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| Greetings. I have been lurking here for some time, and have learned much of what I know from these forums. I appreciate the quality of information here, so when I encountered an issue recently, I joined. This is my first post here, so I'll introduce myself as part of this post.
In March of 2020, I purchased a 2007 Oshkosh MTVR military 6x6 in excellent condition with only about 3,000 miles on it at auction. It appears to have never been deployed.
It is powered by a Cat C12. The engine's serial number range is 9SM.
When it was delivered, it started easily and ran well.
In November of 2020, we drove it 2,000 miles on the interstate to our new home in Central Texas. It continued to start and drive well.
I parked it in my shop, and over the last three years, I have started it for an hour and driven it around our property about once a month.
Over the last few months, it began taking longer cranking times to start, and when it did start, it was missing for a short period of time before running normally. This condition has gradually worsened, until it became a legitimate "cranks but no start" condition.
I have an extensive background in auto repair, but I have never done diesels before. I have owned diesels, and the symptoms I'm experiencing, appear to be consistent with losing prime.
I took some time to try to educate myself as best I could using online resources (to include this forum), on EUI diesel fuel injection system operation, and the Cat C12 in particular.
I looked in the tank, and I've got about 20 gallons of normal-looking diesel fuel in a surprisingly shiny clean aluminim tank, with several tablespoons of brownish "mud" type stuff separated in the lowest contours of the tank, away from the fuel pickup tube.
Because the brownish "mud" is laying on the bottom, underneath the diesel fuel, I'm going to assume it's water-based.
I physically inspected the fuel lines and fittings from the tank to the primary filter, and from the primary filter to the lift pump, and found no concerns.
I cracked open the tap on the bottom of the water separator on the bottom of the primary filter, and got diesel fuel coming out first, although I could see a small amount of the brownish "mud" laying in the bottom of the water separator bowl.
I observed that the primary fuel filter was painted to the vehicle in the military CARC coating, so I'm assuming it is the original filter, with only about 5,000 miles on it, but now 16 years old.
I crossed the original primary fuel filter to a Racor filter, and also ordered the correct Cat 1R-0749 secondary fuel filter directly from my local Cat dealer.
The 24-volt heated water separator bowl was inseparable from the old primary fuel filter, so I ordered an NOS military one with identical part number.
I installed the water separator bowl to the new Racor primary fuel filter, filled it with fresh diesel fuel, installed it, plugged in the heater wiring, and used the hand pump on the secondary fuel filter base to prime the fuel system. It only took a few pumps before I could feel and hear fuel at the secondary fuel filter base.
I performed several 30-second cranking cycles, but the condition remains.
I purchased a diesel fuel pressure gauge, and attached it to the fitting on top of the secondary fuel filter base. I believe the top fitting is before the fuel goes through the secondary fuel filter. When cranking, the needle bounces back and forth from 30 to 60 psi, which from my understanding, is not enough.
This engine uses the 129-2878 secondary fuel filter base. The fuel return check valve is NOT located in the "DRAIN" boss on the top- it is located inside the hole that the return line AN fitting screws into on the rear of the secondary fuel filter base.
I disconnected the fuel return hose at the tank, and put a drain pan under it. When cranking, fuel runs out of the return hose, even though I've only got 30-60 psi fuel pressure when cranking. Fuel pressure drops to zero in just a few seconds after I stop cranking.
I disconnected the return line at the secondary fuel filter base, unscrewed the AN fitting, and removed the fuel return check valve and spring. I think the part number for the spring is 104-1729 based upon online research, and should have a free length of 1.19" per the Cat manual I found online. The spring is intact, was under pressure when I removed it, and measures about 1.2". The valve appears intact, and I can blow air through it. It didn't look particularly dirty, but I cleaned it, reinstalled it with the AN fitting, reconnected the return line, and the condition remains unchanged.
When I use the hand pump, I can hear fuel being pushed past the check valve and down the return line.
Based upon where I'm at, it appears that the return line check valve is not holding pressure, but I don't know why.
I am stuck.
I am considering ordering an AN plug to block off the return line to try dead-heading the lift pump, to see if the rest of the fuel system is capable of delivering adequate pressure that far. I believe the lift pump has a non-serviceable pressure regulator valve inside it, but I'd be surprised if that failed at only 5,000 miles, and even if it had failed, it wouldn't explain my fuel return check valve issue at the secondary fuel filter base.
And I haven't replaced the secondary filter yet, but it looks like I've got an issue before that point in the fuel system anyway.
I have pictures and can take pictures, if needed.
I'd sure appreciate some wisdom here...
Edited by MTVR 12/22/2023 09:57
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Cat C12 fuel system diagnosis
