The most complicated Overheating problem yet

Hello all, This is my first post here. I have read this forum for years but have only now had a reason to post. Yes my xj is overheating and , yes I have looked to find threads on the topic. But please read and you will agree there is no other problem like this one. I have worked on cars for over 20 years , from soup to nuts and even ran a little shop out of my garage. I have never been this stumped. Read on:
I have a 1996 Cherokee 4.0. It ran great coolant never went above 210 except on the hottest days ( high 90s- 100) Then maybe 220ish .
In 2019 the engine broke a piston. I replaced the engine with a rebuilt unit from ATK and at the same time had the trans rebuilt as it had 210,000 miles on it. I replaced the engine and at the same time put on a banks header and flow kooler waterpump and a all metal radiator from quadratec I forget the brand , CSF of something like that. All the other parts were stock. This was in OCT so temps outside were cool. Everything was fine (210 temps) until april 2020 when the weather was getting warmer. The temp would easily reach 230 or higher.
Long story short, I replaced that engine again with another one, thinking that perhaps there was something wrong causing the overheating.
After I replaced the engine Nothing changed(still running hot) So I tried a new Factory style radiator with the plastic ends , 160 themostat, 180 , and 195 , Factory waterpump , flow kooler waterpump, and all the combinations of these parts. Nothing made a difference. And this was with the new engine. I tried burping the cooling system to eliminate air bubles, new rad cap. Nothing changed. SOOOOO...... I replaced the ENGINE AGAIN!!!! after 2 months of fooling with this thing. Oh I also did a chemical test to check for hydrocarbons in the coolant, none found.
Onto the third engine, I guess it was July of 2020 at this point. I put the engine in and like clockwork it was running hot. I verified this using a scan tool and it easily was running at 235 just idling after driving it for a while. It seemed , if you could run at highway speeds it would run at 210 ish but if you start and stop it would shoot right up. I disconnected the exhaust and ran a down pipe to eliminate the cat just in case it was clogged. No change. I replaced the fuel injectors , No Change , I replaced the ECU , No Change.
Then the cooler fall weather came. It ran great . wouldnt go above 210 no matter what. That Brings us to now.
Running hotter than ever. Let me retrace my steps . (how many times can I do this) I put in a Mishimoto aluminum radiator, Put in their 3 fan setup, (I had previously had the factory fan and replaced the clutch)Flushed the system forward and backward , I didnt find anything surprising there. Put an external trans cooler with temp gauge, no trans fluid goes through the rad. Good trans temps. Ac works great. Replaced both temp sensors. still seeing temps 230-240 then I shut the car off. I even wrapped the banks header to try to keep under hood temps down.

Like I said If I could keep moving It seems like it would be ok but any stop and go or even worse idling it overheats.
I am not thrilled with the cooling of the radiator . If I check the top and bottom hose temp with a IR gun , there is only a 10-12 deg difference. I feel like it should be more.
I believe the gauge. because the scan tool backs it up and that uses a different sensor and when it shows 240 , it runs sluggish, you can tell its hot.
I believe the head gasket to be ok because of the chem test I did.
At this point its hard to believe that 3 engines would be bad.
As a rule of thumb , If its nice enough to put down the windows , it will run hot
I feel like all the factory stuff is good enough to keep thing cool, after all they made millions of these and they work just fine. But I have upgraded and replaced things because I thought it would help.
I am at a loss I am out of ideas . please chime in. Thanks

Yes a pure factory system will cool a stock xj perfectly fine. That's basically what mine is and I run it hard delivering mail. You definitely got a problem somewhere. At this point I'd try pulling the condenser and see if that makes a difference. Also seal all your gaps in the radiator support so there is no chance of pulling heat from the engine bay.

Yes , I was thinking about pulling the condenser. I even held a smoke machine about 6-8 inches in front of the bumper so that I could see if air was being sucked into the radiator. It was. also if you put a sheet of paper in the grill, the fan suction will hold it there. But never the less I will have to try this. Thanks

I agree with UncleW. If the XJ has been driven in dusty
conditions at some time in it's life, the condenser fins could
very well be clogged, restricting airflow to the radiator.

