The "mall crawaler" is overheating (I SEARCHED)

First off, I am exhausted. I searched and read every single thread that had to do with overheating for the past two days but couldn't find anything specific to my problem.

It overheats when the rpm's get above 2500 for any long period of time. We thought it would be the fan clutch so we installed that last night. The lower hose is not collapsing. The radiator is a 3 row running all anti-freeze. The tranny is not slipping, tested it last night. So to me that leaves - replace water pump, replace thermostat, flush system.

Before I just go replacing stuff to no end, what is usually the problem when it overheats at higher RPM? If i drive it nice and easy it does fine, but when the engine downshifts and the rpm's stay at or above 2500 it slowly starts to creep up to eventually around 240-250. At that point i just turn it on hot vent and it dropps back to normal within a few minutes.

I thought that maybe the main culprit would be the big lights on the front bumper blocking airflow, but it overheats just the same offroad if it's above 2500 rpm's. Then I thought the radiator needs to be flushed, but its running all anti freeze and looks to be pretty new. So that leaves the thermostat right? Well my buddy isn't convinced it's the thermostat... his words were "well, the thermostat is opening i don't see how it could be that"

Look I know this topic has been dicussed to the point of exaustion (i know, i've read every single thread on it) but im just looking for a definative answer or two. Someone to say... "yeah im 100% sure your problem is ____ " thatway I don't have to just keep running through replacing everything.

Thanks

You stated that you're running all antifreeze? You have to mix antifreeze with distilled water, either 50/50, or somewhere abouts there. Unless you bought that stuff that is already mixed when you buy it?

No meaning when I purchased the truck I looked in the radiator and it had antifreeze in it. Meaning the cooling system is not all water.

yea, that's the first thing to change...ALL antifreeze is the worst thing you could do.

could be the thermostat if you have a faulty one it wouldn't open til higher temps or it would stay shut. staying shut would cause the heat spike too.

I would change the T-stat. I had a ranger that did the same exact thing. At relly slow RPM it would be fine. At hwy speed it would spike and pretty quick. Napa has the t-stat and gasket relatively cheap. Also check the t-stat housing.

Yeah 70 at 2000 is about right. I have 4.56's and 35's thats prob right. There is a hill right before my house that I have to climb. The truck goes from about 65 to about 50 as im climbing, and in the process it downshifts and then runs at about 2500 rpm's for about 30 seconds. At the top of the hill the temps are climbing, within 10 secs of clearing the hill they are maxed in the 240-260 range (close to the 3/4ths hash) and at that point i put on the vent and the temps drop.

The continuous rpm's made us think the fan clutch was slipping, or maybe the water pump is going bad....something that is depicted based on engine rpm. But the wierd thing is the truck has a brand new engine so you would think the thermostat and water pump would be new. We thought it was the radiator, but upon inspection last night found out that it has a 3 row. Thats good and all, but at this point im still just playing the guess and repair game.

Does the stock fan flatten out or disengage at higher RPMs to reduce drag, based on the idea that at that high of revs you should be moving fast enough to cool the engine without the fan blowing acrossed the radiator? Just curious, almost sounds like that is happening.

Just a thought, but how about your auxillary fan? It should operate any time the a/c is on, and I also believe it has a sensor that will kick it on anytime you need extra cooling...

Aux fan kicks on just fine, thanks for the reply.
I wouldn't think the fan flattens out. when we were taking it off of the clutch it seemed to be in great shape.

Oh one other symptom.... it gets to operating temp inside of 2 minutes. My 91 takes about 7-10 where this one only takes 2. Does that indicate something?

Are you sure it's overheating? Guage could be off...

what year XJ?

96 sport

Start with basic maintenance first.

Flush the cooling system and use correct mix of coolant/water.

Do this first.

And if that doesn't work move on to chacking/replacing other parts.

If your temp shoots up within 2 minutes, what temperature is it outside for this to happen? I don't think that's terribly unreasonable, as mine takes little time to reach operating temp in the summer.

The vehicle shouldn't overheat at any constant rpms, save maybe prolonged redlining. I could run at 65mph in second gear all day and besides being stupid and a waste of gas, it wouldn't overheat.

I'm still confused about what you mean by "all antifreeze", as that implies 100% ethylene glycol or whatever else and no water. Pure antifreeze is a horrible, and I mean horrible heat transfer agent. It would overheat in any load, unless in freezing temperatures no matter how good the rest of the system is. So if in any doubt, drain it by taking out the lower hose and draincock and refill it with no more than 50% antifreeze, 50% DISTILLED, DISTILLED!! water. And just because it looks clean doesn't mean there isn't scale, rust, crud built up elsewhere in the system.

It could be your waterpump but the symptoms don't seem quite right. Usually if a w/p is bad, the impellers are corroded and withered away, meaning they don't move a damn thing when idling, but move enough when running faster to keep coolant circulating. Meaning overheating would manifest more when idling rather than higher. This is not a definitive answer to this, however.

It is very important to get a GOOD thermostat and a GOOD thermostat gasket. You must make sure the gasket is designed for your jeep and that it has a hole in the side to allow a little coolant to flow no matter what. If you just use a generic gasket that covers that hole, your temperature will fluctuate horribly and the pressure from the waterpump moving more water may force the tstat to stay shut much longer than it is supposed to (especially with a cheap tstat, believe me I know!). My advice; go to the dealer and spend the $9 on a good tstat and gasket, as the tstat has a bleed hole at the top to help with air pockets/other fluctuations and the gasket is correct. Install these 2 (takes 20 minutes max), while the tstat housing is off clean the inside surfaces and reinstall. See if that makes a difference, and if not post back and I'll keep the ideas coming.

After 4 XJ`s and the last TJ It seems 98% of the overheating problems come from a clogged radiator. Clean antifreeze in there dosent really mean its not plugged up or restricted The higher the rpm the harder the engine is working causing more heat remember. I read over at jeepforum a guy had replaced a pump with a autistic zone unit and was having heating issues again soon after, did everything imaginable and after last removing he found the impeller spun loose on the shaft.
Why 4.0`s seem to destroy radiators I dont know, Ive had to replace everone one in every 4.0 powered jeep from 55K to 180K miles, or 1985 to 1998. Its one line the local radiator shop keeps in stock anymore due to replacement/failure rate. You can pull it and have it flow tested or rodded, while your at it swap thermostats and make sure its not in backwards (no offence, Ive seen it done) flush out the block and heater core, even pull the pump and check or R&R if its old or original. Fill it back up with 50/50 coolant and distilled and be good to go.

Thanks for the respones guys thats great.
So tomorrow I will go pickup a new therm. on my lunch hour. I'll get the system flushed too so bewtween those two things hopefully I'll find my problem.

Thanks again, I'll let you know what happens.