M998 Oil Pump Problem

[ogsched]

Gents - I have a 1993 M998 with 49k miles that I purchased from Uncle Sam about 7 yrs ago. After some rehab and TLC (completed 6yrs ago), it has not had one problem. Good PMCS performed a month ago before vehicle entered service for the season, all fluid levels right, etc.

Earlier today, my son was driving the old girl back from a delivery mission when he heard a loud crack from under the vehicle between the front seats, as if a good sized rock had popped up and hit the undercarriage. He stopped the vehicle, left the engine running, and did a visual inspection that could find no issues (nothing under carriage, no loose bolts, no dripping fluid, etc). He remounted the vehicle and proceeded another 2 miles and heard the same crack again and noticed that the engine oil pressure had dropped to zero (normally will read 30psi) so pulled over, cut the engine and performed another visual inspection under the vehicle, engine compartment, etc again to no avail. It was another 1.5 miles to the house so he started the vehicle (no problem, other than no oil pressure) and proceeded carefully home.

I did a quick inspection before shutting off the vehicle and could not find any other issue other than zero oil pressure. Before I start the troubleshooting procedures, thought I should check the group to see if anyone has any good ideas. I’m afraid it may somehow be a failed engine oil pump, but would appreciate any thoughts.


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[Barrman]

Does your truck have a cable driven tachometer? That cable is driven by the same gear that drives the oil pump is Why I ask if you have one and if it is still working.

The oil pressure sender is on the driver side very rear of the block. Following the procedures to verify the gauge and sender are functional should probably be next.

The thunking noise sure seems connected, but it might really just be a rock.

 

[ogsched]

Thanks for the ideas. No Tach on the old girl, it is pretty much all stock condition. My son said he was certain it was not a rock or other road debris either time he heard the loud crack and looked to find remnants of anything with no luck. Will go out and start the troubleshooting later today when the sun cools down…


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[Mogman]

First thing I would do is swap in a mechanical pressure gauge in place of the oil pressure sending unit, any gauge with a 1/4" pipe thread will do.
It only takes a couple minutes and will give you a positive direction to go in.

 

[ogsched]

Yes, my thought exactly. The camp is quite remote with no manual oil pressure gauge on hand, but hopefully will pick one up tomorrow and get to the bottom of the “faulty gauge” issue. Thanks guys.

 

[Mogman]

Look around any old shop or garage and you may find one, air compressor, Oxy/Acetylene regulators etc. (DO NOT return any gauge used with oil to O/A service!)
You can also remove the lead from the sending unit and open/short it to ground, I never can remember which is which because over the spectrum of automotive and industrial use they come in both flavors, anywho just know that normally one way will be full scale and the other will show zero psi.
This will confirm the gauge only and not the operation of the sending unit.
You can swing the 90 away from the head when you remove the sending unit to give more clearance, this will loosen it a little at the block but at this point a small drip is no concern (if it even actually does)