What should the oil pressure be on the 6.5L?

[mgFray]

I finally replaced the not-working oil pressure gauge with a new one. And I'm now getting a reading.

Cold idle, between 30-60.
Warm idle, 30
0-30 MPH - 30-60
50 MPH - 100-120
55-60 MPH - over 120

The numbers at 50 and above seem high to me, but I have no frame of reference for this engine. Are these numbers reasonable? If not what could the issue be, sending unit, oil issue, etc? I was told prior to purchasing this thing about 4 weeks ago they had just done an oil/filter change about 500 miles prior (and I know the person who put the 500 miles on it.)

Just want to know if this is reasonable or if I need to do something to avoid damage.

6.5L non-turbo engine.

 

[Milcommoguy]

Who changed the Sender with What?

Rule of thumb 10 PSI per 1000 RPM hot. Little more is better. Way more, something not right IMO.

Screw in a mechanical gauge to be sure. Military electrical ones not always so good.

That's one tight engine, CAMO

 

[Coug]

for mine, with brand new sender and gauge, is 50-60 cold, 30ish hot. 6.5l GEP
I have never seen it go above 60psi.

 

[Mogman]

As stated above the factory gauges are not very accurate, screw in a mechanical gauge and if it is far different then change the gauge/sending unit until it is anywhere near the mechanical gauge, for the most part the gauges are just a "relative" instrument to tell you something has changed.
I have had brand new sending units even in the civilian world that were way off, usually the gauge itself is close

 

[mgFray]

Brand new gauge, but existing sensing unit. I’ll get a mechanical gauge and test it properly. I’m assuming it’s fine, and probably the sender right now. Crossing my fingers!

 

[Barrman]

60 psi is what I think the TM says is the max. 40-50 hot cruising is what the TM says is right.

 

[Milcommoguy]

Little heads up... Gauges or sender can be flaky, more important check over ALL the connections. Power and especially grounds. They are a three wire circuit, power, ground and sender. A balancing act of electricity...so to say. Resistive change is just a few Ohms from 0 to 33 more or less over the range of 0 to 120 PSI. 16 Ohms is about 60 PSI. Right gauge with right sender right ???

Advance test. Disconnect sender lead. Connect test Ohm meter to sender and a GOOD ground. Initial read about 0 Ohms. Start truck and watch and record reading to match range above. Anything over 33 ish is going to be + 120 PSI. Again, 10 to 20 Ohms is the happy spot.

Flaky gauge, sender, wires BAD. Flaky crust good, CAMO

 

[Action]

The sender range needs to match the gauge range. If you have a 0-60 sender and a 0-120 gauge, the gauge will show 120 when it is really 60.

 

[mgFray]

Hmm I will definitely need to check it’s the right sender, but I’m suspecting more and more the problem is the sender.

I ordered the big Humvee service kit with all of the tools and gauges. Hopefully it has a pressure gauge that can be used or I’ll pick one up…

 

[Mullaney]

Hmm I will definitely need to check it’s the right sender, but I’m suspecting more and more the problem is the sender.

I ordered the big Humvee service kit with all of the tools and gauges. Hopefully it has a pressure gauge that can be used or I’ll pick one up…

.
If there isn't a mechanical gauge in your kit, there are liquid filled gauges available from McMaster-Carr and any number of other on-line retailers.
I would suggest that you spend a few extra bucks for a little better quality. After all, it IS your engine oil that you want to watch.

You might even decide to install a mechanical gauge to replace or supplement the one in the cab.

 

[BKubu]

I have a 1997 civilian Chevy 3500 pickup with a 6.5TD. The gauge reads about 45 pounds when cold and at idle. While running (revving up) it reads over 50 PSI. When hot, at idle, it reads about 18 PSI. The truck runs beautifully and does not burn oil. I am NOT worried about that in the least.

 

[BKubu]

I have had the truck for about four years and have driven it long distances (3-5 hours). It has 92K miles.

 

[mgFray]

I just looked at the pictures I took of the back of the engine (inside the cab), the oil pressure sender looks exactly like the 60 PSI units I see on ebay. Argh, I replaced the dead 0-120 PSI gauge with a new one, but the sending unit never matched.... so a previous owner must have put in the wrong gauge. This explains the pressure problem! We're reading roughly 2x that actual pressures, which make far more sense.

I still would like to get a manual pressure gauge at least to verify this. Do you know what side fitting is on the engine? 1/4 NPT? And do you have a suggestion on which model McMaster-Carr gauge to go with. (I also like the mount in your picture, hand fabricated or was this an off the shelf item?). I assume this it goes engine to a Tee, then the electronic gauge to the side of the Tee, and a hose/pipe up to the fluid filled gauge?

This 1988 M998, did it originally come with a 60 PSI gauge or a 120 PSI. (I'm guessing the former.). And then when they upgraded to the 6.5L in 2004 would they have re-used the 60 PSI sending unit or might they have moved to 120 PSI at that point? Really just trying to figure out when the mismatch happened, and which of the gauge or sending unit I should replace to have the 'right' pair in the vehicle.

Thanks for the insight, this is starting to make much more sense.

 

[Andyrv6av8r]

Sometimes pictures are worth a thousand words. That's a mechanical gauge.

 

[Mogman]

IIRC the min spec for oil pressure on the 6.5L was 10psi hot and at idle, seems kinda low to me but I did find the spec on the Duramax and it is min. 14psi@720rpm and 38psi@1800prm.

I ran an old 345 International that pretty much showed zip zero nada at idle on a mechanical gauge for about ten years until the body gave out.

 

[Retiredwarhorses]

You installed the wrong sending unit I do believe…

 

[mgFray]

You installed the wrong sending unit I do believe…

I just replaced a non-functional dash gauge with the same one. I'm pretty sure the previous owner of the vehicle mismatched the sending unit and the gauge, so now I need to correct that.

 

[mgFray]

Ok, I've got a manual gauge, and also was able to hook up the tack. So I've got some "real" numbers.

6.5L (2004)

Cold idle - 600 RPMs - 35-40 PSI

Warm idle 650-700 RPMs - 30-40 PSI

55 MPH - 2800 RPMS - 55-57 PSI

55+ MPH - 3000 RPMS (roughly) - 57-58 PSI

The engine never exceeded 60 PSI.

In neutral (no load) at 2800 RPMs was about 52 PSI.

So at a _MINIMUM_ it appears to me that the idle oil pressure is WAY too high. I have no idea what could cause this. Any suggestions?

At speed/load the vehicle seems to match what others have told me to slightly high.

I know very little about Diesel engines, especially this type of problem. I'm looking for any idea what I should look at/investigate next.

I'm tempted to simply do an oil change/filter replacement. (The oil in the engine is _very_ black. I was told it was changed about 1000 miles ago when I bought it, but if it was a gasoline engine I'd certainly be doubting that based on how black the oil is.)

Edit:

Found this in the service manual:

STOP 6.2L IDLE (650 ± 25) 6.5L IDLE (700 ± 25) 6.5L DETUNED IDLE (700 ± 25) 2000	0 PSI 10-15 PSI 10-15 PSI 10-15 PSI 40-50 PSI

So stop and 2000 RPM are about right, but idle is _way_ off.

 

[Action]

What grade oil is in it?

 

[mgFray]

What grade oil is in it?

I don't know. I've messaged the dealership I bought it from to ask. If I had to GUESS it would be 10W30 or 15W40.

I know with winter coming soon, I need to move to 10W30.