Thread: fresh rebuilt 350 overheating rapidly and extremely low oil pressure

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Heater hoses can't affect your cooling except in extremes. Your heater valve might not open against reverse flow, but the only impact would be no heat. I'd say that you have air trapped in the cooling system, which will block coolant flow. Some Jeep models are known to trap air and have to be nose high to get the air out, either parked on a steep uphill slope or jacked up to an extreme. Later models had a 1/2" pipe plug in a high point in the radiator hose, and factory instructions were to fill, start the engine and remove the plug to allow all air to escape, then quickly put the plug back in place and run the engine at 3000rpm for a minute or two to purge any remaining air. I always drill a ~1/8" hole in the t-stat plate to let the air out of the block during fill, but you mentioned you've pulled the stat so there's no restriction there.

You mentioned the cooling fan clutch. The thermostatic clutches are known to fail over time, but you should be able to hear if it's engaged or not by the roar of the extra load. I can't say this is right, but from another forum:

The way I diagnose a fan clutch is pretty simple. Start with the car off. Jiggle the fan forward and back. If there is any play at the clutch shaft, its bad. Give the fan a spin. It should move freely, but stop almost instantly, as if the clutch were filled with mashed potatoes. Start the engine. The fan should turn pretty briskly; almost as fast as the crank. Now use a rolled up newspaper to gently try and stop the fan. You should be able to when its cool. Throw away the ruined newspaper Rev the engine, you should notice that the fan increases speed, but not proportionally to the crank speed. The clutch is full of viscous fluid, so the air friction on the fan starts overcoming the friction of the fluid in the clutch and it stops accelerating with the engine. The fan won't roar. Then let it get hot. You might have to drive it around some stop and go streets to get it to lock up the clutch. Now when you acclerate, you should hear the fan roaring a little louder. If you stop the car, shut off the engine, and try to spin the fan, it shouldn't move.

A bad fan clutch won't come into play quickly. If you're seeing a rapid temperature increase you've got a flow problem, IMO.