QuestionHi Chris. My 98 Cherokee was overheating and losing coolant. I discovered a leak at the thermostat, and I also felt the top rad hose was very hard. I assumed the thermostat was seized - thus blowing the housing gasket. I changed out the thermostat and gasket. The temp gauge now goes high, the check gauges light comes on, then the temp gauge goes back to normal. A few minutes later it comes on again then goes back to normal. I'm thinking that the sensor may be faulty..
We're also not getting much heat from the heater fan, despite my backwashing the heater hoses, and the radiator.
Any suggestions?
Thanks in advance.
AnswerDana,
Sorry for the delay. First off, these 4.0l engines are notorious for air pockets. Once it gets hot, shut the car down and let the natural properties of contraction suck in extra coolant from the overflow, after the thermal expansion gathers the air at the highest point (which, theoretically, should be the fill point -- radiator cap). On older Jeeps, we'd add coolant through the upper radiator hose. On a 98, this really isn't necessary, but running the engine while the cap off is a good way to ensure the radiator is flowing -- by looking into the core and verifying flow. CAUTION: this is extremely dangerous, and not really suggested under normal circumstances. Still, a great place for air to hide is right inside the T-stat housing, and guess what's right there?? Yep, the Coolant Sensor. Oh, and if it's not enough, the radiators have been known to partially clog as well.
Did you check the radiator cap? If you pressure tested the system, the second thing to do is plug on the adapter and pressure test the cap. A cap unable to keep system pressure will allow the mixture to boil at a lower temperature -- which ends up being somewhere around the 230 degrees the system can reach on a normal day. I'm assuming that you have a solid 50/50 mix, and that the replacement thermostat was the 195 degree one. Anything off of this stuff can cause big problems.
The fact that you're not getting much heat, yet the system is overheating leads me to believe that air is in the system. Purging air can be a pain sometimes. Still, you should ensure the gauge is operating properly -- and the only accurate way is to monitor the temperature with a scantool. The signal to the Instrument Cluster is fed by the PCM (Powertrain Control Module) through the CCD bus, with data gathered by the sensor. Comparing the values from the gauge and the scantool can verify its accuracy. Bear in mind that those gauges are somewhat inaccurate.
If you're running out of ideas, consider getting a Combustion Leak test done -- to check for the presence of hydrocarbons in the coolant, which is a sure sign of a headgasket problem. Sure, most of these problems generally have air bubbles in the overflow, but not all. We've had Jeeps with slight overheating, even running hot, and had a headgasket problem -- not very common, but more common if the engine overheated before hand.
Lastly, verify that the fan is coming on at the proper time -- usually around 212-228, depending on the model. If it's not, operate the car with the A/C on -- this should cause the fan to engage, particularly when the high side reaches a certain pressure (it always does). It should cycle when the ambient temperature and humidity is reasonable, and stay on longer if both are up high.
Good luck!
