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My '97 Jeep XJ Cherokee w/ 4.0L inline-6 engine runs a little hot and it overheats when driving on the highway for longer then an hour. The coolant temp slowly creeps up until it boils over. I've seen it boiling and puking out of the radiator cap, so I know it's not a bad sensor/gauge. There's two fans: a mechanical fan w/ clutch, and a small electric fan that only comes the coolant temp rises above 215*. The electric fan works and is running when it's overheating.

I've replaced the:

  • radiator
  • water pump
  • both radiator hoses
  • radiator cap (tested that it works and holds the right pressure)
  • mechanical fan clutch
  • thermostat (new one is Stant SuperStat 195*)

I've also flushed the cooling system more times then I can count. The iron block has rusted because the old coolant sat in there too long. I used, in order:

  • Blue Devil Flush additive
  • Prestone Flush additive
  • Oxalic acid (wood bleach mixed with water)
  • Evapo-rust Thermocure (did this just a few weeks ago)

After all that I'm sure that the rust has been removed from the system. I replaced the coolant with about 30% anti-freeze and 70% distilled water and 1 bottle of Water Wetter coolant additive.

Last Thanksgiving, it was unusually hot the day I drove 3 hours to my parents house, and the damn thing overheated. So... problem not solved! I was able to get home by airing up the tires to 40 psi and driving at 65 mph.

Maybe the head gasket is blown?

To test for a blown head gasket:

  • I used a radiator pressure testing tool (rental) to see if there was a leak. It held the pressure perfectly fine for several minutes.
  • I used a testing tool (rental) to test for combustion gases in the coolant. It looked fine.
  • There is no white smoke coming out of the tail pipe. No milky fluid in the oil.
  • The coolant seems to get brown very quickly, but I don't know if that's because of the rust or from exhaust gases.

Do you think I could STILL have a head gasket leak?

I have a compression gauge at home. Can I use that to check for a blown head gasket??

When I was flushing the cooling system a few weeks ago with the Evaporust Thermocure, I was finishing up putting fresh coolant back in and getting the air bubbles out. Coolant started flowing out of the radiator cap port before the thermostat opened (normal) so I put the cap on and waited for the stat to open. Once it did I took the rad cap off and when I did I noticed some smoke and/or steam came out of the radiator. ???

Winter is almost over and I want to fix this problem before it gets worse. If it's not a blown head gasket I'll replace the radiator again - I replaced it before doing all the rust flushing so maybe rust has plugged it up a little - and swap the mechanical fan for an electric one.

Thanks!

[update] I sent an oil sample to Blackstone Lab and they said there was signs of coolant in the oil. I took the head off and it was warped so I had a machine shop fix it and look for cracks. Put it back on with a new Felpro head gasket. Went for a test drive and it

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    Yes, check the compression on each cylinder. You could be leaking from the combuston chamber into a coolant channel but not losing any coolant. Also, you should be using a minimum of 50% coolant and 50% water when doing your own mix. It's possible that the water is slowly boiling off because there isn't enough coolant mixed in. – finleyarcher Feb 28 '18 at 16:13
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    Does the heater blow hot? Putting the heater on full hot and full speed can help prevent or at least control overheating. Does the system bleed properly - are you sure it does not have an air-lock in it? That can easily cause overheating... – Solar Mike Feb 28 '18 at 17:24
  • @SolarMike The heater does blow hot now that I've unclogged the radiator core (back in December). It's not very hot though especially on super cold mornings. It was getting clogged with rust and crud. After the EvapoRust Thermocure treatment, I tested to see if the heater blew any warmer and it really did not. If it suddenly did blow a lot hotter, that would have told me that the radiator also was freed up of rust just like the heater core. Also, I'm pretty sure that I've bleed all the air out of the system, I've gotten pretty good at it after flushing it so many times. – Dan Mantyla Feb 28 '18 at 17:35
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Taking out the thermostat can be an emergency fix. Even when open it is a restriction in the cooling system. Removing it allows the coolant to flow easier and faster, so more cooling. But it will take longer for your engine to warm up. Putting the blower on full hot also seriously helps the engine cool down.

Now for the solving. First, are you sure the fan clutch is ok? You shouldn't really need it when driving highway speeds, but still. That thing can trick you easily, when it looks like it properly works while it doesn't.. Is the grill free from any restrictions? If it was always running hot-ish, it can be that the engine's internals are just badly rusted, insulating all the heat. Lastly, it could be running lean, or ignition can be retarded. I don't really suspect those, but they can easily overheat an engine. Check the engine bay for any heatshields that may have come loose or have disappeared.

  • thanks! No I'm not sure the fan clutch is OK. I replaced it with a new one, but it behaved exactly like the old one. What I would do is I would stop the engine and watch how long it would take for it to stop spinning. I can't remember how long it was but both new and old took the same time. Grill is free from obstructions and I've actually removed the AC condenser heat exchanger. The engine runs well and has good power and the spark plugs don't look like it's running lean. – Dan Mantyla Feb 28 '18 at 20:11
  • BTW I'm going to replace the mechanical fan with an electric one. That's 100% decided – Dan Mantyla Feb 28 '18 at 20:11
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    @DanMantyla Removing the AC condensor can also have huge improvements, good idea. You can try to fix the fan so it's always spinning at the same speed of the engine, see if it makes a difference. Reading spark plugs is dark art, don't rely on that for mixture indication. And the ignition could maybe still be retarded at cruise situations, and not noticeable under acceleration. – Bart Feb 28 '18 at 20:19
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    By the way, i'm personally a fan(pun intended) of mechanical fans. They are much more reliable/robust, tend to be more powerful for the same size, and make much less noise as electric ones. Especially if you already have an additional electric fan, i wouldn't replace the mechanical one if it's in good shape. You can build a fan shroud to dramatically increase effectiveness, if it's not already got one. – Bart Feb 28 '18 at 20:25
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    While taking a thermostat out of an older vehicle was a "quick fix", it's often not so for modern fuel injected vehicles. The thermostat can not only control the temperature of the coolant, but also the direction of the coolant. Taking the thermostat out could cause more issues than it solves. I'm not sure if the 4.0L Jeep engine is one of those, but you really need to know before you pull it out. Food for thought. – Pᴀᴜʟsᴛᴇʀ2 Feb 28 '18 at 21:12

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