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Another overheating thread...


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1988MJ Pioneer 4x4 with 4.0L, AW4, 78K miles. Truck was given to me when the waterpump quit. Replaced that and inspected the inside of the block hoping to see all kinds of muck, mud, and crud, but the coolant actually wasn't that bad. Replaced waterpump and continued to drive it. Had the idiot light gauge cluster so I have no idea what my temps were. Replaced with full gauges and the truck would stay around 210-220 degrees at idle, but after I drove it around for a bit and put some load on the engine (and probably got the auto tranny fluid temps up), the truck will continue to increase in temps when I'm sitting at a stop light or idling in my driveway. Seems to stay around 210-220 as long as I'm moving.

 

Figured it was the mechanical fan so I swapped one in from my other MJ that I currently have torn apart. That MJ never had any cooling issues, but the "new" fan didn't fix it.

 

Decided to replace the thermostat with a Stant SuperStat 195*. Burped the cooling system by raising the back end of the car up and removing the temp sensor in the head. Ran the truck until coolant poured out, replaced sensor, and nothing - still overheating.

 

Noticed that the hose on the bottom of the expansion tank was seeping coolant. Removed stock clamp, put on a new one, tightened, and no more seepage, but we're still overheating.

 

I see no evidence of seepage or leaking from the expansion tank cap, but I do have a spare tank/cap I can replace it with from my other MJ which never overheated on me.

 

Anyone have any other suggestions?

 

Also, at what temp is the electric fan supposed to come on? Mine seems to come on when the temp gauge hits the red portion of the gauge, which I feel like is too high. I usually leave the truck on defrost mode, which is a double-edged sword since it kicks the A/C compessor on, but the electric fan comes on.

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I did some more experimenting tonight. Drove the MJ around some local 45MPH roads. It gradually warmed up to just under the 210 mark on the temp gauge and sat there for about 5 minutes or so and I thought it magically fixed itself...but then I came to stop at a traffic light and I saw the gauge steadily rise to the tick mark after 210 and even as I was cruising at 45MPH again, it just sat there and wouldn't fall back to 210 where it had been sitting previously.

 

Pulled back into my shop and parked the MJ next to my '01XJ which has a functioning cooling system that keeps temps where they ought to be. Pulled out my infrared thermometer, let the XJ get up to operating temperature and began taking measurements at similar locations on the two engines. The XJ held fast at 195-210 on the thermostat housing whereas the MJ was up around 225-245 at the same spot. Shut both engines down and tested the mechanical fan clutches by spinning the fan. To my surprise, my MJ fan had more resistance than the XJ fan, which tells me that the mechanical fan is probably not the problem.

 

The MJ has yet to really overheat, but it is running too warm for my comfort, but I just can't figure out what it is. The coolant isn't boiling and there isn't steam spurting out from anywhere. If I didn't swap the full gauge cluster in, I probably wouldn't have even known this was an issue as the engine runs fine otherwise...

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The electric fan is supposed to kick on at 220 or whenever the A/C is on. Just follow the wires to the temperature switch. Its easy to find. If you have an IR temp gun, you can determine for sure if its kicking on at 220 or at an actual hotter temp. The Temp switch could be misreading. Did you flush the rad good?

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Any chance someone switched the lower hose at some point and didnt put the spring in the new one? and it is collapsing when the truck warms up? If not, I would swap the radiator from your parts MJ, along with the temp sensor. Since it hasnt actually overheated maybe the sender isnt working right and sending wrong temps to the gauge

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I checked the lower radiator hose to see if it was collapsing and it is not. In fact, both hoses are completely engorged with fluid when it's up to operating temp (and beyond) so I have no doubt there is liquid flowing through the radiator. The temp sender is brand spanking new, but that's not to say it can't be bad, that's why I busted out my infrared temp sensor to make sure it wasn't just a faulty sender. Sure enough, the truck is running warmer than it should.

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I'm starting to think there is something impeding flow somewhere. I'd take the temp gun and measure the input and output rad hoses and compare to another vehicle, unless its obviously not cooling well. In that case, I'd say its flowing through the radiator, but not well enough and the rad needs a good flushing or replaced. Get two readings, first with no E fan, then with the E fan on.

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Quote:

 

Burped the cooling system by raising the back end of the car up and removing the temp sensor in the head. Ran the truck until

 

coolant poured out, replaced sensor, and nothing - still overheating. End quote.

 

 

 

Did you try burping it without the truck running?

