Losing fuel prime many things done already, still won't run unless bled

[drewzee87t]

Hey All

I had posted this previously in a mods forum here. I am posting my latest and will also post link to the original thread. I am having problems where truck is getting air in the fuel system preventing it from starting and I have not yet located the source of the leak.

Could use some help.
Thanks

Hey fuel system peoples!

I have same problem even after replacing the fuel filter with a custom setup. I am still taking on air and after a day or two after running, it will start right up, for like 5 seconds and then dies. Will not start again unless charged and cranked enough to push the air through, then it starts and runs great, but loses prime again. Now loses prime just overnight.

So it's not leaking in filter housing anymore (old housing was leaking from water sensor).

How can I find out if my leak is on the supply line/lift pump side, or the post filter lines to the IP? I am ready to tear out the intake manifold on this and thinking it could also be the line between the lift pump and filter, or even the mech pump has a bad check valve, etc. There are a million ways to lose prime it would seem.

Can anyone get me a way to prove out which side of the system is leaking or bleeding back to the tank?

I also am aware of some scary GM tech bulletin saying you need to replace the fuel pump AND housing pressure cold advance assembly on the IP itself must be replaced due to check valve issues allowing fuel to bleed back to and de-prime. This seems extreme to me, as truck runs fine once primed to run.

Please let me know if you can think of anything? I will probably have to go through all of this and replace it, but how can I test, say, if my mechanical fuel pump is leaking back to the tank?

or if there is a return line problem? I have been suffering with this for years and thought I had it figured out. Please help.

Thanks

Drew

Link to other thread: https://www.steelsoldiers.com/showthread.php?36435-My-CUCV-spin-on-fuel-filter-mod/page26

Thanks guys I really need some help here. I am at the shop ready to start doing procedures, but I don't want to go pulling intake manifold and stuff for unnecessary work....

D

 

[Tinstar]

My first thought is have you replaced every single piece of rubber fuel hose?
The ones at the fuel tank, under cab, etc?

There are threads that describe on how to park the truck to help diagnose fuel prime issues.
At an angle, nose down, etc.

 

[cucvrus]

Have you examined the throttle shaft seals on the injection pump? They are known to leak. A $5. set of small O rings and about 4 hours later the problem could be solved. Finding an air leak is hard. It will NOT always leak fuel. Adding to the issues is modifying the fuel system. Every time that happens it just adds more equations into the mix. I would follow the given advise. I seen the metal lines under the cab rubbed thru on several trucks. At 35+ years old and hacked by everyone the sky is the limit on places for loss of prime. I would replace the fuel pump just to do it. It is a less than $50. item and will make less work later. Good Luck. Just trying to help. Have a Great Day and Be Safe. I still would think injection pump. It cures a lot of ills and is danced around many times to avoid the cost. About $450. and you will golden.

 

[Barrman]

Remove the hose from the lift pump to the fuel filter at the fuel filter. It should be under pressure and squirt fuel so have a rag handy. If you just get a dribble, then pressure is being lost either back through the mechanical lift pump or from a hose or at the IP.

Does your new fuel filter set up have a bleed screw? Besides the obvious of making sure it is tight. It can be a very good diagnostic tool. Look all around the throttle shafts as suggested in the above post and the valley for fuel. If all dry. With the pink wire off the Injection pump open the bleed screw and have someone turn the engine over. Do you get air for a few seconds and then fuel or fuel right away?

Air for a few seconds should correlate with no pressure when pulling the hose off the filter as described a paragraph above. Look at the filter drain, lines between the filter and lift pump and the lift pump. Because, if the valley is dry the fuel is leaving the system somewhere outside the valley.

Fuel right away at the bleeder when cranking means the filter is full and fuel is being pumped by the lift pump right away. This should go along with a gushing line if removed in the first paragraph.

