HMMWV 6.2L Odd Oil Pressure Behavior Now

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
My 1987 M998 has been doing fine. We went to Graves Mountain and had a blast last month.

Acted odd yesterday, so bear with me while I try to give you experts all the information I can...

Coming home Sunday evening from TheCaptain's place, I noticed after a long haul (25 miles maybe) at 50-55MPH on the Fairfax County Parkway that when I stopped at the first traffic light to enter suburban driving near home that my oil pressure dropped to barely 5PSI. I watch my gauges carefully, and this was definitely new behavior. Kept the idle up to get 10PSI standing, and it rose to 20PSI or greater when driving the remaining 2 miles to home. Got home and it was late and I was beat so I punted on any work until morning.

Yesterday morning I fired her up, started quick and easy as always, with 30PSI oil pressure on cold startup, rising to 60PSI at speed (settles down once warmed up). Checked the oil, was low, which I've noticed seems to be a trend, that I consume oil slowly. I have made a mental note to check every fill up from now on. It is only with highway driving I think, that this sometimes happens, but I will check more often to be sure. I have had to add oil twice since May over about 1,600 miles.

I topped off her crankcase with a fresh quart of 10W-30 (yes, rated for Diesel engines) and drove the 10.5 miles to work. Seemed OK, it was a cool humid October morning and she settled down to 30PSI at speed, dropping to 10-15PSI at idle at stoplights. This has been normal since getting it, and I read in the OM that this is acceptable. The pressure might have dropped a little lower at idle just as I arrived at work, but it was not enough to stand out.

Driving home, about halfway through my stop-and-go afternoon commute (morning is more direct as I go in early) I noticed she dropped to ZERO PSI at idle from 30PSI at speed. I immediately pulled off to an apartment parking lot and shut down. Checked oil level, was perfectly fine, and looked for any obvious signs of problem. Couldn't find anything obvious so I fired her up again and watched the gauges. She started fine and showed 15PSI at idle, 30PSI at higher idle, i.e. seemed back to normal.

Began driving the remaining 5 miles home. Oil pressure slowly but surely began falling, even at speed, though it was >10PSI at speed. I limped home, keeping the idle up at stoplights to maintain pressure. Once home I shut down and started back up, 15/30PSI again, then fell slowly. I shut her down and will leave her there until I have a plan.

By the way, I also happened to notice that her coolant temp was edging over 220F at times, but I have noticed this before when she is hot and I'm idling a lot and not giving enough power for the fan system to kick on. Maybe the cooling fan should kick on at idle, but I haven't seen mine do that, she likes to have some RPM going first. Noticed this while mountain climbing in Graves with long waits while trucks ahead of me negotiated the trails (side note, my HMMWV never flinched and ate that mountain up like candy).
Now, when I say higher temperatures, I mean 225F, not any more. But I call that Hot, hotter than normal anyway.


The oil gauge is crusty, and hard to see through along the bottom edges of the glass. I can read the pressure level easily enough, but it is the ugliest of my gauges, and I have been keeping an eye out for one online that is being sold for less than the spot price of gold... I'd intended to replace the sending unit at the same time as well, when the gauge was replaced.


Other stats: mileage (if original) is a touch over 68,000. Have put I guess about about 1,600 miles on her since taking delivery in May. Did full lube order, all fluids and filters, etc., oil air and fuel. Did put some STP in the oil, and I add a 1/2 qt of fresh motor oil to the fuel tank to supplement my local low sulphur diesel every fill up. Have no obvious blow by out oil fill cap.

She sat maybe 10 years in surplus before I bought her, or at least the air filter was dated 2006 when I replaced it, so she was neglected for a while. However, after doing all the lube order and cleaning out hubs, etc., she has performed well, though I can tell she's more tired than TheCaptain's similar M998 which is a few years younger and has 1/3 the mileage (more recent maintenance too by the looks of it).


So, what do folks think?

Is the oil pump giving out? Is it safe to drive? Could it be the gauge or sending unit being 29 years old and getting erratic?


I welcome your thoughts and suggestions.

Thanks in advance.

Bulldogger
 

M543A2

New member
Steel Soldiers Supporter
1,064
9
0
Location
Warsaw, Indiana
My first thing to do would be to buy a mechanical gauge and install it to see if it agrees with the in-dash gauge. Usually problems like this are with the gauge, wiring (especially grounds), or sender, but to be sure it needs checked out. The mechanical gauge is the best way to do it. If the mechanical gauge indicates reliable pressure you can drive the truck with it until you debug the electronic gauge system or install the mechanical where the electronic gauge is in the dash and leave it there.
Regards Martin
 

Retiredwarhorses

Well-known member
Steel Soldiers Supporter
3,463
1,493
113
Location
Brentwood, Calif
here is my list of items that I have found to be the culprit...and never once once was it a motor issue.

1. Bad Gauge
2. Gauge wiring
3. Oil pressure sending unit
4. Oil pressure sending unit wireing/ faulty connection

As has been suggested, hookup a mechanical gausge direct to the oil pressure transmitter port on the left rear of the engine and see what's really happening. I did this to a truck that read 5psi and it was actually 60psi with the mechanical gauge.

on another note....loose the 10/30 oil, these motors and 99% of diesels are made to run on 15w-40, it's formulated to run
in Diesel engines and has detergents. I've been a diesel mechanic for 25yrs and wild not run anything else.
but hey....it's your truck.
 

