Strange oil pressure experience

[Stelth]

I have a full-size GMC van with a 4.3l v-6. For most of the time that I've owned this vehicle, my oil pressure has been 20-25 psi at idle and 45-50 psi at speed, when fully warmed up. Not long ago, I had the oil changed by a quick-lube outfit with 5w30 Castrol and a filter that I can't remember the name of, but it was an e-core. Oil pressure was what I've come to expect until about 2000 miles into the OCI, when it dropped to about 20 psi at idle, and 35 psi at speed. I drove it for a while like this, but it made me uncomfortable, so I changed the oil again, this time with Max Life 5w30 and a Napa Gold filter. Oil pressure is back where I expect it to be, as above.

What gives?

 

[JHZR2]

Sounds like the filter media collapsed or had a big pressure drop or something...

 

[HTSS_TR]

At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.

 

[dave1251]

Originally Posted By: HTSS_TR
At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.


Yes you are correct GM's minimum spec for this engine is 1PSI per 1,000 RPM.

 

[chubbs1]

It isn't the oil, I will tell you that much. I am getting slightly higher oil pressure with Castrol Edge FST SN over Pennzoil Conventional SN.

 

[Stelth]

Originally Posted By: HTSS_TR
At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.


I don't know the rpm since I don't have a tach, but at the same road speed I was getting 10 psi less oil pressure, so something funny was going on. And if I put it in neutral and revved it way up, the oil pressure would not go past 35.

If the oil pressure sending unit is after the filter, I could see it being a filter problem. I've got about 4000 miles on my current oil change, and the pressure has been consistent throughout.

 

[The motor guy]

Originally Posted By: dave1251
Originally Posted By: HTSS_TR
At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.


Yes you are correct GM's minimum spec for this engine is 1PSI per 1,000 RPM.


That makes no sense. So they think that 5 psi at 5000rpm is ok?

 

[tinmanSC]

Originally Posted By: motor_oil_madman
Originally Posted By: dave1251
Originally Posted By: HTSS_TR
At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.


Yes you are correct GM's minimum spec for this engine is 1PSI per 1,000 RPM.


That makes no sense. So they think that 5 psi at 5000rpm is ok?

One assumes this was a typeo. 10PSI

 

[dave1251]

Originally Posted By: motor_oil_madman
Originally Posted By: dave1251
Originally Posted By: HTSS_TR
At what RPM that the pressure at 35 psi ? If it was 3000-3500 RPM or less then it is okay.


Yes you are correct GM's minimum spec for this engine is 1PSI per 1,000 RPM.


That makes no sense. So they think that 5 psi at 5000rpm is ok?


Thanks for the catch is should of been 1PSI per 100 revs. So at idle 10 PSI would be good and if his RPM'S where less than 3,500 it would have been good at 35PSI. But I have another question is this on a real oil pressure gauge or a factory unit?

 

[Stelth]

OK, let me try this again. The only person so far to offer any sort of a possible answer is JHZR2. Mostly I'm getting what I think are irrelevant questions.

So. What would cause an oil pressure drop of 10-15 psi when a vehicle is fully warmed up, with only a couple of thousand miles on the oil? And then the oil pressure goes back to normal with new oil and filter, and remains at the normal pressure for thousands of miles.

 

[Triple_Se7en]

They may have used 5W-20 instead of 5w30. Maxlife 5w30 is one of the thickest 5w30s.
(or)
Your prior filter wasn't working properly at 2K & beyond.

 

[Stelth]

I was under the impression that when the filter bypass opens, the oil pressure would remain the same as when the oil is flowing through the filter. Is that not correct?

This is a GM product, so the bypass is in the block rather than the filter.

 

[cp3]

The pressure difference from 5w30 to 5w20 would have shown up right away though, not after 2000 miles.

Thinner oil or less restriction is about all that could cause it isn't it? If the sending unit is past the filter and bypass I would think you would see some reduction in pressure when in bypass. To be constant like that the filter would have to be completely plugged I would think.

 

[mechtech2]

Sounds like oil shearing down thinner.

It still happens, and this would fit your situation and experience perfectly.

 

[Jim Allen]

What about fuel dilution?

 

[JR]

it sheared down some, filter, or fuel.
i have had a few 4.3 vortecs

98 w/197k Quaker State 10w30 winter and SAE 30 summer

93 w/295k Quaker State 5w30 or 10w30 used to 190k, mc 5w20 used to 275k, vr1 SAE 50 last 20k, also ran e85 last 50k engine still running in cousin's blazer, he uses it off road runs vr1 SAE 40 200hr oci's. I never noted any change in pressure with the 5w20 over SAE 30

94 w/275k oci's unkown before 225k, aunt used Quaker State 10w40 or castrol gtx 10w40 to 275K 3k oci's i have three cases of mc 5w20 laying in the corner may use it, i plan on 75-100hr oci's heavy dust low speed lots of idle, its an off road bogger now

 

[stxonall8]

what about if it wasnt really castrol and some b.s. bulk oil like most quick lubes do. They are most all crocks IMO.

 

[aquariuscsm]

Maybe a bad op sending unit?

 

[yonyon]

JHRZ2 is probably right. Something went wrong in the filter causing it to be bypassed. You ran some unfiltered oil for a little while, then you fixed it.

 

[hattaresguy]

The same thing happened to my 4.3 awhile back, an oil changed fixed it but it wasn't the oil. I'm blaming it on a filter failure, in my case it was a K&N.

Make enough of something and your bound to get bad ones.