Monday, November 20, 2006

VW - Cooling System

Although the Fresh Air blower housing does not use engine cooling air for cabin heat, as is shown by the geometry of the blower housing and the position of the internal air vanes, leaving the Fresh Air heater outlets open will cause a pressure drop that will reduce the engine’s capacity to cool itself. The Fresh Air outlets must be blocked or connected to functional heat exchangers (that is, heater boxes that aren’t rusted-out).

Hoping to improve the cooling of up-right engines, I ran some experiments with the Fresh Air outlets blocked off, different oil cooler configurations and so forth. Blocking the heater ducts provided no additional engine cooling, whereas leaving them open caused a sharp rise in CHT and oil temps.

Removing the screen from the upright oil cooler, a fairly common practice at one time, reduced the CHT measured at #3 cylinder but produced a rise in oil temp. Apparently the screen acts as a turbulence generator, allowing the air to pick up a bit more heat as it passes through the fin-less cooler.

Removing the up-right oil cooler from the blower housing and mounting an external cooler over the cooling air inlet produced dramatically lower cylinder head temps for #3 & #4 cylinders as well as lower oil temps. A small air dam must be installed in the blower housing at the bottom, near the location of the missing cooler to insure adequate air-flow to the #3 & #4 cylinder head.

Mr. Gene Berg’s claim that removing the upright cooler caused a rise in temperature may have been due to his failure to include the air dam. I was not able to reproduce his results with the air dam in place. Even without it, oil temp was lower than before although CHT on #3 rose slightly.

Assuming the timing and carb to be correctly set and the lower cylinder air deflectors are in place, if an early engine has a chronic cooling problem it’s wise to inspect the oil pump for wear. A worn oil pump reduces both pressure and rate of flow and the effectiveness of your oil cooler depends on flow rate. Replacing the up-right style cooler with the later model dog-house cooler is the best solution of all. (The dog-house style cooler appears to be about 4x as efficient as the up-right cooler.)

The other most common cause of overheating is failure of the engine compartment seal. When in motion the air under the vehicle is at a higher pressure than the air over the rear window and deck lid, allowing the engine to re-circulate the heated air from under the vehicle.

If the vehicle is habitually parked near trees the cooling problem my be due to leaves blocking the fins on the cylinder heads.

-Bob Hoover

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