See if a flashlight will shine through it...

Do you hear your fan clutch kick in?

Good thoughts guys. I will look at that condenser but remember , I had no overheating issues with my original engine and that of course was with this condenser. Im thinking of removing it all together and seeing what happens. As far as the fan clutch, I am sure that was working fine but I have since replaced it with the mishimoto 3 electric fan set up.

Might not be a bad idea to check the timing with a timing light, just to make sure it isn't advanced to far.

A few things that are most common that cause overheating. One is poor or restricted airflow. Another is restricted coolant flow. Another less likely, is the coolant flows too fast and the heat exchange doesn't have the time to shed the heat. And lastly, your fuel burn is way off, the motor is running too lean. I'm going to leave out exhaust getting into the coolant.
Lean can be easy to diagnose, plugs burning white with no signs of soot (black or grey). Causes are wrong fuel flow either due to a faulty sensor, faulty programming, wrong injectors etc.
You can also look at the exhaust exit. Unlikely to be much soot in a hot running engine.
A vacuum leak will do it, intake not sealing correctly. A vacuum line hooked to the intake that is malfunctioning. One way to have too lean is too lean on one or two cylinders. Usually, the O2 sensor will correct but the balance between too rich and too lean is a fine line. The programming isn't perfect. Faulty O2 sensor? Faulty MAP or faulty TPS. Any of these sensors common to the multiple engines? I ask myself what is the common denominator, same O2 sensor etc, same motor management (PCM)?
I did some extensive testing on falsely timed Renix motor (initial distributor timing) I have not idea if it is even possible on a 96 seems unlikely, but like I said no experience, never tried it.
Last thought too hot of a plug can cause overheating and may even burn a piston top. Champion plugs are cheap enough, why not try out a colder plug and see what happens. The best way to read your plugs is to run the motor warm up the interstate at around 60 MPH and pull into a rest stop and pull them hot, you don't have to pull them all, maybe just the easy half. Read the plugs hot. Carry a spare plug or two with you in case you crack one.
The least likely is maybe the thermostat is installed backward, it is possible if it is installed at just the right position. I'm grasping here but if it isn't the normal stuff it is likely the exotic. I've seen the thermostat installed backward, either crushed or less likely whole but still backward. I've also seen the thermostat gasket blocking the heater outlet, only the OEM or OEM type gasket will work correctly. Blocking this flow will make the thermostat function sluggish and it may cycle between too cold and two hot in a retarded manner. Not having the spring in the bottom radiator hose can cause issues, depending on how flexible the bottom hose is without the spring the motor at higher (more than idle or slow) can suck this hose partially shut.
Out of ideas

Many ppl battle overheating... Lets go with the basics first... Cheap spectra radiator. Stock fan setup. Stock style water pump ( I like gates) stock temp decent quality stat. New rad hoses (these can get weak with time and compress restricting flow) also is the efan cycling correctly?


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SlowXj said:

Many ppl battle overheating... Lets go with the basics first... Cheap spectra radiator. Stock fan setup. Stock style water pump ( I like gates) stock temp decent quality stat. New rad hoses (these can get weak with time and compress restricting flow) also is the efan cycling correctly?


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Click to expand...

Ditto.

I am not a fan of e-fan set-ups. I would much rather hear that you are running the stock configuration and could clearly identify when the fan clutch is kicking on.

Diagnosing cooling problems from afar is tough.
I have some comments based on my XJ experiences:

