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
- 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: