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My vehicle is a '97 Jeep Cherokee XJ with the 4.0L straight-six.

I'll try to keep this as short as possible, so I'll just ask that you please assume that there is nothing wrong with the cooling system. (I will elaborate the details of my cooling saga at the bottom of this question post). Driving around town all day, it's fine. Driving at 75+ mph, it starts to run hot, the temp slowly creeps up until it boils out of control. I slow down to 60 mph and the temp goes back down.

In theory, is it possible for air to flow so fast through the radiator that - due to some physics phenomenon possibly involving big words like laminar flow and boundary layers - heat transfer from the coolant to the air is reduced? And at what speed could this begin to take place?

My Jeep has the aerodynamics of a trash bag and the front grill is pretty much vertical, and there is nothing in front of it blocking flow, like offroad lights or a winch. I still have the factory air dam underneath the front bumper, the fan shrouds, and all other aerodynamic parts. I have added custom hood vents to the front of the hood above the cooling fans (for the hot air to escape) and I removed the AC condenser. So, what I'm saying is, the air should be flowing pretty damn fast through the radiator!

If you think there may be something wrong with my cooling system, you can go through it all here - http://dannix.net/lib/truck/cherokeeper-cooling - or you can take my word that everything that can be done has been done, except for upgrade to "heavy duty" parts like a stiffer fan clutch or larger radiator.

Last week I replaced the head gasket and had a machine shop flatten and rebuild the warped head. I went on test drive and... still running hot!

In short, I have replaced:

  • the head gasket
  • the radiator, twice. Now running a single row aluminum radiator from Spectra
  • all the hoses including the heater core hoses
  • fan clutch
  • thermostat
  • water pump
  • radiator cap

And I flushed the crap out of the cooling passages. There's no rust and flow is great. Here what it looks like inside:

enter image description here

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  • sounds like excess back pressure from a mostly clogged catalytic converter can cause that problem
    – Edward Evans
    Commented Jun 29, 2018 at 15:30
  • @EdwardEvans I haven't thought of that. How can I diagnose that or can a local garage check that for me? The previous owner had it replaced several years ago, iirc 8 years go, when it started making noises.
    – Dan Mantyla
    Commented Jun 29, 2018 at 15:36
  • The catalytic converter is not making noises now, BUT I will say that there is a lack of high speed power and the intake manifold vacuum is only 15 psi...!
    – Dan Mantyla
    Commented Jun 29, 2018 at 15:42
  • That's some hefty documentation going on! That's also one clean coolant passage. Holy crap. You've ran the gamut trying to fix this thing. I wish I had a good answer for you. I wonder if there are any hoses which are put in wrong, or if the water pump is actually a reverse flow from what it should be. I've seen stranger things.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jun 29, 2018 at 15:43
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    @DanMantyla - To check the cat, use your touchless thermometer to check the inlet/outlet temps. The outlet should be ~100°F or so higher when the engine is fully warm. If they are about the same, or the outlet is actually lower, there are issues with your cat. I doubt this is an issue if you are able to run at 70mph down the road though. A plugged cat usually doesn't allow your vehicle to gain much in the way of speed.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jun 29, 2018 at 15:46

4 Answers 4

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You can't go so fast that air passes though the radiator too fast. In general faster is better as heat transfer is more efficient when there's a large temperature gradient, so you want cooler air moving though the radiator and warm air moving away as fast as possible.

Something else is going on. I would check ignition timing (including checking the advance is working) and fuel air mixtures are correct.

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  • Thanks! Timing is controlled by the PCM and can't be adjusted, but maybe I should check it to verify that its working correctly. I replaced the distributor cap, rotor and spark plugs and wires when I rebuild the head. I don't know how to measure the air fuel ratio but the spark plugs look good if a little on the lean side and the Long Term Fuel Trim levels are on the negative side but I think that's because I'm using slighly higher-flowing fuel injectors than stock
    – Dan Mantyla
    Commented Jun 29, 2018 at 15:47
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Is it possible for air to flow too fast through the radiator? No. Physics is clear on this, the higher the airflow the more the cooling effect. It isn't until you get closer to the speed of sound that you can get weird compression and boundary layer effects.

As for what could be going wrong that isn't part of the water coolant system:

  • You could have the engine running too lean
  • The oil cooler may not be working efficiently, or oil may not be circulating properly. Water cooling is only part of the story, a substantial part of engine cooling comes from oil cooling, if the oil cooler is plugged or your oil pump isn't working then you lose a lot of cooling
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  • Thanks! There is no engine oil cooler but their is a transmission oiler cooler mounted in front of the radiator. Engine oil pressure is great, always around 40 at startup and while driving
    – Dan Mantyla
    Commented Jun 29, 2018 at 15:43
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    It may not have an oil cooler, but there is still oil cooling, especially in the sump, airflow is important there @DanMantyla
    – GdD
    Commented Jun 29, 2018 at 16:09
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Sounds like you have replaced most items suspect of the cause of overheating at high speed. The return hose from rad to water pump may be collapsing when water pump reaches high speeds. This will cause restricted flow of coolant hence less volume therefore overheat of block fluids. If you have been around vehicles as long as I have you may have removed "lower" radiator hoses and noticed a coil spring inside the hose. These were placed to prevent the hose from collapsing under high speed or hot conditions (older hoses get soft with age).

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Your engine is working harder at 75 than 60 ,so more waste heat. And the cooling system is marginal by deterioration or possibly design. It probably require 50% more horse power/waste heat at 75 than 60 mph.

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