'84 Lift Pump Replacement

[bmjwright]

My 84 is having a starting issue which I believe is the lift pump. It'll start fine, run for about 20 seconds, then die. It'll restart after more cranking and run fine. The truck will start fine the rest of the day. I figure a weak diaphragm in the old pump is leaking down after not being ran for a day.

So, any tips for replacing the pump? Or is it just like a gasser (swap the pump out and crank until it starts)? I'm also going to replace the return lines while I'm at it.

 

[tRustyK5]

Not quite like a gasser, although the swapping is...

Get it swapped, pull the big pink wire off the injection pump. Pull the fuel supply line off after the filter, crank engine til you have fuel coming out. Put fuel line back on, cycle the GP's and fire it up. It might run a bit rough for the first few minutes as it works out the residual air from the lines...but should clear up quickly.

Double clamp the lines too. All it takes for problems like you're describing is a slightly loose clamp. Not enough to visibly leak fuel, but enough to draw in air on shutdown. In fact I'd go through all the fuel line connections first before blaming the lift pump.

Some guys have had good luck running an electric pump as well as the mechanical one...and report easier starts. I never got around to doing that to mine though.

 

[bmjwright]

Thanks, I'll probably change the other rubber lines too while I'm in there.

 

[kennyw]

You can also bypass all the bleeding issues by blowing air into the fuel return hose at the injector pump with the bleed screw cracked at the fuel filter. Turn your air compressor output down to 5-10 psi while doing this to avoid a large mess.

 

[diesel4me]

I would think if the diaphram was bad and not holding a prime,then some diesel would migrate into the crankcase over time,and raise the oil level on the dipstick..also why would it start right up THEN die,if it lost prime it should need to be cranked over a while to build up pressure before it would fire at all..unless the check valves in the fuel pump let the fuel go back into the filter and tank or something..

Before you replace the pump I would look closely at the steel fuel lines ,any pinhole from a rust flake can cause air to get in them,yet not always let diesel leak out..if the fuel filter is clogging up that can cause that too..changed that recently?..is it the square box style with the leaky O-rings ?..also I have read if the return line gets restricted it will cause it to stall too,maybe its got some crud blocking it up some..

 

[bmjwright]

My theory is that it'll start, then die because there is still fuel in the injection pump and lines...just enough to make it run for half a minute or so before it dies.

It hasn't been long since I replaced the fuel filter (box type). I'll check the steel lines while I'm in there.

 

[tRustyK5]

Exactly. The remaining fuel in the injection pump and lines is enough for it to start and run for a few seconds.

 

[bmjwright]

Lift pump follow-up:
After scouring through several forums I decided instead of replacing the mechanical pump, I'd go with an electric model. I'd settled on the AIRTEX E8153, but went with the Precision brand model on the advise of the counter guy due to Airtex failures. I also picked up a Wix inline filter and some hose clamps. I keep 10' or so of fuel line on hand.

Install went easily. The pump instructions required a filter and to be mounted as close as possible to the fuel tank (electric pumps push better than they pull). Several posts indicated that there was a rubber section break in the metal fuel line, not the case on my truck. I ended up installing the filter and pump right below the fuel selector valve. I had to cut about 10" of metal line, grounded the pump to the tank, and ran the + wire to the pink wire on the injector pump.

Under the hood, I ran new hose from the metal line that was going to the old mechanical pump to the stock box fuel filter. I capped both ends of the mechanical pump just in case the diaphragm was to break or if I someday wanted to go back to the mechanical pump.

The new pump made priming the system very easy and after checking for leaks, I fired the truck up. It started instantly and kept running. Drove it again today, started instantly (and stayed running).

 

[diesel4me]

My truck had an electric fuel pump mounted on the crossmember behind the cab when I got it,but it was not connected or hooked up!..looked brand new too..

Someone had butchered the dual tank setup on it--they removed the switch on the dash,and the valve under the truck was also gone,and someone had mickey moused the plumbing to the two tanks with lots of rubber hose and a manual three way valve..one fuel tank leaked,so I decided to chop out all that jerry rigged crap and run the truck on just one tank--and I found that electric pump worked fine,so I decided to wire it up and use it as a booster to feed the stock mechanical lift pump..(which looked new BTW)...

After I got everything installed and bled,I noticed the truck ran smoother and seemed to have a bit more power with the electric pump switched on--I can shut it off and the truck will stay running ,so I guess the lift pump can pull fuel thru the electric pump when it is off--I leave it on all the time though,its wired to the ignition so it works only with the key "on"...

I do have to keep an eye on my oil ,because if the lift pump did decide to lose the diaphram,the electric pump could well fill the crankcase with diesel fuel...so far so good ,its been 10 years so far and no issues..I like having the electric pump because in the event you ever ran out of fuel or had to open up the fuel system and let air in it,it self bleeds after you open the bleeder on the fuel filter and turn the key on--once fuel comes out there,it'll start up quickly after closing the bleeder..