Varying oil pressure guage?

[ImRich]

On a trip out this weekend, as I was going through a no-wake area, and the engine was freshly started, but up to 150F or so, I noticed the oil pressure guage bouncing (slowly) between about 20 PSI and 40 PSI.

I had checked the oil level so I didn't think it was low oil.

I upped the RPMs a bit to 1000 and it appeared to go away. After the engine was used for a while (so it was good and warm), the pressure didn't fluctuate when I was back at idle.

If this was my fuel guage, I would have known it was just the fuel sloshing in the tank, but I didn't expect to see my oil pressure varying like this.

Is this indicating an issue that is sneaking up on me? For example, is my oil pump about to fail?

I hadn't noticed this effect on the oil pressure previously. My engine is a 1990 7.4L Mercruiser with an unknown number of hours on it (but it runs otherwise smoothly).

[CountryME]

Was it jumping quickly or slowly and was it in a rhythm? Check your voltage and see if it is jumping too. Sometimes a bad voltage regulator (in the alternator) or a bad ground on the specific gage can cause a drop in voltage that affects the sending unit output. Could also be a loose alternator belt causing the same thing. I like to rule out the easy stuff that doesn't require an engine tear down first.

-JP

[pascavone]

i always believe the gauge, i think your low on oil or your have water in the crank case.

water will foam, and make the pressure go up and down.

look under your filler cap for cream foam, and the dip stick for oil and water.

engine wear will make it lower pressure, but the gauge would always stay low, like max 20 lbs.

[ImRich]

The guage was moving quickly from 20 to 40 psi, not slowly as a pressure would bleed off. Also, the volt meter was steady, so I don't think it was the alternator.

I haven't seen any evidence of water in the oil, but I'll look again closely.

I'm leaning towards a loose or corroded electrical connection, but the reading never went below 20, and stayed steady as I throttled up and it didn't happen at the end of the ride when the engine was warm.

[ss3964spd]

I'm in with JP.

If the gauge was mechanical I'd be a little concerned. My first guess would be the sending unit on the engine. Inexpensive to replace.

Dan

[ImRich]

It's an electical guage.

[ss3964spd]

Understood Rich.

Was just saying if it were mechanical I'd be more concerned. Mechanical gauges don't rely on various senders or other electrical connections to give a reading so are far less prone to false readings.

The oiling system on small or big block GM engines is stone reliable. If the oil level isn't low I'd suspect the sender first. If the oil level is too low I think you'd be seeing low OP readings all through the RPM range.

Dan