Low Oil Pressure

[GeneM]

I have twin 6BTA-370 5.9 Cummins 1998 mechanical and the port engine is somewhat worn (two cylinders have marginal compression). The blow-by seen from the oil fill cap opening is substantial.

This means I'm putting a bunch of carbon in my oil. After about 40 hours, the oil pressure drops substantially. I think that happens because the filter gets plugged and the filter bypass opens. I'm just curious if anyone else has experienced this.

I plan to overhaul the engine soon and install bypass filters, so no need to talk about a solution.

[sailorchic34]

I would find out where the oil pressure relief is and make sure the valve is not fowled with crud. (remove, clean, replace) Sometimes it's integral with the pump (Hard to get too), sometimes installed inside the center of the oil filter mount.

Oil pumps are positive displacement pumps and they require a pressure relief valve to prevent over pressure when the oil is cold. This is different then the oil bypass in a oil filter, which is there to prevent break through of the filter media when cool.

Check that first. Blow by could be a blown head gasket. OK most times it's bad rings but check the easy stuff first.

[Lepke]

Low oil pressure is caused by worn oil pump, open or semi open pressure relief valve, worn bearings, oil diluted with diesel or water, light weight oil, etc.
If you have 2 cylinders with low compression, it could be a head gasket, or crack. It also could be bad injectors. Move the injectors and see if it changes.
The oil filter bypass is designed to open when flow isn't enough to supply proper lubrication. It shouldn't restrict pressure.
Keep the soot in the oil under control. Install a bigger filter or dual filters. Too much soot in the oil will restrict and eventually clog smaller oil galleries. Usually those going to the head/valves and turbo. Low flow in the turbo bearing will not remove enough heat and the turbo will fail. Oil also removes excessive heat from the rockers and valve tops.

[GeneM]

Thanks all. I knew I'd get some good advice. I'd better get that engine done soon.

[a64pilot]

Real easy to find out, once pressure drops, put a new filter on, if it recovers, then you have most likely found the issue.
I’d be astonished if the filter is plugging up myself.
You can also cut open the filter and inspect.

More likely I think is fuel dilution of the oil making it lose viscosity.

[GeneM]

Maybe it is fuel getting into the oil. Next time I'll change the oil and leave the filter. That will tell me. When I change the oil and filter, the pressure recovers.

[wsmurdoch]

If fuel were getting into the oil, wouldn't the oil level be rising?

[GeneM]

The crankcase is pressurized and is pushing oil into the Air-Sep and turbo. It loses a quart after a half day running.