High Oil Pressure

[Jeb]

Two days ago, out of the blue, I started seeing oil pressures in the 70-80 range while at 1500-2000 RPM.

Prior to that, I would have 30 or so at idle and 45-55 when at speed (cruising).

Any ideas?

This is in a 110,000 L98 with no previous problems other than a blown head gasket about 8000 miles ago. That was fixed with two new gaskets and heads that were gone over and rebuilt to spec. The only thing done recently to the mechanics of the car was that I had one of the exhaust pipes on each muffler plugged to get rid of resonance. That was over a month ago and I doubt if this is related.

 

[n8dogg]

What oil filter are you running on it? might be the by-pass valve failed and is stuck closed

 

[boomdriver]

Two days ago, out of the blue, I started seeing oil pressures in the 70-80 range while at 1500-2000 RPM.

Prior to that, I would have 30 or so at idle and 45-55 when at speed (cruising).

Any ideas?

This is in a 110,000 L98 with no previous problems other than a blown head gasket about 8000 miles ago. That was fixed with two new gaskets and heads that were gone over and rebuilt to spec. The only thing done recently to the mechanics of the car was that I had one of the exhaust pipes on each muffler plugged to get rid of resonance. That was over a month ago and I doubt if this is related.

Its an electrical issue, not the engine.

I have been chasing that problem for several weeks now. Originally I thought it to be the sender, but its not. New sender did not solve the problem. next I thought it would be the wiring near the sender because wiggling the wire in the right way sometimes got results...but not everytime.
The manual ends up saying the gauge or cluster is at fault after a series of simple test demonstrate a predictable result by process of elimination. The only test thats left is to install a mechanical gauge and observe that for a day or 2 to rule out the filter bypass valve. I juct can't see the bypass jumping from 80 lbs to 10 or even going to zero while the engine is running. Shut down and immediate restart gets totally different readings.

I'm almost to the point of accepting that.

Its all about electrical resistance. 80 lbs (system max) is the same as the plug being disconnected. The closer to 80 the worse the connection (but where?) I'm starting to think in the cluster.
Because my old sender was all over the place, I replaced it. The new one seemed to be ok for about a week. Then it started the same old stuff.Exact same behavior.

It will also go to ZERO lbs oil pressure....don;t freak out ! Just shut it off, and restart the motor and I'm almost positive that you'll have some kind of reading. Remember that the oil pressure switch will shut off the fuel pump if there was really zero pressure....its supposed to, anyway.

I've been thru the grounds, and inspected the harness behind the motor. Nothing there.

 

[Jeb]

I've killed it and re-started with no change. It still reads a about 30 lbs high. It's never gone to zero or even close.

When cold, the OP reads upper 50s at idle. I'm thinking sensor or loose wire so I'll just start digging. I really don't think it's in the dashboard but it's possible.

 

[vetteoz]

Remember that the oil pressure switch will shut off the fuel pump if there was really zero pressure....its supposed to, anyway.

But the ECM still has the fuel pump relay powered up so it will make no difference.
Engine will still continue to run. :ugh

OP switch is not a safety switch ;
Is there purely as a backup should the pump relay fail ( and to give pump operation indication to ECM when cranking )

 

[boomdriver]

But the ECM still has the fuel pump relay powered up so it will make no difference.
Engine will still continue to run. :ugh

OP switch is not a safety switch ;
Is there purely as a backup should the pump relay fail ( and to give pump operation indication to ECM when cranking )

I was wondering about that after having posted that...

I know that is what it was described as, for many yrs. Then after seeing the drawings I could see that the OP switch could be laying on the shop floor and the car would still start with no pressure.

Somehow cutting fuel when/if the OP got too low just made sense...seemed like a good idea.




JEB,

if you remove the wiper motor you can have good access to the OP sender, switch and the harness section.
I found that pulling on the larger harness section made a difference. Moving the plug on the sender did at first, but stopped making anything change. This week mine wants to start out @ 80 and go down as it gets hot, while last week it was the opposite, cold starts at zero or 12 lbs and maybe get to 30 on the freeway.

I purged the oil system and changed the oil,. no effect.
I notice a small amount of dash flicker with no run ning lites, so I'll pull the cluster and look at grounds and see if I can spot the path for the gauge telltale.

 

[Jeb]

Thanks a lot. I'm not able to get to this right now because it's too humid for me to do much outside but I'll get on it as soon as I can. I'm not too worried about the engine getting fried but it is unnerving. I'm keeping a eye on it and if it suddenly drops, I'll kill the engine stat.

UPDATE: I was just out and about and the OP dropped to the high 30s when I was idling waiting at a drive up. Normal idle OP was 20-30. When I started moving again, it went up to the mid-50s. While at idle the OP moved a pound or two up and a pound or two down from about 38 lbs. It didn't stay steady like it used to.

I'm thinking I'll try an oil/filter change and see if maybe that helps. It's a cheap fix if it works, especially since it's almost due anyway.

 

[Jeb]

What oil filter are you running on it? might be the by-pass valve failed and is stuck closed

I'm going to have my mechanic change it out and look at that end of the system this week. That was my initial thought but I really don't want to drop the pan unless I absolutely have to.

I use AC Delco filters. I won't touch a FRAM. I wouldn't put a Fram filter on my lawnmower.

 

[n8dogg]

Jeb
I have had all types of oil filters fail yes fram is the worst one by far just a cheep fix if it works

 

[Jeb]

Oy. It appears to be the sending unit. Will be replacing it next week.

 

[JAKE]

I was wondering about that after having posted that...

I know that is what it was described as, for many yrs. Then after seeing the drawings I could see that the OP switch could be laying on the shop floor and the car would still start with no pressure.

Somehow cutting fuel when/if the OP got too low just made sense...seemed like a good idea.JEB,


Glad to see you guys caught that before it went ay further. I've seen it passed around on other Forums too, including that ". . .laying on the shop floor . . ." part. LOL

Yea, it does sound logical and would be a GREAT IDEA especially on LT1/4s since those engines will continue to run when the oil pump drive gear fails and OP drops to zero. Happened to me on my 96.

I was pulling out of a parking lot and just happened to glance as the dash - CHECK GAGES - plain as day. Then when I looked a bit closer, ZERO OIL PRESSURE! Gear died.

 

[Jeb]

Confirmed, it was the oil pressure sender. Replaced it and all is now well.

Thanks for everyone's assistance.

Damn! These cars are fun!