So I can finally give my insight into the XJ cooling issues as I now have some empirical information due to direct experience. Translated… it sucked being me last Saturday.

To get right to it, I was tootling up to the Rim north of Payson AZ on the hottest day so far here, (102. ~118 in Phoenix), going 55/60mph with the AC off (dumb but not stupid). Temp needle at 210 and responding to any down grades by millimetering lower, but inching higher while going up. Just when my tiny pea brain started getting concerned as I was approaching a 4% grade to reach the top, the infamous Check Gauge light went off, accompanied by the beep (actually a chuckle as I later interpreted) I immediately pulled over and killed the engine, popped the hood, jumped out in time to see coolant viciously escaping the vent as I raised the bonnet. 3-4 seconds later a load pop and spray from somewhere went everywhere, while me being unable to run and investigate at the same time. After my 2001 XJ’s hissy fit subsided, I was able to determine right away and with certainty clarity, that I was screwed.



Called road service, waited 4 hours for the tow, blah, blah, blah, every jeep owner knows the rest.



This post does have a point to it as Im seeking input from this forums experience, and what Ive seen, some excellent suggestions gained as I can only explain as, ‘at the university of been there done that’.



1) My ‘01 XJ has 99k well maintained miles on it. Bought from my sister who drove it like a girl for 19 years and never put it in 4wd. Go figure!

2) The rad is an OE (Spectra style) last replace in 2011 and 19k Florida miles on it.

3) When I got it to AZ in Jan, I did a complete flush, bypassed the heater core for a later date replacement, new 180 tstat. Everything else looked fine.



What I discovered during surgery…..

1. The coolant I drained was still green, tranny fluid still pink, engine oil still clear (as it had just been changed) and my wallet going from black to red.
2. The loud pop I heard at the scene of the crime, was the hot end plastic end cap splitting on the Spectra, up near the top. See pic
3. I don’t know if the efan ever ran on temp, as I didn’t have the AC on. At 60mph, it wouldn’t matter anyway.
4. The jacket behind the water pump was just lightly rusted with green residual coolant. Typical for a clean system.
5. The Spectra cool side header design has the tranny cooling header right up against the 1 ½” outlet connector, Possible flow restriction. More on that later.
6. The water pump was fine, looked new except for light deposits.
7. Hoses all good, AC was off, rad & condenser fins clear and can see through them. Mech fan shield in tack.

My questions are….

1. What made the engine overheat to begin with while going up the hill. I cut the plastic headers off and water runs freely thru the tubes. Light deposits on the interior.. See pics.
2. Any experience with a high flow tstat. Speedway or Mr Gasket?
3. Does a ZJ fan clutch do any good a 60mph? I think not.
4. Anyone try shielding the exhaust pipe that wraps 2 sides of the tranny pan to see it theres any effect? Maybe more critical at low speed.
5. Anyone try installing an auto air vent at a high point in the system? High pressure steam systems have these and are reliable.
6. The only way air could get out is through the overflow and only if the pressure exceeds 16psi and ~48d above 212d. Is this common? I read a lot on this that folks don’t know where there coolant is going if theres no leaks. May just be air in the system. What am I missing here?
7. Anyone experiment with ram air induction that bypasses the radiator, straight to the engine compartment. Not as effective a liquid cooling but 102d air flow across a 4.0 cant do any harm. Lots of variables here that can be sorted out by trial & error.

So where Im going with this as my wallet hopefully recovers…

1. Install a ZJ fan clutch. Sounds good but to me more effective an low speed or idle, not 60mph
2. Im replacing the Spectra with a 2 row CSF. My investigation into heat transfer pointed to 2 row and all metal. Good explanation here. https://www.dewitts.com/blogs/news/1...%20of%20tubes. Just couldn’t bring myself to go all aluminum and could repair a pinhole in a CSF myself. To me the science in this link debunks a lot of what I read in these forums. I apply the same information when designing cooling coils for HVAC systems.
3. New Delco water pump, (belt, hoses and alternator) while Im at it.
4. Hayden tranny cooler to replace the stock one. Reroute the inlet to the cooler from the rad for better flow.
5. There a whole different conversation about efan vs mech fans. Didn’t want to confuse the issue here.

I should mention Im a mechanical engineer and to address flow through the circuit, I can see a high flow water pump working, BUT only if the restrictions and coefficients of all the fittings, passages, hoses, restrictions etc, were either too restrictive to begin with (don’t see that) or improved. Its curious that the split occurred right were the hot engine water entered the 9 year old radiator. It was just its time I guess. If I had let the engine run instead of turning it off, would that have prevented the failure? Who knows.



To improve the flow restriction I may install a Speedway high flow tstat ($36!) but measurably better. The stock tstat is designed for temp management and not for high flow.

