2000 GC 4.0L radiator fan problems
 

My 2000 GC 4L Radiator fan runs only intermittently - that is to say that sometimes it doesn't go on at 208-215 degrees as usual and the engine overheats. Shut it off and let it cool down and it goes back to normal. Sometimes. Sometimes it may take a drive cycle, running hot, and another shut down.
In the past 2500 miles, I've replaced the fan, waterpump, altenator and in the last 500, the fan relay(under the pass. headlight) and temp sensor.
Any other suggestions? My girlfriend was stuck on the highway for an hour yeasterday, waiting for it to cool down.
Nice car but becoming a PIA

Just a guess but my 2015 Ram 1500 v-6 was doing the same thing and it turned out to be an air pocket trapped in the cooling system. Had to jack the front of the truck up in the air to get the air pocket to fill with coolant and resolve the problem. It could also be a defective temperature switch, bad thermostat, or a broken wife-loose connection in the circuit, or a relay going out. Just because a part has been recently replaced does not mean it isn't bad. I've installed quite a few parts over the years that were total junk straight out of the box. If your o2 sensors are going bad they could be jacking the fuel mix and timing around to make the engine run hotter to burn more emissions off. In this case the radiator cap may be weak and loosing coolant at the higher temperatures which can also create an air pocket in the system

Thanks for the reply.
I don't think it's coolant- just had to drain and refill the system from the overheat 10 days ago ,when I replaced the fan relay under the headlight.
Things were fine and then the fan didn't turn on two days ago, temp gauge at 250 and climbing. Shut it off, let it cool down an hour, start it and no troubles, fan working as it should- on at 215-218 deg., off at about 208. About 4 driving cycles and then it overheats on my girlfriend while she's getting on the highway.
Same deal, once cooled down, no problems, fan turns on and off.
I did read the instructions for the relay and it said the relay itself can overheat and not work properly if not tight to the fenderwall. I know the screws are tight, but I'm thinking I should take it apart, clean the surface well and re-install. Also thinking about some thermal transfer paste under it, if I can find some- has anyone tried this?
I did put a new thermal sensor also and checked the wiring, tested good, but maybe there's something else in the circuit?

Well good luck with it. The only other things I can think of is that a lot of vehicles have two fan relays, not one. The second one is for the air conditioning and kicks in sooner and at a higher fan speed when the AC is on. The other is the fan itself may be going bad. When you find the problem please post the cure. I got an 04 GC I'm building and your probably uses the same cooling system setup as mine does.

Okay Mark, here's how your system works. The fan relay is under the headlight for a reason. It does get hot! The idea of a thermal bedding paste is a good idea and can be found at Radio Shack for use in bedding CPU chips in PCs. The relay is controlled by the PCM and it's "pulsed" by it when it needs to run at low speed, gives full power for high speed. There is a bypass pressure switch on the high side of the A/C to turn the fan on high when the compressor turns on.

I have this identical system on my WJ and the way it functions is the fan doesn't run at all until the engine temp gets above 210*, then runs on high until the temp drops down to 195*, then runs on low until you shut the jeep off or the temp goes up again, cycle repeats. The fan has only 2 wires where the 2002 and newer have 3, controlled by 2 separate relays. Because the fan relay is cycled at a high rate (milliseconds), it is prone to failure if the quality is compromised, as in an aftermarket part. Try replacing it with a genuine Mopar part and you should have no problems.

Bad bearings in the fan can cause a high amp draw and burn out the relay, but you've already replaced that. I suspect the relay is a piece of crap. The fan is fused at 40 amps, so you know what kind of draw we're talking about. When my relay died, I was on the road and luckily I had a piece of wire in my toolbox so I ran a hotwire direct to the battery to get home. Until I could get a relay, I ran a switch and relay to it and just turned it on on startup and let it run all the time. Actually the relay trigger was wired to ignition hot, so it shut off with the key, but I could turn it off on the highway if I chose. Funny, when I went to replace the relay, SOMEONE HAD ALREADY CUT THE HOLE!

Thanks for the detailed explantion, always good to know how the system works.
I've read the relays are "notorious" for burning out.
The hole was cut on mine also, so it's not the first time the relay has been replaced. I also suspect the car had been crunched on the pass. side, theres paint overspray where there shouldn't be, etc., so the wiring may be suspect here. Seems I've replaced all that can be bad but that.
Yes, I did use a NAPA relay;
it was all they carried and I've had pretty good luck with most of their parts. I know the guys there well and they steer me clear of junk and always warrantee anything that's a problem. The relay was a little pricey, $58, can't wait to find out what a Mopar will cost:001_unsure:
I'm going to set up a bypass switch and have it ready to plug in should the problem continue.

Why not wire it in and only use it if it overheats? That way just a flip of the switch and you can continue driving.

What kind of relay did you use in your switched circuit? And do you think I'll need to wire it to cut the factory circuit out or just a simple jumper?

I just used a headlight relay because I had one laying around. I just used a 3M splice connector and fed it into the power wire for the fan. I put a 40-amp fuse in the line and connected it to the big red terminals in the front of the PDC. The relay trigger was connected to the cigarette lighter supply so it would turn off with the key. I used slide-on terminals on the relay and wrapped the whole thing in electrical tape, sticking it behind the headlight panel. It was only going to be temporary anyhow and the switch I put in the dash would be used for aftermarket fog lights so that was permanent. The switch was just so I could turn it off on the highway when it wasn't needed.

You know, for less than $30, you can buy a fan controller complete unit with temp probe from rockauto.com

Well, it just never ends-
Car overheated again, fan won't turn on ,replaced the relay, still won't work, going to try jumping the fan to see if it has failed......

EDIT
Fan seems to have died, new one ordered for tomorrow am