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Old Yesterday, 10:45 PM
indigo indigo is offline
 
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Back of the Envelope

I think we can get a little better idea of Clark800's cooling situation with a simple thermodynamic analysis. Here goes my shot at it:

You can reasonably assume that an A4 running at 2000 RPM in neutral (I think that was the operating condition) is at most consuming .4 gph of Fuel. With gasoline rated at about 115,000 BTUs per gallon, the energy input is about 46,000 BTUs per hour.
If you do a simple energy balance on the engine, energy IN has to equal energy OUT. The tricky part is the energy out in that it takes the form of heat rejected to the cooling water, heat in the exhaust gases, heat rejected to the engine compartment, power actually generated, friction losses, and any output by the alternator.
By and large, most of the energy OUT will be in the cooling water, especially with the engine being in neutral. We know that there are 4 gallons per minute of water flowing through the engine and the exiting cooling water temperature is 84° F. I found out from Clark800 that the inlet water temperature in the San Francisco Bay area where he was operating is 65° F. The heat rejected to the cooling water is found by simply multiplying the mass flow rate of water, in pounds per hour, times the temperature increase or delta T. Doing that math yields heat rejection to the water of about 38000 BTUs per hour. That leaves about 8000 BTUs per hour for everything else which, given the operating conditions, seems perfectly reasonable.

What I am suggesting is that the cooling system is doing its job quite well and the real overheating problem is a matter of an erroneous temperature indication. Either there is a problem with the instrumentation itself or there is some blockage or cooling water flow stagnation in the area of the sensor causing the high temperature readings. Without pulling the head, you really cannot ascertain how clean the head is or how open or closed the oval shaped cooling passages on the manifold side of the block/head are. The openings closest to the flywheel are the ones that provide coolant flow across the temperature sensor. If they or their coolant source are totally occluded, no amount of acid flushing will help to provide any flow through those passages and the temperature indicated will continue to be too high.

Tom Stevens
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