6.9 not running right

[wiguy]

Hello, I am having a problem with my 1986 F350 with the 6.9idi and manual 4 speed transmission. I have read many other posts on this site and they have given me great insight on these trucks as i have never owned a diesel and just bought mine this summer. Anyway, the truck was running fine until it started getting cold, then it started running rough- surging and sometimes dying. I replaced the "olives", return lines and lift pump but it is still doing the same thing. One thing I noticed is there inever seems to be any fuel coming out of the valve on the filter even when it is idling ok, I hooked up a fuel pressure/vacuum gauge to it and it showed negative fuel pressure (vacuum) when it is running, could this be my problem? Thanks in advance for any help!

 

[79jasper]

Lift pump could be bad. It's said here that the airtex/carter pumps are junk. (Never had a problem myself)
Have you changed your filter?
Also bypass the water separator. Search here for it, you'll see why.

Sent from my SM-T537R4 using Tapatalk

 

[wiguy]

yep I should have said that, I did replace the fuel filter with a wix and my water seperator has already been deleted, I took off the line that goes to the filter head from the lift pump and turned over the engine with the glow plugs disabled and the wires disconnected to the IP and there was a healthy pulse of fuel on each rotation of the engine so I assume the lift pump is working ok?

 

[dieselpower33]

i had a 6.9 a few years ago and it did the same thing when it started getting cold. i did all of what you did then i put a new IP and a after market water sep and while i was at put all new injecters on while i had it apart. timed the IP and that truck ran like it did brand new.

 

[wiguy]

Hoping not to have to change the IP if at all possible! Wanted to rule out everything elsefirst, from what i understand if I depress the shrader valve on the filter head when it's running fuel should actually come out, am I correct? In my case air is being sucked in for some reason I don't understand.

 

[Matt33]

i've heard of many people who installed their lift pump wrong and as a result broke of its arm. the IP will suck enough to run without it once its primed, which may have kept it running from then till now.
since you don't seem to be getting anywhere cranking it, i would pull off the lift pump, even test it by hand - should suck at the intake port when moving its arm. or maybe just go buy an electric lift pump, plenty of threads about that around here, those WILL prime your system in just a few seconds, even with air in the lines.
however if all is good its your IP i know it sucks! but after you replace it you will be so happy they run like you wouldnt belive!

 

[wiguy]

Oh it starts no problem and runs, just rough, when i pulled the line from the lift pump to the filter head off and cranked the engine over quite a bit of fuel came out of the line so I assume the lift pump is working fine.

 

[BDCarrillo]

The schrader valve test port on a 7.3 style filter head taps into the unfiltered fuel. You could rule out a clogged filter then. It does seem like the lift pump isn't supplying an adequate flow if you have vacuum.

I concur with 79 Jasper that the lift pump is the likely culprit. Make sure you haven't gained any oil as well... when the pump diaphragm fails it will push fuel into the crankcase.

 

[wiguy]

Yes i read about lift pump failure, I don't seem to have anything extra on the dipstick, seems like if the lift pump diaphragm went I wouldn't be getting any fuel from it or am I wrong?

 

[Matt33]

i myselfe would lean to the IP but im just a welder!

 

[BDCarrillo]

If the lift pump isn't providing pressure, per you measuring a vacuum, it's either dead or you have an issue upstream of it.

 

[wiguy]

hmmm... that's what's baffling me I seem to be getting plenty of fuel from the lift pump and the truck runs ok at idle, but why am I having vacuum at the filter head???

 

[BDCarrillo]

The appearance of adequate free flowing volume from the pump does not equate to proper pressurized delivery through the filter.

 

[wiguy]

hmmm... yeah i wish good electric fuel pumps were available at parts stores, was planning on installing one anyway so imight as well order one and see what effect that has, looks like the carrier transcold is the one of choice?

 

[BDCarrillo]

I'd suggest the carrier over a mr. gasket green pump... the mr. gasket appear to be hit and miss on longevity, while I've read very few complaints on the carrier units. Most failures were attributed to alternative fuels.