CM870 Fuel ump

[Burgerjones]

Hi Everyone,
I have scoured the site looking for info on replacing my mechanical fuel pump and can't seem to find what i'm looking for. I try to research all i can before ever posting something already discussed.
My problem is as follows, ONLY when its single digits or below do i have this problem! I start the truck, it runs for a couple of minutes and then chokes out and dies. If i wait a few minutes and then try again it will start right back up, runs out of fuel and then dies again. So what i have done is turn the key on and let the priming pump go through it's 2 minute cycle and then check for fuel pressure(It has good pressure at this point). I then start the truck and let it run until it dies, then i turn the key off immediately and check for fuel pressure again and it is at zero. Am i correct in assuming that my primer pump is operating as it should and the mechanical pump is bad? As it will only run from the fuel the primer has put in the system. I have checked for bad fuel, changed out both filters, and checked for fuel pressure before and after start up. What gets me is if its 20 degrees or above it runs with no problems. If i am correct in my diagnosis how difficult is it to replace the mechanical pump? Do i have to change the entire IFSM or can i just change the pump?:huh: I have good mechanical aptitude but have no experience with replacing this particular part. Thank you in advance for any help i can get with this issue.

[Rawze]

register your enigne on quickserv.com and read the instructions for replacing the gear pump. its free.

when it is refusing to start up,.. what is the fuel pressure reading while cranking?

You sure your not sucking air into a fitting, or gelling up in a line somewhere when it gets cold enough? Have you tried pressurizing the fuel tanks to 10 psi and trying it?/looking for leaks?

from what you describe, is kind of unlikely for it to only happen at low temp, but not impossible for the gear pump to do that. usually when it is the gear pump failing, it runs for a few seconds like you describe and dies,.. but it does it consistently, not just in the cold.

[Burgerjones]

I am not sure what it is when cranking, I'll check tomorrow morning when I go back to the truck. I've checked the lines and fittings for air leaks, I'm not sure how to pressurize the tanks, not gelling up from what I can tell either, fuel looks nice and clear, lines seam clear. It never refuses to start up, just won't stay running. It starts everytime. I'll register on quickserv.com and read all I can.

[Signature620]

Your symptoms make me think it's a shutoff solenoid, lift pump or air in fuel issue.

[stevesparts]

My 2 cents... if it was your pump it wouldn't run, i've had 2 go out on me and they both snapped the drive gear off the pump.thankfully no other damage. as for changing it out its 4 torx(#20 i think it was) and undo the fuel line. and did 1 on the side of the road.
But check to make sure that is your problem as the pump is $$ and they want your old one for the core. plus there is a thing/valve that keep your fuel from flowing back away from the pump so look into that being messed up/plugged. Just my 2 cents :)

[Chamberpains]

I'd also check your bleed back valve and other pressure check valves. They could be getting gummed up. Here's the video for your part of the fuel system.

https://youtu.be/HvvFElrc5ak

[Nilao]

Not knowing what type truck I'm going to throw my opinion into the ring....my truck uses voss fuel fittings. When its super cold outside the plastic contracts at a different rate than the metal fitting it connects to. This allows the fitting to suck some air after they get some age on them. The orings get hard and won't take up the larger gap anymore. If your fuel line are quick connect like mine you may need new orings...