96 ZJ 4.0 overheating, help! - JeepForum.com
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post #1 of 14 Old 08-09-2018, 08:12 PM Thread Starter
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96 ZJ 4.0 overheating, help!

This is a 96 Grand Cherokee, inline 4.0 six, near 200k miles, very few problems to now. Good old machine. Running real hot suddenly.

Before the big write up, is there a definite test for bad head gasket or cracked head? Whole lot of R&R work there.

Driving the Jeep and noted temp gauge at 220 or so, much higher than normal. Engine checks hot with IR meter but the top of the radiator is at 160 or so. New thermostat, 195 degree with the little bypass hole. Test drive and engine got really hot, boiled the antifreeze into the exp. tank. Radiator still not real hot.

Spent the whole day going through the usual checks, nothing that I can see is wrong.
I took out the spark plugs, all dry, looked fine. Rolled the engine over several revs, nothing sprayed out. Tops of pistons have normal carbon, nothing looks like an internal coolant leak. Pressurized the rad. to about 15#, listened in each cylinder, no sounds, rolled the engine over again, still dry. Took out thermostat, reverse flushed the radiator, filled the radiator with clean water (no t-stat). Started it up and let it run. Didn't note any bubbles in the coolant that might show a compression leak into the water jacket. Rev up the engine and could see the water level go down a bit, seemed normal there. Temp came right on up, got to about 220, checked radiator with heat gun, not over 160 as before. Water started boiling, Old Faithful out the rad, neck. Odd thing, clutch fan seemed to be working fine until the engine got just ready to overheat point, then it went quiet, minute later it erupted. Everything checks out except that there doesn't seem to be enough circulation through the radiator. Tore into it, took off water pump, looked fine. Radiator flows fine on a second back flush. Didn't notice hoses collapsing. I'm beat on this one.

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post #2 of 14 Old 08-09-2018, 08:23 PM
riverzj
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Sounds like coolant isn't flowing properly. Run some flush cleaner through the system. Pump could be bad radiator could be shot/clogged. If it overheats while just idling it's a flow issue. Yank the fan around and if it moves up down in or out the bearing is bad. Pull the belt when doin that. I had same issue replaced whole coolant system ended up being cap. Sometimes you have to just work through it replacing things until you find the problem


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post #3 of 14 Old 08-09-2018, 08:25 PM
kg6mov
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Boiling in the overflow bottle is often a bad radiator cap. I would also check the fan clutch, that sounds like it's gone bad.
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post #4 of 14 Old 08-09-2018, 08:52 PM
GOLDWING
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Are you positive that you got all the air out of the cooling system? I have a Lisle radiator funnel .https://i.ebayimg.com/images/g/cPQAA...ycc/s-l640.png that lets you be certain that the air is out.

"Before the big write up, is there a definite test for bad head gasket or cracked head? Whole lot of R&R work there."

A compression test showing two low compression cylinders adjacent to one another will indicate a bad head gasket.

A radiator pressure tester will show pressure rising on a relatively cool engine.

GW

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post #5 of 14 Old 08-10-2018, 07:57 AM
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Take the radiator cap off when the engine is cold. Make sure that the radiator is full to the top. Start the engine than gun it a few times and check if the antifreeze level changes a little.

If there is no change in the level I would check the impeller on the water pump.

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post #6 of 14 Old 08-10-2018, 09:02 AM
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Sounds like you might have a couple of things happening at once. I've found on mine of the same vintage that sequential failures happen a lot - e.g. maybe fan clutch causes pressure cycling causes rad cap to go causes bubbles in system causes overheating.

Cool radiator seems to imply water isn't circulating. Make sure you've burped the top hose. This isn't normally an issue but if you get a steam bubble up there it could be a problem. You might have a bad radiator cap too, causing coolant pressure loss and the resulting failure in circulation.

Your radiator might be clogged but usually you'll notice it just less efficient over time, not suddenly overheating.

Don't run it with just water long-term, you'll corrode the water pump bearings etc.

These older 4.0s don't tend to crack heads or things. If it does you're going to notice eventually some big symptoms - missing, oil in coolant or vice versa, burning coolant or oil in exhaust, blowback through intake.
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post #7 of 14 Old 08-10-2018, 12:14 PM
RedRiverT
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"Pressurized the rad. to about 15#"

I would be interested to know if it held pressure for a while.

"took off water pump, looked fine"

Did you check to see if the impeller could be turned on the shaft, or if the front hub could be turned in relation to the impeller? I agree with others, it sounds like maybe no / low coolant flow. Also the fan clutch question.

With the cap off and only water in it, Old Faithful upon heat up is normal. IIRC, when attempting to burp mine a few years ago by driving it with no cap, it went past 210.

