93 Grand Cherokee high temperature
 

Hi,
My 93 grand cherokee temperature guage has been displaying very high temperatures.
It has only been in the last few days, and when I noticed it getting hot, I checked fluid which was slightly below the ADD line on the reservoir. I filled it back to FULL and went for a drive. Same issue persists and was hoping for some suggestions to try to diagnose the issue:

-The temperature climbs to just below the Red dash, never above. Just below 125 C.
-After a minute of climbing that high, it drops back down to around ~100 C
-The temperature will climb like this regardless if driving (with good cold airflow) or sitting at idle. Seems to do this every 5 minutes?

Any simple tests to run to try narrow down the problem?
Thanks

Check the level in the radiator when it is cool, the reservoir is kind of useless for an accurate fluid level.

Sounds like low fluid or air in the system. Might try to heat cycle the motor while adding fluid. Squeezing the upper hose helps to get the air out too. Also, might be the thermostat sticking, IMO if you live in the south just yank it out.

Thanks for the quick replies.
Was just outside bleeding the radiator. With the engine cool, I took off the cap, started engine, and blasted the heat. After a few minutes, I checked the rad and some of the coolant was beginning to leak.
Replaced the cap, and watched as the reservoir rose past the FULL line, so I need to let a little more leak out.

I did notice that the temp still ran high.
Checked the oil which was pretty low, so replenished it.

Going to take it out for a drive and see if the issue persists.

How do I check the thermostat? (If its stuck?) Unfortunately, I don't live in the south so I can't just pull it out.

Thanks in advance for any advice.

It is starting to sound like a head gasket issue.

Ran engine with all fluid levels ok.

Ran it for a while and watched the temp rise to just below red and then watched it drop slightly.

After shutting it down, I checked around the engine bay to look for signs of leaking. I noticed some drips that appear to be coming from the bottom of the thermostat housing. It must have been doing this for quite some time cause there was a bit of a build up collected ontop of the water pump housing.

With the engine running I checked the upper rad hose and it didn't feel as 'hot' as the engine felt. It was warm, but I think that was just from being so close to hot metal of the engine.

Is it possible that the thermostat is acting up ('almost' stuck closed and opening at too high of a temp)?
I have NO idea when the last time it was changed.

Is there a correct method to test for faulty thermostat?

Also, if it were a bad head gasket or bad water pump, wouldn't the car just continue to heat up into the red zone? Having a hard time understanding how it keeps cooling itself down but only at the last minute?

Thanks

Just replace it, you have to fix the leak anyway.

Replace the stat, it's cheap. A head gasket will give you those sypmtoms, also you should be able to feel the water flowing through the upper hose. If you can't feel the water flow for whatever reason, just yank if off and turn the car on. Water should come out quickly. No water, change pump. Water come out, change head gasket.

Thanks,
Changed the thermostat. Removing the bottom bolt on the thermostat housing is a massive PITA.
I flushed the cooling system at the same time.
Can't believe the difference! The engine sounds a lot smoother, temperature is steady, and seems a bit more responsive.
The cooling system must have been pretty plugged up because there is a night and day difference now.
Not sure if it has anything do with a previous problem, but the car is no longer stalling shifting from reverse to drive??

Thanks to all for the help. It's motivated me to start tackling some other annoying issues with it.