Did you replace the 1/4" ID line between the filter and the IP? This one is a pain to do so most people just look at it and leave it alone. However, since air molecules are smaller than liquid fuel molecules. Air could be getting in and no leaks. I have found removing the air filter, putting a lawn chair padded cushion on the radiator, alternator mount and intake make it ok to lay down on the engine. Then give it a big hug and use a 1/4" short handle socket and ratchet to get the hose off. Be sure and use a NEW green letter replacement hose that is biofuel compatible.

I am chasing the exact same demons on my 6.2 powered M715. I have all new hose to do my 7 year fuel system refresh and then go from there. You are looking at 35 year old hoses so no matter what you find is the ultimate cause. New hoses will sure help out on a cold dark rainy night in the middle of nowhere 10 feet out of cell phone range.

 

[drewzee87t]

Thanks everyone for thoughtful responses. I am going to be able to look at it probably Sunday, busy right now. I will check the supply line to the filter first and see if there is any "missing" hose fill. Based on responses, I am going to start with the intake removal and see what's visible under there. Hopefully there will be something obvious. If I can't find anything visible wrong with IP, I will replace all the hoses and put it back together. Retest.

If it's still losing prime I will replace the lines and the fuel pump. Retest.

If it's still losing prime I will probably then tear the IP out for service.

I probably should just do the whole thing, IP, new everything and be done with it, but that's expensive for me right now.

Thanks again, I will keep posted on what I find.

Drew

 

[Barrman]

No need to pull the intake. Just pull the air filter and use a flashlight to look around.

I was motivated by this thread and cooler weather to come home and crawl around my M715. I found a probably original house between the fuel tank and lift pump. I replaced it and all is well again. I can’t believe I never changed it out before. I sure thought I had. 2 engine swaps and multiple fuel tank removals and I never changed it because it looked ok I guess.

Replace any rubber hose in the fuel system and use new clamps. Then see what you have is my suggestion.

 

[nyoffroad]

I've found air leaks by pressurizeing the fuel system. You will need a couple of shop rags, some clean card board, and an air compressor, regulator and hose and blow gun. Place the cardboard under the truck all the from fuel filler to core support, then set the air pressure regulator to about 10-15lbs, put blow gun in the filler neck and 'seal' using the rags and apply air. DO NOT TRY THIS WITH A FULL TANK!! FUEL MAY SPRAY BACK AT YOU! Apply air for a minute or two then look for spots on the cardboard and the look up.
BTW this can also 'pop' out dents in the fuel tank.

 

[cucvrus]

I found a probably original house between the fuel tank and lift pump.
WOW I know them trucks are big underneath but an original house. I would put it on the market or since its so close by I would rent it out. Tell them the upkeep on the M715 is the tenants responsibility.

And putting 10 PSI of air in the fuel tank. How will that find leaks that are beyond the fuel pump. I suppose thru the return line but doubt it. I still side with injection pump issues. Start from the front and work your way back on the entire fuel system. make it right and reliable the first time. You always look back with confidence and a smile. Beats broken down in BF Egypt. I would say $500. in parts and you have a very reliable unit. Provided the vehicle is worth the effort. Step back and do a complete analysis and estimate of what the vehicle needs to be a good runner and reliable. Include everything and make the decision from there. Good Luck. Be Safe. At over 30 years old the trouble with the starting can be the start of big money spent. The cascading CUCV expenditures.
​

 

[LT67]

Thanks everyone for thoughtful responses. I am going to be able to look at it probably Sunday, busy right now. I will check the supply line to the filter first and see if there is any "missing" hose fill. Based on responses, I am going to start with the intake removal and see what's visible under there. Hopefully there will be something obvious. If I can't find anything visible wrong with IP, I will replace all the hoses and put it back together. Retest.

If it's still losing prime I will replace the lines and the fuel pump. Retest.

If it's still losing prime I will probably then tear the IP out for service.

I probably should just do the whole thing, IP, new everything and be done with it, but that's expensive for me right now.

Thanks again, I will keep posted on what I find.

Drew

If it's losing prime, then it's getting air in the line somewhere.