These water pumps are not positive displacement so a high flow pump doesn’t necessarily equate to higher water flow, if nothing else has changed. They spin at the same speed but increase downstream pressure. Possible create cavitation if air exists. Not sold on these.

At a stop or idle, a higher flow rad will help move more water, but its effectiveness is dependent on the volume of air across the coil and its temperature. I see a efan override switch as a plus.

As mentioned the Spectra tranny header does restrict the outlet from the rad. No doubt about it. Do others manufactures do this as well? Ill see when I receive the CSF. Could help explain where the split occurred. On a older rad, the water couldn’t get through the tubes as easily as the pressure differential is greater than design.



Hope Im not rambling too much. Hope someone can help me with their insights.

Lessons learned are the 4.0 really does need a clean system to stay at the ragged edge of reliable. Not much wiggle room built in. I agree with folks that the stock system, well maintained, should be fine.

Theres real opportunity to add some tech to predict that ragged edge with averaging stats across the rad, water temp measuring at strategic points, side stream filtering, etc. May try some of these in time.

Header split on the hot side of the 9yo radiator. The top is on the right side of the photo.

View of the tranny header inside the cool side of the radiator. Cut piece off. Hard to see the restriction it creates

View of the tranny header inside the cool side of the radiator. Cut piece on, darkening the photo..

Tranny header. Not to bad for 9yo. The right side is the top. Notice the cleaner side is where the flow leaves the radiator at increased velocity which scours the header clean.

Hot end. the right side is the bottom of the radiator. There was flow through all the tubes but the bottom appears to be more restricted.. Settling when the engine is off, etc.

 

Oh man, sorry to hear this. Definitely a bad time.

One thing I didn't see you mention was an additional transmission cooler. I put one of on a previous 98 XJ I had and it worked great. You could install it before the radiator to reduce the load the transmission fluid is placing on the radiator for instance.

 

The only questions of yours I can accurately answer are:

I've never seen a difference either way with a high-flow t-stat, pump or housing.
ZJ Fan Clutch will help cool at low speeds / idle. Doesn't do anything at high speed, also reduces your MPG

Last note, I've used a 2 Row, All metal, CSF radiator and it was actually worse than an OE Spectra Premium from NAPA for what it's worth. I ran the 2 row for about a year and was still getting warm (not overheating but warmer than I expected with a new radiator), after swapping out for a new Spectra Premium the temp stabilized for me.

 

Have you checked for exhaust gasses in the coolant?




Thank you for that interesting link. I have been hammering this message for years. More rows does NOT mean more cooling. It's not anywhere near that simple. I haven't read the article yet, though. I got as far as the first sentence and started banging my head:


It wasn't true then, either. It was a simpleton's bromide and it still is. It's just not that simple. There are many other factors that affect how well a radiator cools.


Replace the stock one? OR maybe in addition to the stock one? Plumb it in front of the radiator to dump some heat to the air instead of to your coolant. Yes, I know about the efficiency difference.

Interesting note on that - I bought a Ford reman transmission for my E350 van some years back. The Ford website spelled out some conditions for validating the warranty. THey wanted TWO additional tranny coolers. The usual -in-the-radiator like the XJ, then one air-cooled BEFORE, and one air-cooled AFTER.

 

Look at the rust in that rad....there's your problem lady

 

Thanks, Gmac, I always wanted to see the inside of a Spectra radiator (I am running one now)

Good luck with your issues.

 

Thanks. Im replacing the stock cooler I have and put the new one on the passenger side in front of the mech fan. You know of anyone thats installed an aluminum flex fan in place of the clutch fan?

 

Im looking at this from a pure hydronic perspective hoping to get nuance input from experienced xj owners. Pump laws are the same whether in a jeep or a chilled water system. So are heat exchange rate for materials and air flow across a cooling/heating coil. Trouble is HVAC has real criteria to design a coil or pump but autos dont (at least whats available to the masses). Im sure jeep spent many hours and tests getting to what was installed so you have to trust it somewhat. I read a post about the same issue of overheating on a highway but not around town. Sounds like my problem. Ill conclude that the spectra I had just gave out due to age (bless its heart) Im just not sure why the engine overheated to begin with. His resolution revolved around cleaning the system several times seeing some improvement but no jackpot. I know mine is clean to a point with a 9yo rad, but you can only do so much when you cant see inside. Once the new one is in, Ill reassess all that.

