Oil pressure dropping a lb or 2 a day

[bobmook]

OK I wanted to track this long enough to make sure I wasn't being alarmist or paranoid. I've had the car (84 vette 60k miles) a month and a half. When I got it at highway speed the oil pressure was 55-63. About two weeks ago I noticed it was running around 44 at highway speed. I figured it must need a change so I had the oil changed, synthetic 5w 30. Since then oil pressure (at times and for as long as 5 minutes) dropped to 42 then 40 and this morning was running at 37 @ around 60 mph. At times it still jumps up to about 55 psi.
Here's where it get's weird. At idle the oil pressure is still excellent, between 27-32 once the car is warmed up. The problem only seems to manifest itself at highway speed. I know of nothing the previous owner could have done to have temporarily raised the oil pressure for three weeks or so in order to sell the car and the maintenance records show oil changes like clockwork every 3k.
Does anyone have any ideas? This is making me nuts (reminding me of when I bought a Jaguar XJS w/ excellent records and spent $4000 rebuilding the V-12 two months later)

 

[bradvette]

could be the oil sending unit on the fritz.

 

[bobmook]

That's what I suspect as well but I'm confused as to the difference between the sending unit and the oil pressure switch.

 

[Vettefan87]

I would get a analog gauge and hook it up directly to the motor and compare the different readings you get. When you say that sometimes it still jumps up to 55 and then back down, makes me think that, the sending unit is bad, because mine did that.


Justin

 

[G Winter]

The oil pressure sending unit has a very small hole for the oil to pass through. A very small piece of varnish could come lose and end up flooting in front of it. The sending units can also get sticky with age, replacement might be a good idea.Old sending units have a bad habit of leaking too.I would go for the analog guage and check presures. If oil presure was really droping that fast, I would think the engine would be history already . Good luck .

 

[Hib Halverson]

That's what I suspect as well but I'm confused as to the difference between the sending unit and the oil pressure switch.

The sending unit sends a signal to the IP which reads out in oil pressure. The oil pressure switch, cuts power to the ign. if oil pressure goes away.

 

[bobmook]

Well based on some experience with electrical circuits and limited knowledge of the combustion engine it appears to me I'm developing an open in the oil pressure circuit. I've never ever seen an engine in any condition where the oil pressure jumped up and down so widely at constant rpm, but I have seen a LOT of electrical/electronic devices where either the device was going bad and/or power/ground was becoming intermittent with results a lot like this. I hope to replace the oil pressure sensor (If I'm right that's the one w/ one terminal as opposed to 3, please please correct if I'm wrong) this weekend and see what I can see.

 

[brookman]

The sending unit sends a signal to the IP which reads out in oil pressure. The oil pressure switch, cuts power to the ign. if oil pressure goes away.

Not to be augumentative - but - the oil pressure switch is only there to apply power to the fuel pump in the event that fuel pump relay has failed. so - unless your fuel pump relay has indeed failed the engine will run until it burns up.

I have a friend that experienced this first hand.

 

[geekinavette]

The oil pressure sending unit and switch are in the same unit, the three-terminal sender behind the intake manifold. The single wire unit by the filter is the oil temp sensor.

The oil pressure switch provides power to the fuel pump relay, not directly to the fuel pump itself, nor the ignition system. When you turn the ignition on the ECM will energize the fuel pump relay for a couple of seconds, when you start it the oil pressure switch takes over and keeps the fuel pump relay energized.

Bill

 

[bobmook]

Just a quick follow-up the oil pressure has since returned to normal on my car. Either the sensor is failing slowly or there was some sort of blockage. Having the engine flushed and replacing said sensor.