixing valve too while you are at it. After reading and hearing here about that you will be lucky if it does not come apart when replacing the heater hoses. When removing the heater hoses from the heater core near the firewall cut the old ones length wise, don't try to pull them off and if you use dealer supplied OEM hoses they come with new clamps on them too, plus the lower hose already has the spring inside it and both the rad hoses have those protective covers over them.

collegeoffroad said:

I've checked around on this one and done a few searches of the site, but haven't seen anyone answer for a jeep older than a '91. I have an '89 XJ and had some issues overheating on a long trip to SC last week. It was probably close to 95 outside, driving about 75/80 mph, and my temp. gauge ran about 230/235 :wow: Thermostat is pretty much shot, as well as the water pump. I was looking at replacing the water pump with a high performance pump and the thermostat with a 180 degree one.
I have heard that this may affect my mpg (which is already bad) but that the effects are on the computer in the jeep. Is this applicable for me, considering my jeep's age? Also, I have been told that the computer goes into closed loop at 1000 ohms. At 180 the sensor is at 1100-1200 ohms reading.

I'd appreciate any help y'all could give me on this one and TIA for the answers! :wave1:

Click to expand...

By 180*, the computer should already be in closed loop. I believe the transition from open to closed loop is when the resistance across the coolant temp. sensor is 2000ohm (approx. 165*F), so you should have no rich running problems with a 180* t'stat. I've been running one for the last 7 years with no adverse effect on gas mileage nor emissions.
I doubt that your water pump is a problem (unless it's leaking from the weep hole) but if your lower radiator hose is old, it could be collapsing under the suction of the pump.
Since your engine's overheating on the highway, your radiator could be clogged up with debris. A thorough flush might help but if it's still the factory original, you'd better think about replacing it with a 2-row Modine unit and convert to the '91+ open cooling system..