Question1989 Jeep Cherokee is overheating quickly - after about 5-10 minutes - steam starts spurting out of the reservoir cap. Started a few weeks ago, sometimes getting hotter, and taking a while to get there or not at all. As time went on, it took shorter and shorter time to steam up. Where do I start to troubleshoot - is it the thermostat, the water pump, a diversion valve, or something else? How do I tell or test each component?
Thanks, Henry.
AnswerHi Henry,
First of all, you should figure out what pressure the system is reaching. With a coolant pressure tester (and adapter), connect it to the reservoir (check it for cracks -- they are under system pressure), and let the car go through its warm-up cycle. You should see a minimum of 15psi out of a 50/50 coolant-water mix, and anything less might cause the mix to boil off -- if the mix is mostly water, it boils a lot quicker. Regardless, get an idea of system pressure -- the radiator cap, which I believe is hidden from view under the core support, is notorious for not maintaining pressure.
Did it start after servicing the cooling system? These Jeeps had horrendous trapped-air problems, and most times you could avoid that by filling the system from the upper radiator hose, at the thermostat outlet. Filling it from the reservoir usually gave you a hard time releasing air bubbles.
Checking the thermostat is most effective by feeling the upper hose. It should be consistently HOT the whole length, and often times the t-stat will stick closed, and the hose will be luke-warm towards the center, while the housing remains HOT. You can also go old-school and yank it out to test it with a mini-torch or lighter, just be sure the wax pellet is not directly touched by flame. It should open and close relatively easily, since its a simple device, and I doubt this is the problem since they have a tendency to stick open (car takes forever to warm up).
The radiators in these things are a pretty poor design as well, and can be fully or partially plugged, internally. They'd drop like flies all summer long in the shop, and next to water pumps, were replaced with regularity. You can tell they're bad if you compare the temperature between the left and right tanks. Similarly, check between the radiator and condenser -- if it's filled with mud and leaves, no airflow can cross the core.
Is the cooling fan working? This system relies heavily on the fan, so make sure it works -- the common failure with these is the coolant fan relay. It should engage at around 222-228 degrees, or any time the A/C is on. Again, make sure it's drawing air across the radiator core.
The water pumps fail often, but normally for leaking. Has the system been leak tested? Is it leaking coolant? A low coolant condition can be the culprit here as well. I've seen one or two water pumps with sheared-off impellers (no flow), and even a few with opposite rotating impellers (wrong part replaced), but they are a rarity. In these two cases, only removal would verify the problem. Normally, when the pump goes bad, you can't help but notice it.
Lastly, does the reservoir bubble? If so, you might have a blown headgasket. The only accurate way to check is with a headgasket leak tester, which checks for the presence of hydrocarbons in the coolant system. The tool isn't generally cheap or easy to find, so a trip to the shop may be in order.
Good luck!