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
Thanks gents, I appreciate the feedback. I can easily hook up a mechanical gauge, and happen to have more than half the setup handy from a motorcycle diagnosis project earlier this year.

I am happy to change to 15W-40 RWH, especially considering the advice comes from you. I bought it by mistake for my old Honda motorcycles, badly worded WalMart listing made me think I was getting 10W-40 at $3/qt. So I have a whole case of it. My recollection is 10W-30 is recommended in the Tech Manual, which is why I used it. But I defer to the voice of experience in situations like these.

Will slap on a mechanical gauge and change the oil this weekend and report.

I welcome other suggestions if any.

Bulldogger
 

DatGuyC

Member
537
11
18
Location
Essex, Maryland
The fan is definitely supposed to turn on at idle. With the motor off can you move the fan at all? It should be locked in place when there is no pressure from the PS pump.
 

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
DatGuyC, the fan is locked when the motor is off. It only seems to kick on high speed when the vehicle is moving though.

I installed my grounding harness last night, and that included a ground to the EESS, so I will see how/if her behavior changes as I drive her. I took a 15m test drive last night and couldn't trip the fan, but I think she simply hadn't gotten the radiator hot enough, so the coolant was able to keep the motor at 220F well enough.

Perhaps my steering pump isn't putting out enough pressure at idle for the high speed fan. It does leak a little, but not enough to notice in the steering. I have a new pump, but lack the enthusiasm to do all the tear out necessary to get the old one out... Working on it.

Bulldogger
 

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
On the suspicion that my HMMWV has less than perfect grounding, I went ahead and put on my homemade grounding harness last night.

First thing I noticed digging around is that the ground lug above the fuel filter was loose. This is the ground lug that on the inside of the dash happens to connect to the grounding wires for the instruments. I took the time to pull the gauge cluster so I could get at the back end of this bolt and clean all connections as well as tighten the lug. Then finished the install.

The oil pressure gauge functioned as I remember it, and in a 15m check ride it stayed at or above 10PSI, 30PSI at speed, which is what I'm used to since getting her running.

I am still going to install a supplemental mechanical gauge to double check my gauge accuracy before putting the motor under any further strain or distance. I will also change the oil to the 15W-40 recommended by RWH before making any judgements.

Bulldogger
 

DatGuyC

Member
537
11
18
Location
Essex, Maryland
Just for some information, my 6.2 with I assume 40k miles on it has around 25psi hot idle if I just got off the road, around 30-35 if I let it idle for a couple minutes. At speed the oil pressure seems to top out around 45-50. This is all with Rotella T 15w40 oil and a mechanical gauge.
 
Last edited:

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
I haven't made time to install the mechanical gauge, but just changing the oil to rotella 15w40 and a new filter has doubled the idling hot oil pressure. Funny what the right (or better) components will do. I will still put a new gauge in but so far things are definitely improving. I'm glad for the grounding problem that made me change the oil. Am very glad for it.
 

Bulldogger

Well-known member
Steel Soldiers Supporter
1,263
330
83
Location
Quantico VA
Well gents. The problem seems much more clear now. I plumbed a manual pressure gauge and did some driving. Took a little more time as I gathered fittings so's I could have both the factory electric gauge and new manual gauge connected at once.
After some highway driving, the dash gauge would drop to near zero at idle while the mechanical gauge was always over 21 psi.
Seems clear that the factory setup is steadily degrading after years of sitting and now sudden routine use.
The dash gauge is reading low by 15-22psi depending on the conditions at present.

After warming up fully, the manual gauge displayed 22psi at idle and over 40psi at speed. As high as 50 under heavy demand.

I've ordered a new sending unit and gauge from Erik's. Will leave the mechanical gauge in at first for comparison. Why not. It isn't long to swap or replace them. Probably one of the more convenient repairs I've made on that machine in a while.

Many thanks for the help folks.

Bulldogger
 

kfrosty

New member
267
5
0
Location
Belmont, NC
Thanks gents, I appreciate the feedback. I can easily hook up a mechanical gauge, and happen to have more than half the setup handy from a motorcycle diagnosis project earlier this year.

I am happy to change to 15W-40 RWH, especially considering the advice comes from you. I bought it by mistake for my old Honda motorcycles, badly worded WalMart listing made me think I was getting 10W-40 at $3/qt. So I have a whole case of it. My recollection is 10W-30 is recommended in the Tech Manual, which is why I used it. But I defer to the voice of experience in situations like these.

Will slap on a mechanical gauge and change the oil this weekend and report.

I welcome other suggestions if any.

Bulldogger
Rotella 15w-40 is what is mainly ran in big trucks and what I run. (Walmart usually has a good deal on it.)
 
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website like our supporting vendors. Their ads help keep Steel Soldiers going. Please consider disabling your ad blockers for the site. Thanks!

I've Disabled AdBlock
No Thanks