Re: 1973 75hp overheating
Since I last posted, I thought about it a little more and I have changed my opinion: First a little practical theory:
The water pump impeller sits off center in the pump housing. Thus, as the impeller is turned by the drive shaft, the vanes, which are riding against the housing, are alternately bent , then released to straighten. If you can picture this, it becomes obvious that when bent, there is less room between the vanes. This is on the discharge side of the pump and is how water is expelled from the pump. This is the way the pump functions as a positive displacement pump at low speed.
At higher speeds, the pressure of the water forces all the vanes backward so they no longer touch the pump casing and the pump acts like a centrifugal pump.
It should be clear that in the 12 vane impeller, there is less room between the vanes so at low speed it would probably pump less water than the 6 vane impeller. At higher speeds it would not matter.
The 12 vane impeller was adequate for new components. I had a 90 with it and had no problems until it wore out.
The water pump casing has a plastic ring in it to locate the casing eccentric to the drive shaft and to keep the drive shaft centered in the upper seal. If the plastic ring is worn, the seal could leak, or if the seal was worn, you would lose water into the lower unit instead of having it pumped to the engine. Thus, with a marginal impeller, at low speed you would lose enough water to overheat, yet when at high speed, the water loss becomes negligible because of the added efficiency of the centriugal operation, and the engine cools off.
At this point, I would say first check the pump to be sure the seal is acceptable, check to be sure the pump casing is properly tightened and the stainless plate and gasket is intact. Do not overtighten the pump screws; this can cause warping or cracking of the pump housing. Also, be sure that the white plastic adapter is present in the pump housing and is not cracked or loose on the brass pipe, causing water loss.
As far as the bypass, it is a diamond shaped cover at the cutout of the exhaust cover. it is at the bottom between the electrical terminal and the exhaust cover. it is on the block nestled close to the lower left side of the exhaust cover. But as I said, be very careful. If you snap those screws. it is a major job to fix because of the inaccessability.
If the thermostat was faulty, it most likely would overheat at high speed when the demand for cooling is greater.
Also, while you have the leg down, pull the water intake screen ( white plastic sitting down in the leg right by the intake slots.) Be sure it is not partially clogged.
