Low oil pressure but engine runs great

[JM in KY]

I had assumed my oil pressure gauge was just wrong, but maybe not…

Recent purchase of 1987 GMC V1500 with 350 engine. Other than paint, nearly everything is original with 114,000 original miles.

Engine has always run quietly for me. No valve train noise, etc.

Oil pressure gauge usually reads about 10psi. Doesn’t matter if it’s idling, reviving, cold, or hot. After reading some posts on here, I deciding I should investigate the gauge and the sender.

To test the gauge, I disconnected the wire from the sender and it pegged above 60. I then grounded the same wire and it pegged at zero. Both correct from what I’ve read. Gauge probably fine.

I replaced the sender. Can’t find ACDelco, so I got the one Advance Auto recommended (Carquest or something like that). No Teflon tape that might prevent grounding. I get exactly the same reading on the gauge.

I changed the oil and filter, thinking the old filter might be clogged. I even changed from 5W30 to 10W30, hoping the increased viscosity might help me. Wix filter. The engine was already quiet, but now I can barely hear it run at idle if I’m in the cab. Again, gauge shows the exact same reading.

Occasionally the gauge will go up to 15-20psi, but this seems random and usually will be like that from startup until I shut it off, regardless of how long it runs.

Thoughts? I’m sure there will be suggestions to check with a manual gauge. I’ll do that, but I don’t own one of those, so I’ll have to get one. Anything else I should be checking or planning to work on?

 

[Memaloose]

My '75 350 always runs at 30-35 PSI. If it's over 100 degrees out it runs about 25 at idle. I run 10W30. I have 2 mechanical gauges, factory in dash and SW in a panel. It seems low to me and I'm always watching the gauges but like you, the engine runs quiet and smooth.
Try hooking up a mechanical gauge where your sender threads in and check it that way as well. Otherwise, I wouldn't worry about it.

 

[Big Ray]

Put a good aftermarket guage on it. See what it's actually doing.
Factory guages aren't known for accuracy.

 

[TotalyHucked]

I agree with these guys. If you don't have one, see if a buddy has a known good mechanical gauge and throw it on to see what it says. If not, just buy one and hope it works correctly. Stick with a name brand like Stewart Warner so you have the best chance of it being good out of the box

 

[Radiohead]

Cam bearings worn?

 

[Rusty Nail]

Are you  certain the correct sender is installed? Worth looking into.
There is more than one possibility.

My second thought is Castrol sludge in the oil passages. Marvel Mystery Oil in the crankcase never hurt nothin.

 

[Ricko1966]

I'd definitely put a good mechanical gauge on it to verify your oil pressure. Rule of thumb is 10psi per thousand rpm so at say 3000 you should have a minnimum of 30 psi. If pressure truly is as low as you say,now is the time to rebuild it. If wait too long the price goes way up. You know I stepped up with that answer a little too quick, if your pressure us really that low I'd put an oil pump in it if you have the ability.Then recheck your pressure.

 

[fast 99]

Bought a reman 350 engine once that barely moved the pointer at idle and never went above 30. Not something I would have desired but didn't hurt this engine. Was still going at 125k when I moved to another job.

 

[Gray Beard]

In 1987 the TBI trucks had two sending units. One for the gauge, the other for the TBI fuel shut off. One is bigger the the other, can't remember which is which.
Make sure you changed the one for the gauge.
Oil pressure: 10psi for every 1000rpm is fine for a stock engine taking in account for the relief valve cut out at around 60psi. A good mechanical gauge can be tapped into where the the sending unit is for a good check.

 

[bucket]

In 1987 the TBI trucks had two sending units. One for the gauge, the other for the TBI fuel shut off. One is bigger the the other, can't remember which is which.
Make sure you changed the one for the gauge.
Oil pressure: 10psi for every 1000rpm is fine for a stock engine taking in account for the relief valve cut out at around 60psi. A good mechanical gauge can be tapped into where the the sending unit is for a good check.

Minor correction: That other sender does not cut off the fuel pump, it's actually a redundant power supply for the fuel pump. The way GM did it, a failed pump relay only causes an extended crank time, rather than a non-running vehicle.

Sorry to harp, but it's an old wife's tale that is a pet peeve of mine.

 

[JM in KY]

Going to close the loop again and update everyone on my problem here. You can read my initial post, but basically checked everything I knew to check, put in a new sender, and changed the oil.

I finally got a good manual gauge and plugged it into the port for the pressure sender next to the oil filter. I was pleasantly surprised to see it register 40psi at cold idle and rev above 50 psi. That was a huge relief after only getting 10psi on the electric dash gauge.

Since I knew my pressure was ok, I decided to put the original sender back in just because I was curious. Somehow, my dash gauge now reads pretty close to what the manual gauge read.

Long story short, tinkering around and ultimately putting the original parts back in seemed to fix it. I can’t explain that, but I’m glad to know the oil pressure is normal and to have a gauge that shows it.

 

[GTX63]

The last 3 squarebody trucks I bought have all had an aftermarket oil pressure gauge. I'm seeing a pattern.

 

[geocrasher]

Minor correction: That other sender does not cut off the fuel pump, it's actually a redundant power supply for the fuel pump. The way GM did it, a failed pump relay only causes an extended crank time, rather than a non-running vehicle.

Sorry to harp, but it's an old wife's tale that is a pet peeve of mine.

Could this be the reason my truck sometimes starts immediately, and sometimes cranks for a long time? That sounds like an easy fix...

 

[Gray Beard]

Minor correction: That other sender does not cut off the fuel pump, it's actually a redundant power supply for the fuel pump. The way GM did it, a failed pump relay only causes an extended crank time, rather than a non-running vehicle.

Sorry to harp, but it's an old wife's tale that is a pet peeve of mine.

Bucket,
Thanks for the correction!

 

[bucket]

Could this be the reason my truck sometimes starts immediately, and sometimes cranks for a long time? That sounds like an easy fix...

There could be another issue, but yep, it's 100% a possibility.