Oil Pressure

[Pmcb]

I have a 1991 Comanche Pioneer, 117000 miles, great shape, was show truck for many years with previous owner.  Coming back from long highway trip, oil light would come on at idle.  I had done oil change about 300 miles before so oil was good.  Short trips no problem but more than 20 minutes and light on at idle.  Changed sending unit and it stopped.  About a week later after 50 mile highway run light on again at idle.  (The light flickers on, never constant). I changed it over to pressure gauge.  Stay around 30lbs maybe up to 35 on highway, but after warm it droops to 10-15.  

No heat issue, never knocks, engine sounds fine, not burning oil.  I changed oil from 10w30 to 15w40 but no difference in pressure. Oil filter is located on left side of motor, up high and sending unit is attached just below.  I think this may be aftermarket mod, there are others.  

Could the oil reading be off because of location?  The engine seems to be running fine with no noise is 10-15 ok?

Attached a couple pictures of oil filter location and one of truck.

[MiNi Beast]

Honestly thats about right. 

[scaleless]

Are you saying its 10-15psi when warm on the highway?  What RPM is that?  If that's 2500+ RPM that's VERY low. I'm pretty sure the spec at idle is 13+psi and I assume the oil light comes on when it dips below 13.  Ideally I would like to see much more than that, though.  Before you do anything, though, rent an external guage from your local auto parts store and slap that on you verify your guage and sender.

[Pmcb]

The 10-15 is at idle after the ride, while still in drive but stopped, once I put the truck in park it goes up a couple pounds (I guess the gauge moves up slightly.

 

[Minuit]

Specification is a bare minimum of 13 psi at any time, and at least 37 psi at 1600 rpm. Maximum pressure is 75 psi set by the oil pressure relief valve. The oil pressure idiot light coming on at any time is bad news. A healthy 4.0 in my experience produces roughly 30 psi at warm idle and roughly 60 psi at cold start.

 

That oil filter and sending unit setup is correct for a '91. Make sure you're using a decent filter. No orange Fram cans of death. The higher end Fram filters are fine.

[AZJeff]

1 hour ago, Minuit said:

A healthy 4.0 in my experience produces roughly 30 psi at warm idle and roughly 60 psi at cold start.

A lot of what you say there is dependent on ambient environment.   Here in PHX,AZ, even a brand new 4.0 won’t run 30psi in the summer at warm idle.  With 10w30 oil, it “might” make 25psi, and most of them will be closer 20psi.

 

For that reason, I always recommend using the shop manual specs to judge engine oil pressure as to whether it is “good” or not.

[scaleless]

10 minutes ago, AZJeff said:

A lot of what you say there is dependent on ambient environment.   Here in PHX,AZ, even a brand new 4.0 won’t run 30psi in the summer at warm idle.  With 10w30 oil, it “might” make 25psi, and most of them will be closer 20psi.

 

For that reason, I always recommend using the shop manual specs to judge engine oil pressure as to whether it is “good” or not.

Ambient temperature plays a big role in viscosity at startup, but once the engine reaches operating temperature it should be extremely negligible.  It's been nearly 100F outside every day and my new 4.0 is running pegged at max pressure until it's warm, but drops a little to 55 or 60 when warm at idle.  It does have a brand new Melling pump though - not sure how much that impacts it despite it being labeled as standard volume.

[Minuit]

1 hour ago, AZJeff said:

A lot of what you say there is dependent on ambient environment.   Here in PHX,AZ, even a brand new 4.0 won’t run 30psi in the summer at warm idle.  With 10w30 oil, it “might” make 25psi, and most of them will be closer 20psi.

 

For that reason, I always recommend using the shop manual specs to judge engine oil pressure as to whether it is “good” or not.

That's why I said "roughly." My posts run on long enough as it is. To really say what I really mean and account for all of the "well, actually" situations, digressions, and your-mileage-may-varies out there, I would have to make encyclopedia length posts full of footnotes. Ain't nobody got time to write or read that.

 

My '91 has an aforementioned healthy 4.0 with very nearly 200,000 miles on it now, and it produces those numbers quite consistently. Temperature might have an effect, cold start oil pressure showing the biggest difference, but it's a small one and well within the limits of "roughly".

 

As an aside, look at the OP's previous posts - one is about getting 11 mpg in highway driving in their truck, which I would say is indicative of an engine in a poor state of tune in one way or another. In the pictures, note the horrific placement of the air intake. The only way that air filter would be sucking in hotter air would be if it was tucked under the exhaust. The OP's engine is not at the top of its game. If it's been running bad enough to get 11 mpg on the highway, who knows what other abuse it's been through to have low oil pressure at such low mileage. Based on the numbers OP provides (if they are trustworthy), their engine is towards the end of its life, but it's not dead yet.

 

 

[MiNi Beast]

End of its life indeed. 

[75sv1]

I change the oil pump on my '98 XJ. I went from where the OP's oil pressure is at to roughly 25-28 psi at idle and 205F and 35-50 psi at 2000 rpm. I did have to replace the crankshaft and bearings as the harmonic balance went bad. 

[ruralandalone]

I put the new sensor in this morning.  Seems to work fine.  The needle seems to periodically fluctuate, but it is certainly working - and is a great improvement.  Thanks for all of the help!  All I have to do now is find the short of the instrument lamps, and I'm good to go!