1. It appears to me that your predominate cooling problem is either a bad radiator and/or lack of air flow through the engine compartment.
2. If all three engines, you dropped in, were "new", they will run hot , from additional engine friction, until broken in properly.
3. Most likely, all three engines had "use-once" head bolts. If the head was over-heated and had some "hot-spots", a few head bolts yielded/"grew in length" and lost/reduced their "clamping force". I had an engine that overheated and passed a static pressure-test. Then one hot day, I stuck my foot into it and saw a small puff of white smoke in the exhaust gas. That indicated a momentary pressure leak of the head gasket because a head bolt or two was acting like a relief valve which then resealed. A tear-down, resurfacing of the head, and ARP higher strength bolts cured that problem.
4. If air-flow into the engine bay is "backing-up" and not getting out (i.e. in stop and go driving), the radiator will not cool properly (i.e. air-flow through radiator slows down). The biggest radiator in the world WILL NOT if no air flows through it to cool it down. Likewise, the header heat is not getting out of the engine bay and increasing the engine heat load. A quick thing to do; make two(2) spacer blocks to raise the rear of the hood to increase ALL the hot-air flow out of the engine bay. Drive your XJ and see if your engine is running cooler in stop and go driving. Increasing air-flow through engine bays is a very common problem when "stuffing" high horsepower engines into small compact race cars.

Best regards,

CJR

Be careful with the rear hood raising though, at higher speed it acts like cowl induction and your problem switches, it cools better at slow speed, worse at high speed!

A brief comment of cowl "induction". At speed, an aerodynamic pressure builds up near the Cowl or base of a windshield. Occasionally, this developed pressure is used to flow a little more air into the engine INTAKE to get a slight boost in engine horsepower. In order to do this properly, separate ducting and sealing is used to get this air flow directly into the engine INTAKE from the windshield/Cowl area. This is typically done with a specially designed Cowl Induction hood. If you just raised the rear of a standard hood, there would be no air flow to the engine INTAKE from the windshield/Cowl area. So, in my view, unless you're making a 200-300+ mph pass at the Bonneville Salt Flats, XJ cooling, with a slight raised read hood, would not be a problem at higher highway speeds.

Finally, let's try to solve this XJ over- heating problem first and just start another thread to discuss muscle-car cowl induction.

Best regards,

CJR

Try bypassing the heater core, this should force the coolant through the radiator instead of flowing through both. If this works then you would need a restrictor on the heater core. I know you been stock and high flow pumps, but IIRC high flow pumps needs a restrictor in the heater core lines

Ok guys , Thanks for the ideas so far. Give me a few days and Ill try the things that were suggested. I will report back with my findings and hopefully have a soulution.

Freggie:
I did bypass the heater core . I forgot to mention that in my intro. It didnt make a difference. That was a good thought though. When I thought of that a few weeks ago I was thinking" OH Yeah this has to be it!" Then I tried it and it was the same. More frustration.

CJR said:

A brief comment of cowl "induction". At speed, an aerodynamic pressure builds up near the Cowl or base of a windshield. Occasionally, this developed pressure is used to flow a little more air into the engine INTAKE to get a slight boost in engine horsepower. In order to do this properly, separate ducting and sealing is used to get this air flow directly into the engine INTAKE from the windshield/Cowl area. This is typically done with a specially designed Cowl Induction hood. If you just raised the rear of a standard hood, there would be no air flow to the engine INTAKE from the windshield/Cowl area. So, in my view, unless you're making a 200-300+ mph pass at the Bonneville Salt Flats, XJ cooling, with a slight raised read hood, would not be a problem at higher highway speeds.

Finally, let's try to solve this XJ over- heating problem first and just start another thread to discuss muscle-car cowl induction.

Best regards,

CJR

Click to expand...

I said cowl induction effect, and I was speaking from experience in dealing with the overheating on my previous xj. Helped low speed cooling, started gaining heat at 65. Removed spacers, went back to how it was, just saying.

You've been through several water pumps, but are you sure they were the right ones? Seems to me I recall reading around here years ago that the pump for the 4.2 bolts up and looks right, but the impeller's configured to be spun the opposite way (due to different serpentine belt routing compared to the 4.0).

Confirm proper heater hose routing as well. I installed a new radiator, water pump and a 195 thermostat awhile back and the engine ran hot.
I printed a schematic from the FSM and discovered my heater hosed were reversed.
I did a heater valve delete on my 90 XJ. The valve self destructed when I took the heater hose off of the heater core. Somehow I reversed the heater hoses.

Reversed heater hoses make no difference; at least on later models.