M998 Fuel Issue - Runs great then dies - HELP!

[TG5446]

Hi All,
I have searched the forums and have not found the exact issue I am experiencing. If I missed it please forgive me! Anyways here it goes:

I have a 1985 M998 with the NA 6.2L motor. For the most part, it is in excellent condition, really well maintained. When I got it, it would start but it would not stay running for more than a few mins. (I think it had been sitting for a while) So I started troubleshooting, here is what I have done so far:

-dropped the fuel tank and checked for debris in the sock and cleaned the tank
-checked the fuel water separator
-replaced kinked high-pressure hardline and installed an inline fuel pressure gauge (getting about 7lbs consistently from the mechanical lift pump to the filter)
-replaced #4 fuel injector line (cracked)
-checked all injectors for cleanliness, they are good
-checked for continuity and resistance on glow plugs and controller
-replaced the IP with a remanned unit and advanced the timing slightly

After doing all of this, the 998 now starts perfectly and runs seemingly like it should at idle. It will drive down the road for a few miles but then will start to lose power and eventually stall out like its getting starved for fuel. The pressure to the fuel filter seems constant. Could it be that I have some air in the fuel system? I was thinking of doing a sight tube at the IP to observe the fuel for air bubbles while at idle. Has anyone run into this before?

Any help would be much appreciated! Thanks in advance!

 

[NDT]

I would check that the return line is free flowing, check pressure on the IP side of the filter, check for bubbles in the supply line, if all is good, I would send the IP back for bench testing.

 

[Dock Rocker]

Did you replace the fuel filter when you checked the water separator?


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[Wire Fox]

I too am banking on poor return line performance. That could cause a pressure issue in the pump and lead to a stall, but naturally would take a short running period to lead to failure. You mentioned having a fresh reman pump in there as well, but I'm wondering if you're having a solenoid problem? Any chance that you have the run and cold advance solenoid wires reversed-once your engine is up to temperature, it's going to try and shutdown that cold advance solenoid, but if it's on the wrong solenoid, you just cut off the fuel and promptly die.

 

[Retiredwarhorses]

Make sure you didn’t swap the 2 wires on the injector Pump...one is cold advance, one is fuel cutoff solenoid, when reversed, the truck will die as soon as it hits approx 120 degrees....

 

[TG5446]

Thank you, guys! I did not replace the fuel filter, so that is on the list. I was also thinking of adding another inline pressure gauge to the line coming from the filter. Might be nice to know inbound and outbound pressure in the future. I will check the wiring, it's completely possible that I reversed those two - very good call that I hadn't thought of, but it does want to die as soon as it gets between 120 and 140 degrees and runs flawlessly until then. (I am going to kick myself if that's the issue!). I am also going to install that sight tube tonight on the IP to see if I have any air in the lines. I will keep you guys posted! Thanks again!

 

[Retiredwarhorses]

Thank you, guys! I did not replace the fuel filter, so that is on the list. I was also thinking of adding another inline pressure gauge to the line coming from the filter. Might be nice to know inbound and outbound pressure in the future. I will check the wiring, it's completely possible that I reversed those two - very good call that I hadn't thought of, but it does want to die as soon as it gets between 120 and 140 degrees and runs flawlessly until then. (I am going to kick myself if that's the issue!). I am also going to install that sight tube tonight on the IP to see if I have any air in the lines. I will keep you guys posted! Thanks again!

Sounds like they are reversed...the cold advance plug has raised ribs on both the plug and connector

 

[papakb]

Good call Steve! Goes to prove there's always new tricks to learn.

 

[TG5446]

Ok Guys,
A quick update. Unfortunately, I did not have the wires reversed, I thought that was going to be it! I even referenced the wiring schematics to cross-check the metal ID plates on the wires and they were correctly connected. So, now I'm thinking more about the restricted return lines building back pressure and I'm wondering if that problem led to a premature failure of the old IP that I replaced. Of course, the two might not be connected at all, but its just a thought. Anyone know any good tricks for purging the return lines? I'll be working that angle later today. Thanks!

