Oil pressure decreases with engine speed???

[Ranman]

I was running my boat the other day and noticed that the VDO oil pressure gauge needle actually travels backwards with added RPM. At idle it reads around 40psi, at 3000 RPM it reads 30 psi and at 4500+ RPM it reads 20 psi. It is an electronic gauge and I don't have a mech. one to test it with yet. My guess is that something is wrong with the gauge / sender or ground. Has anyone seen or heard of this? I don't think it is physically possible for oil pressure to decrease so drastically with added engine speed. Any ideas?

[Forrest]

How old is the engine?

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Forrest

[Ranman]

Forrest, The boat is a 97' with the original stock Merc MAG 350. The hourmeter "says" 110 hrs. The condition of the boat seems to support that.

[Forrest]

First, if you in any way suspect the gauge, you need to eliminate it from the equation. Pick yourself up a mechanical "wet" gauge and thread it in the hole where the present electric sender resides. Run the same tests again and note the readings on the mechanical gauge. If you see basically the same readings on the mechanical gauge as you saw on the elecric gauge, well then . . . let's not go there for now and hope that it's just a gauge problem.

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Forrest

[This message has been edited by Forrest (edited 08-03-2000).]

[Murphy]

Ranman,

I have a similar situation with the VDO oil pressure gage on 22 with the Tempest 300. I've spoken with two other guys with the VDO/Tempest combination who say it happens to them too. At 3k RPM I can be at a solid 40psi all day. Shoot up to 5k RPM and watch the gage drop down to 25psi. VDO makes their own sender. It's available through Freeport Marine 1-800-645-2565, part #360-003, $28.99. The VDO oil pressure gage description says you must use the VDO sender. I tested some stock GM senders and the resistance specs were different. Could be someone replaced yours with the wrong one at some point. GeneD is right though, the only way we're ever going to know for sure is to test with a mechanical gage. If that holds pressure, I'd suspect the sender.

Murph

[Murphy]

Oops, sorry Forrest, thats your thread. Got some previous advice on this from GeneD.

Murph