 

[Tinstar]

If he hasn’t replaced all the rubber fuel hose on the truck yet, then everything else is a waste of time until that’s done.

 

[Tinstar]

If he hasn’t replaced all the rubber fuel hose on the truck yet, then everything else is a waste of time until that’s done.

 

[Small Prepper]

Evening All
I am having the same problem. Starts, runs for about 15 - 60 seconds, then stalls, no power when in gear. Sometimes it would stay running a little longer, letting the engine rev up, in park.
Looking to replace all of the hoses.
Questions:
Is their a kit I could buy to replace the hose, if so, part numbers and store
Does it have all of the sizes?
ls there somewhere in the TM's that tells you which ones to replace?
Where in the TM's are ALL of the hoses located?

Thanks for any help
S.P.

 

[joshuak]

Evening All
I am having the same problem. Starts, runs for about 15 - 60 seconds, then stalls, no power when in gear. Sometimes it would stay running a little longer, letting the engine rev up, in park.
Looking to replace all of the hoses.
Questions:
Is their a kit I could buy to replace the hose, if so, part numbers and store
Does it have all of the sizes?
ls there somewhere in the TM's that tells you which ones to replace?
Where in the TM's are ALL of the hoses located?

Thanks for any help
S.P.

Both the parts manuals have the hoses and metal lines, so -20P and -34P.

Here is a screen shot from TM-34P of the rear fuel lines and associated parts (M1009).



How's the exhaust coming? Exhaust thread.

Sounds like you're doing a some work to your truck, consider a build thread.

It would help others follow along, fix similar issues and for you to keep track of what you've done and whats on the to do list.

 

[drewzee87t]

Okay, so update here. I finally got a chance to get the intake removed. I don't like the clamps very much that are used on the metal injector pipes .

I had a giant rats nest under the intake and I cleaned some of it out by hand. It smelled of diesel. I vacuumed it all out and cannot see any source of leak. I will replace the hoses under the intake. Is there anything in particular I should inspect on the IP?

A couple interesting findings here -

The engine had a army refurbish tag floating around under the intake. I am not sure where it was supposed to be fastened to. It claims my engine was refurb'ed at Ft. Polk with standard pistons and etc. Not overbored. The dates kind of funny it seems to say 02 3 9. I think this would probably either March 2, 09 or feb 3 09. I suppose it could be 1999, not sure if the army phased in Y2k compliant dates for such things.

There's a refurb tag on the IP from FT. Polk. It just says 03-11, no year.

I bought the truck in early 2010, and paperwork says it was from Ft. Riley KS.

I have put maybe 3k miles on this truck since I have had it, and that's probably pushing it.

If there is nothing that I NEED to check or replace on the IP, I am going to just replace the fuel lines and zip it back up. I should then be able to see if I am missing fuel in the supply side. I do not plan to rebuild the IP unless I get a compelling reason to do so, and given it probably has less than 5k on it since rebuilt, I think I am being reasonable with that.

I will get back with any findings from replacing the fuel supply and filter to IP hoses and then move on to other hoses on the front side of the truck between fuel pump and hard lines, and maybe replace the fuel pump itself. I will keep up with updates here.

Thanks for your help!
drew

 

[cucvrus]

Mileage does not kill injection pumps. Time does. If you have diesel in the valley you have a fuel leak. Check the throttle shaft seals. You can run te engine with the intake off. It will be loud but it will run. Run it and shut if off. Check back latter and see if it is leaking. Don't drive it with the intake off. Just clean the valley really good and run it. You will see where the fuel leaks after you shut it off. Good Luck. Report back. Getting the fuel line clamps on is easier with the nozzle nuts off. Use some grease on the valleys of the clamps and slide them in place.

 

[drewzee87t]

Hey all,

Sorry for delay here. Life and other chores. I scraped up and cleaned all the gaskets and vacuumed again. It's about clean. I tested at the fuel filter for air, it's been sitting about 2 weeks since last attempt to start:

Clear line from bleeder port on filter to container. Open valve and puts out some pressure of fuel, no air
Leave valve open, crank motor over to push some fuel, get clean fuel, a couple seconds and then a big bubble in the line. Clear the bubble until clean fuel.

