I've been experiencing a seemingly random problem overheating on my 2000 3.3L GC. On most days, the temp gauge indicates mid range and all is well. Occasionally, the gauge ramps up to indicate overheating (and yes, it is overheating).

Coolant level is fine. (except, of course, when if overheats)
Water pump was replaced last winter.
Radiator and thermostat replaced last summer.
Serpentine belt is good and tensioner is working correctly.

Too rule out a faulty thermostat, I replaced it again and the problem is still occuring. Question: Is the thermostat position important? By that, I mean, is the clock-postion orientation of the thermostat critical? (I'm pretty sure I have the thermostat's "right-side-up" correct else I would have temp problems all the time, right?)

Could the lower radiator hose be collapsing and resrtricting flow?

Does this sound like an obstruction problem?

 

so when it overheats, the coolant level is..........

Most of the time they overheat because of a leak and coolant is low or the fan is not working.

 

The coolant level is at the proper level. But when it overheats, it spews coolant through the recovery tank.

There are no leaks.

I'm curious about your suggestion about the fan. Both come on at what appears to be the right time. Could they not be running at an adequate speed. They are the original fans. I always thought the fans' function was to provide airflow across the radiator when the vehicle is not moving but I have been wrong in the past.

 

the fans supply additional airflow whenever needed. How often does it overheat? Are the fans running when it overheats?

 

It was exhibiting higher-than-normal-temp tendencies a time or two per week last spring. This behavior would then just mysteriously disappear. It first overheated last summer 20 miles into a 300 mile trip. As I was without tools at the time. I guessed it was a sticky thermostat and just gave the housing a few light whacks and it was fine for the remainder of the trip.
It overheated this past friday, so I replaced the thermostat as I had a new spare lying around. I filled it with coolant and let it fully warm up and topped off. I filled the recovery tank to "max" level.The following morning I checked the coolant level and it needed a little in the radiator and a little in the recovery tank (it had recovered some when it cooled, i guess) which I topped off. There were no leaks or puddles. My wife drover the van on saturday and called me to say it was overheating. She let it cool down and then drove home 15 or so miles.. It was overheating as she pulled in the driveway.

I'm not sure if the fans are running when it overheats. I'll make this observation....

 

So, after putting in the thermostat, I added coolant and brought it up to temp and waited for the fans to come on (which they did). I then topped off and put the cap back on. I forgot to fill the recovery tank (which I normally do keep to proper level) . A few hours later, I drove three miles to a friend's house. About 5 hours later, I drove 20 miles. As I pulled into the driveway, the temp gauge was starting to creep up above normal. Before shutting off the engine, I got out of the vehicle and observed the fans were indeed running. I shut off the vehicle and let it sit. This morning, I took off the radiator cap and observed the coolant was a bit low. There were no drips/puddles under the vehicle. I then topped off with about a pint and a half of coolant and filled the recovery tank to proper level. Was this apparent loss of coolant an air pocket that needed to work its way out?

So now I'm perplexed as to what's causing the overheating. Would a faulty radiator cap have anything to do with this? Could there be an obstruction in a cooling line? Could the fans performance be degrading?

 

Looks like your heading down headgasket lane. If it's not leaking externally, could be leaking into the cylinder and burning it.

Could be a bad cap but you sould see signs of it leaking.

 

It is looking more and more like an intake or head gasket....I recently went through replacing the HG on my moms 01 GC...what a pain

 

I borrowed a combustion leak detector. It's the one that uses a a dark fluid and a device that resembles a "turkey-baster". The device draws air from the radiator air pocket and bubbles it through the fluid in the device. If the fluid turns yellow, the presence of CO2 has been detected which indicates combustion leak into the coolant. This check came back negative. This is good news....

As the radiator cap was another suggestion, and since this cap is many years old (not the original cap, but more than 6 years old), I just went ahead and replaced it. Hopefully, this will be the culprit. I'll report back in a few days....

 

Not sure if this helps or not... I'm not a mechanic but...."overheating" caught my attention.

I recently read through alot of threads on the EGR valve, according to alot of the info, if it is malfunctioning then it could cause the engine to overheat.

Hope this may help you out.

Good Luck!