That being said, my 98 Ford truck was doing this, boiling in the reservoir, gauge hot, but radiator barely warm to the touch. I kept investigating and found small leaks. I think it was boiling due to not holding pressure and then getting a flow-blocking bubble in the t-stat housing, which others have reported is a possibility / problem on them. Now it has a blown head gasket (all the classic signs, none before) which I wonder if happened during one of my test drives when it overheated again, or if was slightly leaking all the time, then failed completely. I also wonder if the PO might have put stop-leak junk in it.
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post #8 of 14 Old 08-10-2018, 07:05 PM Thread Starter
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Thanks for all the suggestions. Put the pump hub in the vise and pulled on the impeller pretty good, on tight. Poured some boiling water on it, still tight. Put it all back together with new lower hose and cap, checked new thermostat in water on the stove, opens up fully. Gave the radiator a really good back flush while out, both cores were pretty clean on the outside. Everything just looks fine in the cooling system. The new thermostat has a bleed hole in it that I suppose takes care of any trapped air in the block. Did notice the water level drops when I rev it up some.
Did find a tester at Advance Auto that checks for gases in the coolant, does a color change if present. I'll see how that works tomorrow.

One question, how do you burp the upper hose? Is this necessary?
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post #9 of 14 Old 08-11-2018, 01:58 PM
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Quote:
Originally Posted by tom3 View Post
Thanks for all the suggestions. Put the pump hub in the vise and pulled on the impeller pretty good, on tight. Poured some boiling water on it, still tight. Put it all back together with new lower hose and cap, checked new thermostat in water on the stove, opens up fully. Gave the radiator a really good back flush while out, both cores were pretty clean on the outside. Everything just looks fine in the cooling system. The new thermostat has a bleed hole in it that I suppose takes care of any trapped air in the block. Did notice the water level drops when I rev it up some.
Did find a tester at Advance Auto that checks for gases in the coolant, does a color change if present. I'll see how that works tomorrow.

One question, how do you burp the upper hose? Is this necessary?
I am sure that there are other ways, but this is how I do it:

First I put in my Lisle radiator funnel in the radiator and fill the cooling system. Next I add a couple of inches of coolant to the funnel so I can easily monitor the coolant level.

Start the engine and let it reach operating temperature while watching for leaks as well as the coolant level in the funnel. (add as needed)

When the temp gauge tops out @ 195 (ideally) you should be able to see the coolant moving in the funnel a bit which tells you that the water pump and thermostat are working.

At this point put on your gloves and eye protection if you haven't already and give the upper radiator hose a two handed squeeze as if to try and flatten as much length as possible.

You will most likely see bubbles in the funnel each time you squeeze for four or five times. The hose is burped.

Now since you probably have a couple of inches of coolant in the funnel you can just put in the supplied plug and avoid the mess on the floor.

Good luck,

GW

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post #10 of 14 Old 08-11-2018, 02:19 PM
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Quote:
Originally Posted by tom3 View Post
Gave the radiator a really good back flush while out, both cores were pretty clean on the outside.

One question, how do you burp the upper hose? Is this necessary?
One of the concerns others previously stated, and I agree with, is that the engine block passages might have gunk in them so you need to do a "system flush" not just the radiator. That's where you buy the flush chemical and add it to your radiator then run the engine / drive around for however long the directions say. Of course if it overheats quickly and you can't run it that long I guess you have to do it in spurts.

Simple way to burp the upper hose is to part uphill on a steep incline with the cap off. I don't know if it is really necessary. Supposedly not per the factory instructions, if your system is working properly (no leaks) it will burp itself by pushing air/steam out into the overflow. A few years ago, mine had a tiny leak at the top of the radiator (new one) under the fan shroud where I couldn't see it. It drove me nuts and I just lived with it. One day after a thunderstorm I had the hood up with it at op temp (maybe running) and due to the humidity I could see steam coming out from under the shroud. O'Reilly's warranteed the rad and now mine burps itself. Take the cap off and it's full up to the hole.
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post #11 of 14 Old 08-11-2018, 09:11 PM Thread Starter
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Fingers crossed, success

Not sure exactly what the problem was, did everything mentioned here. New cap, lower hose, thermostat with bleed hole, couple system flushes. Hard core high pressure radiator flush (clean or blow it up) purged and burped. Radiator sniffer test was a-ok, that was a relief!

After a test drive, idling with the AC on:
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post #12 of 14 Old 08-11-2018, 09:12 PM Thread Starter
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And thanks to all for the help!
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post #13 of 14 Old 08-12-2018, 10:12 AM
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Wow, that's running nice and cool for the summertime. Congrats! I'd like to get mine running at that temperature when the A/C is running at idle. Good job!

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post #14 of 14 Old 08-13-2018, 06:08 PM
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In addition to the other recommendations: With any vehicle, if it overheats, even a little, replace the thermostatic valve.

Snowman
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All seriousness aside => 95 ZJ, 6" Rusty's Long Arm, Body Armor, ARBs front and rear, a bunch of upgrades
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