Except if.................

C'mon somebody.

 

My first reaction was whats going on. Let off the gas & oil pressure jumps up. Speed up to 70 MPH & 3400 RPM & Oil pressure is back down to 25#. So I kept the RPM's down & limped the last 12 miles home. The oil pressure would just kinda dance around almost like the oil was foaming up or may be had some air in line to gauge. Stopped at auto part store bought some 10w-30W PennZoil & changed it out added Z Boost from Motor Head. Cranked her up. 45# with the cold fresh oil. down to 30# after warming up. Ran on a 25 mile jaunt on I 35 to bring up temps. Steady as a rock at 45#. I'm thinking the oils viscosity thinned out to much for the vintage oil pump? Sure was strange thou.

 

 

we're waitin for you

 

Should have read more closely, the OP does have a different type of symptom, and yes I understand that cold oil will have higher pressure than operating temp. It's just that for the first few weeks the pressure stayed basically the same until I switched to the
Mobil 1 10W-30. I still have good pressure, but will probably change to an oil with the recommended ZDDP for our engines.

 

sorry couldnt resist

 

If the pump sucked some air pressure would drop. Oil pressure relief valve spring? Nut behind the wheel?

 

To the OP - did you change the filters with each oil change - and if so, were they all EXACTLY the same filter?

 

ok i will bite......more flow less pressure....

 

I think he's watching the tach not the oil pressure gauge

 

there's a multitude of possible causes, collapsed oil filter, low oil level, slow oil drain back, bypass valve issues,etc.etc.etc. I almost remember one specific to this but CRS kicked in

 

Yeah, I've been kinda thinking it through the same way. Pressure is resistance to flow - so either the flow, or the resistance, changed.

I'm admittedly out of ideas

 

Replaced with same filter both times. Had to be viscosity was lower for a given temperature. Don't know. But makes since if other people are not having any problems with Mobile 1, 15W- 40.
I admit i don't know what was going on with it. Ive had over 30 cars & 5 heavy trucks over my life. Never seen one act like that.
Thats why I posted on forum.

 

Oil pump is losing pressure under rpm. Maybe it leaks, maybe it sucks air? Maybe bearings on this engine are just worn and more slicker Mobil brings this out before the dino oil?

 

Another idea would be to check the actual oil pressure with a mechanical gauge other than the dashboard oil pressure gauge which may be giving you suspect readings!

 

First point, If Mike Ward has information that would contribute to the thread post it. If not why post? I see no sense in playing trivia games in a thread. Or you can sit their until someone comes up with a logical cause then post "Yes! That is exactly what I was thinking". Either way it just seems like ego stroking.

I have been thinking about this and might have a couple possible causes of why the OP is seeing this strange oil pressure issue. The smaller molecule synthetic will drop oil pressure some accross the board running the same viscocity as conventional due to its ability to flow through smaller gaps than conventional.
1. Worn main bearings or thrust surfaces could be the problem. When the engine is under load the crank is pushed down into the bearing cap. Oil feed holes are at the top so this opens clearance up where the oil is under pressure resulting in pressure drop due to the small molecules being able to escape the gap faster than they can be fed in. When you let off the crank is freewheeling with less downforce and more uniform clearance at all points on the circumference of the main bearing journals with less oil (pressure)escaping.
2. Worn oil pump. Similar situation. Gears are loaded different under load than when engine freewheels. Gears or case could be worn more where they mesh under load and less where they mesh when freewheeling.
The smaller molecules could be why these symptoms show up with synthetic and not conventional if one of the above problems exists in the engine.
I don't know for sure if either of the above scenerios would cause the issue but it has been provoking some thought on the subject.
that a heavier weight synthetic may clear the oil pressure drop under load issue up. I would try and figure out the underlying cause though.

 

You really need to get that chip off your shoulder. Gets real boring.

NOBODY here has ever driven a car and seen the oil pressure drop off when cruising then come back up when slowing down? I don't believe it. Nobody here was broke all the time?

 

???

 

I Think maybe Mako is right. Bought the car Used.
5000 mile since rebuild on engine. Don't know who Rebuilt it or what kind of parts they put in it. Just a receipt of when it was rebuilt. Saying it had new cam. Crankshaft ground. 10 under & Cylinders bored 30 over. Heads ported & polished. Runs great thou.
Still want to run synthetic in it. But it'll have to wait until next oil change. I'll try Mobil 1 15W-50 at that time. Let you know how it works out.
Thanks for the feed back. it is what it is, when you by used.

 

Mike, if you've got the solution or a perspective - just share it.

The intent of the forum is to help fellow Corvette owners, right?