Weird cold oil pressure readings

[TallPaul]

Subject vehicle: '95 F150, 4.9L I6, 125,000 miles.

My oil pressure always (since I got a mech gauge in '93) read about 54 psi cold pretty much right after startup, dropping down to about 42 hot at 2000 rpm (using 10w40). Now that cold weather is back (26F this am) it only goes to about 44 on cold start and takes about a mile to get up around 50. Sometimes it doesn't get that high.

Two things happened this summer: 1) I ran 4000+ miles with the PCV breather unhooked from its filter daily through a 10-mile long construction zone (lead was 476 ppm), and 2) I changed my oil using my normal 10w40 but added an entire bottle of VSOT which should about push me to the boundary of a 10w40/10w50 in 100C viscosity.

I did notice that the increased viscosity with VSOT (about 2 higher cSt at 100C) gave me the same oil pressure as I would get without it before the internal sand blasting (PCV breather unhooked episode). So I wonder if the lower than normal cold start oil pressure is due to the thicker oil, the presumed greater clearances somewhere, or both?

Not a problem, just wondering. But I think my next oil change should be just straight 10w40 to see what happens.

 

[Gary Allan]

If your hot oil pressure is about the same ..then there's no difference in volume (aka: you didn't re-clearance your oil pump in the process)...so I would think (pulling my first, most likely, cause out of my behind= swag) is that your pump is in relief and your filter is restricting enough flow to reduce downstream pressure. This would sorta explain the elevation later in the cycle ..as the oil became more fluid. That is, you're probably @ 50 psi in both situations at the same time ..just that, in this case, you're under reduced flow until that point ..when before, you were not.

The way to tell is somewhat complicated ..but you're close enough to a nut like me to actually do it. You need a filter spacer like they use for Frantz tap offs ..and rig a gauge there too. You would then see what the terminal pressure is from the pump ..and the resultant downstream pressure. If you had that setup now, you would probably find the prefilter pressure had terminated/antenuated ..and your post filter was reduced. As you went down the road you would (probably) see the delta between the two converge to whatever was the static PSID across the filter media due to normal loading

.


Then again ..I could be FOS

 

[TallPaul]

Somehow I thought you might have something to say on this. The extra gauge would be fun, but probably make me generate dozens of posts about weird oil pressure behaviour. So maybe I best be content that the thing runs great and not watch the needle so much when cold.

BTW, the pump may have reclearanced a bit as the 16 cSt at 100C gives about the same pressure as the 14 cSt did before. But then again the difference is only a couple PSI and probably not significant to really say anything is absolutely different.

On the bright side, me being a thick head, I managed to make it so I can run even thicker oil in this truck and that ain't all that bad. Next summer I wll be fortifying my 10w40 with some Synpower 20w50.

 

[Gary Allan]

Soon, if you keep up the good work, you'll quailfy for emmigration to Australia. Right now, you're still on "training oil"

 

[moribundman]

"training oil"

 

[TallPaul]

Maybe I should unhook the PCV breather again?