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I would try a mechanical gauge in the line pressure fitting on the back of the block and see what it tells you. That is the only accurate way to know what you have. I doubt it is the pump or the thermostat. I have had this experience on two rebuilds. Most cases of low oil pressure on a Chrysler engine are main bearing clearance, rod bearing and lifter bore clearance being excessive.
The mechanical gauge will let you know for sure. If it still is a problem, it is internal. Oil pump pressure relief valve could be sticky but unlikely, try a can of sea foam in the oil if the mechanical gauge tells you it is internal. I cannot tell you how many problems you can run into with these Chrysler engines, oil pressure and rebuilds. Maybe you are lucky and the gauge or sender is bad
Bill again, internal parts from Napa o.k. Gauge and sending unit you should get from John at Marine Tech phone # is 717-684-4145. they have the originals that match what you have.
I would still put a mechanical gauge in it to make sure I would hate to see you turn a bearing. That old oil in there can eat the bearings up, there is a lot of acid in dirty oil. I still don't understand why people store a boat with dirty oil in it, I guess they just expect things not to fail and run for ever. I hope you didn't get stung on this buy. Best of luck, let me know if you have any other questions about it.
I've had a few sending unit failures. My flutuations happened when warm at cruise -neddle dives to zero and jumps back up intermittently. A mechanical gauge would have been little help unless it was run all the way to the bridge. On tell tale that I hadn't lost oil pressure was that I never got an alarm . My alarms are 9-11 psi so if presure had dropped below that as the gauge suggested, I would have expected at least a chirp. I replaced the sending unit and the problem went away immediatly.
I suggested a mechanical gauge because two people can acomplish the task. One drives the boat and the other watches the gauge. you can rig the gauge on a hose to the cabin and someone can watch the gauge as you run. it just might be the sending unit or gauge, but would you like to know for sure what kind of pressure you really have? I suspect with some shopping you could find a gauge and hose with fittings for $30.00 and have a good tool for future use.
