oil pressure in an 8V71

[Bob & Tracey]

Our 8v71 has less than 10psi at idle and about 30psi with the engine reved, I understand that this is ok but this engine has less than 40,000 mile on it and I would expect better. I just replaced mechanical gauge in the rear of the bus with a new one so I feel the readings are good. I read an old post about an alternator causing a low oil pressure problem so I took my alternator out of the oil circuit and saw 60 psi at idle with the engine cold. Should the alternator cause the engine to lose so much oil pressure? Any thoughts?

Thanks,
Bob

[Melbo]

when I had my 8V71 the idle was about 15 and 50 at speed

I think mine was due to be rebuilt

HTH

Melbo

[Utahclaimjumper]

If I remenber right, the alternator has a restrictor in the line to limit pressure loss.>>>Dan

[Tony LEE]

My alternator has two oil feeds instead of the normal one that tees off at the alternator. The diode chamber is fed from the oil pressure switch manifold and the stator is fed from the front end of the left block.
With the diode chamber blanked off, the oil pressure does rise considerably - as you would expect. This means that the oil pressure at the engine itself is actually a lot higher than what the gauge is reading so you can safely ignore the low reading when the alternator is hooked up. The alternator feed is not dragging the actual engine pressure down, just dragging the manifold pressure down

[HB of CJ]

Maybe you can screw in a temporary oil pressure gage in turn in each head and take a direct reading, both idle and at high rpm, cold and again hot.  I do not know if gear mounted accessories might leach or restrict overall engine pressure.  Figure 10 psi at idle with a hot mill as being low, but OK.

This is unrelated, but my '82 VW Rabbit diesel showed very low oil pressure on the dash gage, but screwing in a mechanical gage directly into the head showed a big difference.   Enough soosss that I/we just compensate and mentally add 30 psi to what the dash gage reads. 

[Barn Owl]

Below is directly out of a GM 4106 operator's manual.


Normal readings with engine hot are:

Idling – 4 pounds minimum.
Governed speed – 25 pounds minimum.

Engine must not be operated when oil pressure falls below 25 pounds at governed speed under full load. If oil pressure falls below 3 pounds, the "LOW OIL" tell-tale will light and buzzer will sound.

[buswarrior]

And what oil viscosity are you using?

The DD books are assuming 40 weight oil, not multi-grade.

Checking oil pressure when the engine is at operating temp removes the variable of what "cold" means.

Check the picture at the left...wonder what the oil pressure might be in that if I fire it up?

Apples to apples, operating temp to operating temp.

happy coaching!
buswarrior

[bevans6]

resurrecting this thread, my 8v71 has 30 psi cold (five minutes after a start in 70 degree weather) at high idle, measured at the manifold at the side of the engine that has the oil pressure sender, feeds the alternator, etc.  I'm using a new Autometer gauge.  The thread implies  that this is OK, but that this is not a real good indicator of what the true OP might be.  Where else can I measure, or it I disconnect the alternator feed temporarily will that give me a better read there?  My alternator feed is dual, like Tony Lee's, one to the body of the alternator from the engine, one to the diode case from the manifold that I am taking the pressure reading from.

The problem is the dash gauge reads even lower and is known to be faulty, so I need to figure out what the conversion factor is.  The PO said it was like that from when the engine was overhauled.  Not that I don't trust him, I just don't know what is in the range of normal.

Brian

[johns4104s]

My 20,000 mile 671 in my 4104 kept dropping oil pressure 40 one time out 30 next time out 20 next time and then ZERO. I dropped the pan everything looked good to me. But what did I know. So someone suggested dropping the oil pump, when I dropped it the tube going from the the pump to the block had a weld failure, you did not see it until I dropped the pump. I replaced it with a new one. Pressure back up to 60Ibs.

John

[bevans6]

I blocked off the alternator line, and the pressure went up to 65 psi.  So I think the basic problem is with the alternator.  Several mentions of restrictors in the oil line to the alternator, which uses a 1/8" pipe fitting with an ID of about 1/4".  mention was made of a .050" restrictor at the alternator fitting.  I have started another thread about that.

thanks for your reply!  Until I found out that the alternator is potentially part of the problem, I was making plans to pull the pan and check the pressure relief valve and generally wiggle things about down there.  Don't think I need to now!

Brian

[JohnEd]

My experience with sender units is that they read very low at idle.  Their problem is that they don't read the lower pressures "at all".  Not all, some!!!  Only mechanical for the proof.

John