 

I believe your supposed to do it when the engine is not running so the air bubbles can accumulate and be let out when you remove

 

the sensor.

 

Another thing I do is to drill a very small hole at the top of the T stat to let air bubles through. Actually some T stats came this way.

 

And last of all, not trying to be insulting, but is it installed facing the right direction?

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Well, the rear end of the truck was lifted and the sensor was out for a few minutes before I ran the engine. Not sure how long you're supposed to let the air bubbles accumulate...minutes, hours, days??

 

I read about drilling the hole in the T-Stat AFTER I installed it so I'm not sure if it came that way. I'll see if I can find a product-specific picture of it to see if there is a hole in it.

 

Yes, I installed it the right way :D Or, at least I installed it the same way as the other 5 T-Stats I've done in 4.0L engines...

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Lift the rear up and take the sending unit out when the engine is not running and cold. Then pour your coolant into the bottle and wait for it to come out of the temp hole in the head. If it doesn't come out then you need to drill a hole in the T-STAT. When you do this procedure you can guarantee there is no air in the system unless you have a cracked bottle or bad cap. Also you may have the wrong water pump on there.

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I'll try your suggestion with the engine off and cool.

 

I replaced the cap with the one off my other MJ, which didn't have any cooling issues - no change.

 

The wrong water pump? I suppose it's possible - what would be "wrong" about it? Pretty sure I got it off of RockAuto, but it's been about a year since I replaced that guy.

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Removed radiator tonight - original AMC unit that appears to be in great shape. Ran water through it for a while - no gunk came out and it seems to be flowing just fine. Don't really think the radiator is the problem, but I was a little disappointed that the original OEM unit was a single row, plastic endtank radiator. The radiator out of my '88MJ SporTruck is an all metal and 2-row (I think) radiator, but it was manual transmission and I can't swap over the fittings from the other radiator since they seem to be built into the endtank. Wonder if I can get a fitting kit for the manual trans radiator to connect the auto trans cooling lines up to...

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Ironically, I found one of the two fittings I need to mate the stock auto trans lines up to the manual trans radiator (came from a junk aftermarket radiator out of an '01XJ). I do have the second of two fittings, but I can't seem to get it to fit onto one of the auto trans cooling lines.

 

Ideally, I'd like to put an aftermarket trans cooler in this thing, but I want to make sure I have the cooling system issue nailed down before I drop any more coin into it. Thinking the cooling issue may be something a little more complex, like a cracked head.

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Dang, I thought I had it...

 

Replaced the stock radiator with a 2-row all metal one. Found another fan clutch in my spare parts bin that is better than the other two I tried. Replaced everything on the truck, lifted the rear end WAY up, removed the rear temp sensor on the head and filled the cooling system until coolant puked out the sensor hole. Reinstalled sensor, tightened up the cap and let her run. The engine gradually warmed up to just below 210 and stayed there. Woo! Took it out for a 15 mile test drive. Drove it around town with plenty of stop lights and stop signs. Temp held steady. Went to a drive-thru and sat behind some lady who couldn't make up her mind about what she wanted. Temp held steady. Headed towards the highway. Thought about all the things I can start doing to the truck now that this cooling system issue is out of the way...new tires on the stock 5-spokes, center console, repaint...this truck would look better than ever.

 

Entered highway...temps rose and continued to rise. Panicked and threw the heat on full blast. No change...we're hovering right around the mark before the RED zone on the gauge. Pulled off the highway and headed back home on 45MPH backroads. Temp held at the mark before the red zone and would not drop. Got it back to my shop and let it idle outside while I opened the doors. Temp held steady at the tick mark before the red zone.

 

So we're right back where we started. The truck still doesn't technically overheat to the point where steam pours out of the cap and the truck starts knocking and sounding like poo. It seems as once it breaks that 210 degree threshold, NOTHING can make the temps come back down except for shutting it down and letting it cool off. So, we're not really overheating, but running nice and HOT.

 

Can anyone draw any conclusions from this?

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I have a '89 XJ doing exactly the same thing yours is. Except for the radiator, I've done everything you have, with the same results. After boiling over the other day I've come to the conclusion it's leaking coolant. A very slow, undetectable leak but a leak none the less. I had the head off and replaced the gasket. A close inspection didn't reveal any cracks but I may have missed one. A crack in the valve chamber would be undetectable without removing the valve. A leakdown test wouldn't reveal it if the valve was seated. I'm seriously thinking about just driving it until it wears out. Then replace the engine. Just check the coolant every morn, or whenever I drive it and top it off and go.

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