 

[NDT]

Yeah, I did not think so. "Lose power and eventually stall" does not sound like the abrupt engine off situation you would get with the wires. As far as the return line, I would get the compressed air nozzle and start blowing.

 

[Action]

Did you put fresh diesel in the tank when put back in?
Is that reman IP a 12v or 24v?

 

[TG5446]

Yes, fresh diesel and the IP was a twelve volt HD but I swapped out the top cover to make sure the fuel shutoff solenoid was 24V and that the appropriate electrical connections were there, other than that I couldn't find any differences in the 2 IP's. All of the other parts swapped over perfectly.

 

[Action]

Maybe the top of th IP was and still is the problem?

 

[lowell66dart]

Do you have a copy of the troubleshooting guide? It will show you how to test the fuel system. If you don't have it send me your email address in a pm and I'll send it to you or some smarter person can maybe link it here. That thing should be a sticky.

 

[TG5446]

Action - If the top of the IP is the problem then I will again kick myself for not listening to you the first time you mentioned it in another thread! So, what are you thinking? Would the fuel shut off solenoid cause such an issue? I purged the return lines with a compressor all the way to the tank, so they are or at least seem to be clear. The problem persisted after doing this although it seemed to run really strong for a few moments after putting it back together, I guess that could also be wishful thinking on my part. So, I'm back to that part of the engine as the possible cause. I guess I'll start hunting around for an IP top.

Lowell - Thanks! yes, I have the troubleshooting guide and I watched a pretty cool video on youtube that I think was made as an instructional video to show to new HMMWV mechanics during MOS training. It walks through the whole process which I've gone through. In the video, the issue they were chasing was a dirty injector, man, if only it was that easy.

 

[Action]

Start it with thr filler cap open and see if it stalls.

I dont know how the IP works inside.

 

[TG5446]

Ok guys, Thanks again for the advice, I think I may have sorted it out. I was thinking about the return line restriction, and after blowing the return line through with pressurized air and getting positive confirmation that they were clear, (hearing bubbles in the gas tank), I concentrated on the one part of the system that I hadn't changed (on Action's advice). The top of the IP, which I had swapped from the old one to the new one when I installed it. I removed the check valve from the top and sighted it, it seemed clean but figured, since I had it off I would see if the "glass ball" and spring were free and moving properly. Turns out, they were not. Using compressed air and cleaning solvent, I was able to get some particulate debris out of the housing, then using a small allen key, I was able to get the Glass ball actuating properly. I reinstalled the valve and fired the engine up and it seemed to run fine. Later, I took it for the longest drive that I have been able to execute since I got the 998 and it ran well. So, fingers crossed, I think I may have beaten this thing. Hope this helps anyone who is having similar issues. Thanks again to all who gave me advice, I truly appreciate it!

 

[jlarovere]

I know this is an old thread, however I thought I would share my experience on my M1123 6.5 NA.

Symptoms-idles fine, starts fine, after driving and under a load would act as if starving for fuel.

First fix was to repair some small leaks on fuel inlet hose to fuel filter, thought it was sucking in air during higher RPM, fixed the leak and it appeared to run and drive fine afterward. Longest drive was about 10 minutes at night up to 55 MPH, ran fine, thought it was fixed.

Second fix was after a repeat of the issue, tried driving to gym during the day, hotter air temps, stalled several times, acted like starving for fuel, would restart after a few minutes and stall again under low load. More extensive troubleshooting this time outlined below.

• dropped tank down enough to remove top plate and sending unit, inspected tank, it was fairly clean, noted missing suction suck on 3/8" fuel line, installed new sock, gasket was missing from fuel sender, installed gasket, reassembled, all good
• removed check valves and lines were cracked and leaking, fuel return check valve moved and worked freely, fuel supply to low pressure fuel pump was partially clogged, probably 75% and check valve sticking, replaced both check valves and reassembled everything, fixed the issue.

I've driven it several times and it runs strong and does not stall even under a load and for longer drives. All seems good.

• Check and clean entire fuel system, open up and blow out the return and supply lines
• Replaced cracked and dried fuel lines, install new clamps
• Make sure suction line has a sock on it to prevent debris from getting into check valve
• Clean or replace check valves
• Replace fuel filter