I close off valve. I wish I had thought of this test before I pulled the intake.

Test two - try to ignite truck:

I attempted to crank the truck enough to clear the air through and get her running -

The cranking became very slow and I shut it off. I went out and found some lingering smoke around the front battery. There is a small negative lead off the battery on mine, and it was starting to cook. I remembered a wire that was hooked up to one of the passenger intake manifold bolts...

I tested battery volts on both batteries are 12.5. I think that wire needs to be grounded to the motor to continue testing, but I also think that the bubble that was seen in the first test says it's not leaking at the IP side, it's on the supply side.

Thanks again for all your help on this. I have observed all the spots where I think it could be leaking down to the fuel pump, that funky rubber line to another hard line that runs on the passenger frame to a junction under a rock guard. There's another rubber there under that I could only feel it up, couldn't see it. It's hard lines there until it gets lost in the tank on the driver side.

Obviously want to replace all lines that need to be replaced, but with no visible leaks at least downstream of the fuel pump, could we say we're good? I am thinking of continuing CUCVRUS test once I ground that line, make sure there's no leaks up top. Then if top is good, just replace the couple hoses up there and do a new fuel pump and hoses that connect to it. Retest.

Thanks again all of you. If you had a TM link for the fuel system hoses and also the parts on the IP that I should be looking at for problems CUCVRUS

D

 

[cucvmule]

The truck starts and runs for 5 seconds.

First I would get some amps going into the battery. Put on a battery charger, with the continued cranking and not running, recharging battery is not happening. The ground cables and secondary grounds need, must be in a+++ condition at all times. Overloading the secondary ground tells you that your main battery ground is not making good contact, other ground points are failing. Clean contact points, or replace with acceptable cables. Battery ground to engine, engine to firewall bus bar ground. Make sure ground strap at rear of engine block to firewall bus bar is making good contact. Core support to battery, to frame.

Just listening to your information about symptoms, the air is at the injection pump already, running for seconds. Sounds like injection pump to me. The more air the injection pump eats, the problem get steadily worse until the pump decides it has had enough at the worst time.

Replacing all rubber fuel line is a positive step for sure, can not go wrong there. The rubber line at the firewall to the frame hardline never gets replaced. When ever the wheel tub is out you should replace just because, I just did mine while I am doing other work.

 

[cucvmule]

Using air to blow out, clean out line of crap or clean fuel tank sock is ok to do, as long as you do not blow off the sock.

Blowing air into fuel line towards tank will clear line. But blowing air into tank and pressurizing fuel line towards injection pump may show a leak. But you will have to close off return line. And 5 pounds will be enough as the volume in tank will be enough to show any wet areas.

And having less than full level of fuel is advised unless fuel bath is desired. Using all safety precautions.

 

[drewzee87t]

Okay, another update. Weather has prevented work this week.

So, it's been sitting for a week now since last test. I had forgotten the ground wire that goes to one of the intake bolts last time, so that was cause of electrical issue. I grounded that.

Put bleeder hose/jar onto fuel filter and opened. Did not get any fuel out, which I normally do. I cranked the truck and watched the air bubbles come through until clear. Cranked the air through the IP until I had fuel/smoke. Glowed it and started right up. Ran it for about a minute and shut her off. In looking for leaks "in the valley" so far nothing. I will check again tomorrow.

At this point I am convinced this leak is before the fuel filter. I am going to next replace the fuel pump and all rubber hoses that I can get to, and re-test. This would be everything that I don't need to pull the truck bed for. And then I will test again and post back.

Thanks!

 

[cucvmule]

Well one problem helped, just make sure all grounds are clean and check with voltmeter that all contacts are in good condition.

Now to conquer all fuel lines, flexible and do not rule out that hardlines may be compromised as well.