As far as using a 2 or 3 row, aluminum or copper, this manuf or that, etc, there anecdotes all over the forums, good and bad for each. One persons experience may not be the same due to location, driving habits, air temperatures, maintenance etc, that part of why Im looking at this from a design perspective. Champion got a lot of bad press 4 years ago with leaks. This is 2020 and they appear to have fixed that process problem as an example. A 3 row that has less surface cooilng area that a single row, just does equate to being better for me. Ive owned jeeps all my life, still have the '73 I still drive so Im not a newbie, only to this XJ. But again the science behind water cooling systems is the same. Im just trying to drill down to actual cause and effect. Sorta like karma for an XJ.

 

Reasonable.

That rust might be the answer. Have you looked at your water pump yet? This all sounds very familiar. If that rust is the remains of your pump vanes, bingo. Everything fits. It will move enough coolant to keep things going under most conditions, but once the heat hits the fan (so to speak), it's not going to get the job done. Sudden overheat is known to be a symptom of this problem.

 

Maybe I missed this, but have you checked for signs of exhaust in the coolant, like BlueRidgeMark suggested? You want to rule out a cracked head or headgasket first, or else you will be throwing away money and time on cooling parts when the real issue could be the engine. You have a 2001, which could have head cracking issues. Do a search on it and you will find plenty of threads on it. You can run a compression test on the engine, or do a chemical test on the coolant when you have it put back together again. You can also look down the oil fill hole and look for the word TUPY, which is supposed to be the better head.

If the head or head gasket is not the issue then you can move on to maximizing or returning your cooling system to normal range. After years of chasing my slow temp creep in summer, I put on a new electric fan with a bypass switch. The new fan is way stronger than the old one (new fan is advertised as 1050 air flow which I think means CFM). As soon as I turned on the new fan, I felt the difference in air flow between the old and new. I can't quantify it exactly, but I would say it is double the old one, which was rated as 880 cfm, but I doubt it was pulling that. The new fan has 10 blades and a very powerful motor.

 

Yea the water pump is fine, just some surface rust on the impeller. Even had the R stamp which the new Delco doesnt have, but is the same reverse rotation. Not the issue.
Water leaving the WP goes through the block and the next 'device' is the tstat. Theoretically a high flow stat (maybe housing too) should allow more water through (less restriction) and the waters residual time in the block is lessened and wouldnt heat up as much. But thats only one point of the circuit and may not matter if there other design restrictions down stream.

I like the idea of the override switch and efan upgrade. I didnt know what the OE cfm was and what model to replace it with. I do now. Thanks. Should this be just an override or an adjustable switch added as well?

Once its back together next week, Ill do the exhaust test. I do remember seeing a few water drips either out the tail pipe or that little hole in the muffler, Isnt that attributed to the cat? From Quora> " catalytic converters do produce water but only if the engine isn't running efficiently and is allowing unburnt fuel into the exhaust. But you shouldn't forget that water is a product of normal engine combustion too"

Also will do a pressure test on the system

 

just a few thoughts. Many moons ago, vehicles used a vertical tube radiator and in those, all the debris landed in the bottom tank and the tubes stayed cleaner longer, plus the trans cooler was an open coil and didn't restrict the flow of coolant as much. IDK how efficient they were at heat transfer either. They also had larger coolant capacity than today's systems and ran at a much cooler temperature. The difference between a 180* normal operating system and the boiling point of a pressurized system gives the engine more flexibility before failure. Today, there isn't much room between normal operating temp and the boiling point. Would installing a relay system to keep the electric fan running for a cooling cycle after shutdown help to prevent afterboil? Maybe with the help of an electric water pump? My 2000 WJ only has an electric fan, so would replacing the clutchfan with an electric fan work better? It runs on high when over 220* and at low speed at 195*. On startup, it doesn't run at all until 220*, then runs high until the temp drops to 195* and remains on low until you shut the engine off.

Also, as we all are aware of, airflow thru the engine compartment of an XJ isn't what it should be. There isn't very much room between the end of the fan shroud and the engine block for air to pass around it. In older cars that weren't unibody, you could vent or remove the inner fender liners for better airflow. I suppose that could be done to a certain extent with the XJ, but it will weaken the structure a bit.

I suppose it all relates to a question my Physics teacher posed. "Why should a heat engine need a cooling system?" Because all the thermal energy produced is not utilized.

 

Just an override. No need to complicate things.

There's actually a better way to do this. Just tap into the fan relay at the PDC. The wire that grounds the relay can be tapped, run to a switch, and ground the other side of the switch. Now it mimics the action of the PCM by grounding that relay. Very clean. I need to write that up one of these days. I can't remember which of the members tipped me to that. I checked the wiring diagrams, and he's right. There is no possibility of a back-feed with his method.


That's perfectly normal, cat or not. The main products of burning gasoline completely are water and CO2.

 

The complete combustion of anything is CO2 and water. It's the incomplete combustion that gets you.

 

Not my wife's casserole. Trust me on this.