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Murphy
07-10-2000, 08:13 PM
I'm getting bad readings on oil pressure. Spec is 30-60 psi. When the engine is cold I'm getting about 40. As it warms up and the oil thins it reads a steady 20... a little low for comfort. At idle it's been holding at about 5-10. Last night as I was coming into dock the gage read 0 and the oil light came on. So far no valve noise or any other symptoms. I know the dash gage is only so accurate and I will test with a mechanical gage. When I suggested to a Merc mechanic that I was thinking my main bearings might be going he asked what kind of oil I was using. I said straight 40W Pennzoil. He said that standard automotive oils are unsuitable for high performance marine applications and that the Pennzoil probably started breaking down after about 20-hours of operation. He told me to replace it with MerCruiser High Performance oil saying it holds viscosity better and may give me back my oil pressure. Of all the things I was thinking, oil blend wasn't one of them. Anybody else ever hear anything like this? Thanks.

Murph

Forrest
07-10-2000, 10:49 PM
You can try changing you oil, but I'll bet you that it won't make five pounds of difference. You have problem. Either with the oil pump, excessive bearing clearences, or a faulty gauge. Just for grins, try sticking a wet direct reading oil pressure gauge where the oil pressure sending unit is now, and see if it reads any different than the electric gauge. That way at least you will eliminate the gauge. Doesn't sound good.

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Forrest

WildBill
07-10-2000, 11:01 PM
I have had the same problem in two different boats. The first had a Volvo 280 with a Chevrolet 350 with only 320 hours on it. At idle the guage showed 0 lbs pressure and sometimes the low oil pressure buzzer would sound. I installed a mechanical guage in the engine compartment right next to the hour meter. It read 10-12 lbs at idle and maxed out at 55 lbs at 4600rpm. I then replaced the sender on the dash guage and same readings as before. I then took the wire to the sender and clipped/installed a new pressed end to go on the threaded stud on the sender and removed and lightly sanded the other end at the guage, and reassembled it with corrosion block. Since the guage/sender works off resistance in the circuit this took care of the problem. Ditto on the second small block chevrolet with a mercruiser. I have always run Castrol oil in all of my marine engines, 20-50 wgt, and tried to stay completly away from parafin based oil, it breaks down quicker. Hope this helps..good luck..

WildBill

RickR
07-11-2000, 01:40 PM
Murphy
I agree with wildbill
Check the sender 1st. If it is leaking oil replace it. When they start to go bad guage reads ok when cold but when engine gets to normal operating temp it will read low.
You engine is not overheating is it?
You said your light came on, what light?
RickR

Murphy
07-11-2000, 06:00 PM
Thanks for the ideas. The engine is not overheating which is in my favor. The red light I mentioned is what we used to call an "idiot light" for temp., oil, and low fuel mounted on the dash. I haven't taken time to trace it but I gotta believe it feeds from the dash gauge somehow. Anyway, it has never come on until that oil gauge bottomed out the other night. Made me real nervous. I'll check all connectors as advised. I'm changing oil Friday and will check with a mechanical gage then. I noticed that the VDO oil gauge requires a special sender that I'll have to order. The automotive ones I've tested don't have the same resistance specs.

Murph

Forrest
07-12-2000, 09:34 AM
Those buzzers and(or) idiot lights are connected to a separate oil pressure switch this is no way connected electrically to the gauge sending unit. The only exception that I know of is Gaffrig Redline gauges. Those oil pressure switches are usually rated to make contact somewhere around 5 to 7 psi. You have two separate indicators telling you that you have low oil pressure . . . but check it with a mechanical gauge anyway.

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Forrest