Back to the Jabsco pump impeller
The larger volume Jabsco and large volume Johnson F6B-9 crankshaft pumps both use a very similar impeller.
The OD of the vanes is identical (A below), the vane count is identical, the shaft spline size and count are identical (shaft diameter below).
Either impeller will insert into either pump body!
The Johnson F6B-9 impeller measures 1.900" in depth (B below).
The large volume Jabsco impeller measures 1.990" in depth (B below).
This is a .090" difference in depth and is difficult to see with the untrained eye.
The Jabsco impeller will not fit correctly into the Johnson pump body (it will sit proud of the body by approx .100").
However, the Johnson impeller will slip into the Jabsco pump body.... but will be shy .090" of the pump body bore depth.
Parts guys will occasionally give out the wrong impeller for the older Jabsco large volume crankshaft pump.
If the Johnson 1.900" impeller is inadvertently installed in the Jabsco pump, the .090" deficit will prevent the pump from creating the necessary suction.
In which case, you will have over-heating problems.
I'd highly recomend that you start at the drive. Make sure that nothing is obstructing the pick up area, including the area that leads to the pivot tube... of which doubles as a water tube!
The water neck fitting is a wear item, and must be routinely replaced. Corrosion takes it tole on these.
The "S" hose must be good and with no signs of cracking, etc.
The seawater pump suction hose must be good and tight at either end connection.
Once past the suction side, any leaks are easy to see.
As said, pull the H/E end caps and look at the tube bundle. The ID of the tube must all be clean.
These can be carefully rodded out with the correct size wire rod.
Pull that splitter fitting out and look at it. It should have a divider plate in the center of it.
Remove the exhaust elbows and look at the seawater transfer ports. These can become rust restricted and can cause a big issues.
At the ends of each elbow, you'll see the scallopped areas where the spent seawater enters the exhaust gas flow.
If the rubber couplers are being compressed into the scallopped areas, it will restrict outgoing spent seawater.
If the seawater cannot enter and leave seawater side of the H/E tube bundle rather quickly, it will not be able to remove heat from the E/G coolant solution.
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