Is it possible for air to flow too fast through the radiator? My engine runs hot at 75 mph but not a 60 mph

Question

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:

Dave Smith · Accepted Answer

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.

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

GdD · Answer

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

james · Answer

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).

Answered by james on April 3, 2021

blacksmith37 · Answer

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.

Answered by blacksmith37 on April 3, 2021

Is it possible for air to flow too fast through the radiator? My engine runs hot at 75 mph but not a 60 mph

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