Weak IP?

[moondoggie]

Good Day!

Is it possible that a weak lift pump could provide symptoms that would indicate the condition (old & weak) of the IP?

The story: My 95 Sub quit running; I don't remember all the details (it was quite some time ago). Prior to quitting & not being able to start, it acted sorta like a gasser with an ignition miss. I replaced the PMD/FSD, & it started & ran normally.

The next weekend, it quit again (at the boat ramp at a lake, of course). As I was loading the boat on the trailer, it again sounded like a gasser with a miss. I ran to try to hit the foot-feed & keep it running long enough to get the boat out of the way but to no avail - it died on the ramp & would not restart.

I tried two known good PMD/FSDs - no change.

I tested the lift pump output while cranking by opening the bleed valve & directing the flow into a can. There was LOTS of fuel flow.

I gave up & had it towed to the local dealer. He fixed it in no time by replacing the lift pump.

I've driven 6.5s with weak or dead lift pumps for many miles. The 1st lift pump I had fail I probably drove on for months (I obviously don't know for sure when it died, so I don't know how long it had been dead). The only symptom I got was bucking (again, like a gasser with an ignition miss) under heavy load, like climbing a hill.

My premise is this: perhaps my IP is weak but functional. Because it's old & weak, it needs the lift pump's full rated output, both flow & pressure (remember, I never looked at the lift pump output pressure) to work right. The mechanic replaced the lift pump & the Sub now starts & runs fine.

What say yea?

Blessings!

[DaveBr]

Here's a theory for you. You are on the boat ramp which is on an incline making the tank lower and the IP higher which may have caused your IP to starve. Maybe the steep incline on the ramp and the week lift pump combination was the perfect storm. Just a theory.

[Robyn]

Very good theory.

How much fuel was in the tank while on the ramp ???

There is a bypass suction port in the fuel pickup tube on these trucks, and if the fuel level is down to below 1/2 tank and maybe on an incline the thing can suck air if the little tank valve is weak/leaking.

My DaHoooley will do this (Stutter) if I allow the tank level to get down to around 1/4 tank.

I replaced PMD, lift pump, optic filter harness and pulled some hair out.
I was back driving the rig and noticed that it needed fuel.
As soon as I filled the tank the stuttering stopped.

A few weeks later with the tank level down again the issues came back.

Your issue may be the same.

The pickup tube has a sock on the bottom and part way up the pickup tube is a little spring loaded valve.
The valve will allow fuel to enter if the sock plugs up.

The other possibility is a small pinhole leak in the pickup tube that was above the fuel during the incident.

If your fuel level was down, I am suspect of this problem.

Another possibility, how is the fuel filter, been changed recently ?? A near plugged filter along with a weak lift pump, the boat ramp incline and maybe the tank issue and things can get real hinky.

If the lift pump is weak and the engine speed is low the transfer pump in the IP will not keep fuel flowing.

Keep us posted

Missy

[moondoggie]

Good Day!

Maybe the steep incline on the ramp and the week lift pump combination was the perfect storm. Could be, but it wouldn't start on the level, either.

How much fuel was in the tank while on the ramp ??? My best guess is that there was, & is, ~ 13 g in the (40 g) tank.

...how is the fuel filter, been changed recently?? No; it's dirty, but not very. Yup, it'll be getting changed soon. (Boy, do I hate it when questions like this get asked.) On the other hand, it now runs great again after being fixed at the Chevy garage in town.

pickup tube sock plugged, pickup tube pinhole: I'll be keeping an eye on things for awhile. I'm probably going to run the tank down to 5 gallons or so a few times to see if anything else rears it's ugly head. The problem is, since I began working out of our home 2½ years ago, I seldom buy fuel anymore; not a bad problem to have, eh? ;)

Thanks for all the terrific feedback. I mainly post stuff like this so folks can search & find it later.

What I also had really hoped to do here was explore my premise: Could what happened be a warning that my IP's getting tired? Remember - there's nothing to fix now, the lift pump installed by the local Chevy garage fixed it.

Blessings!

(signature in initial post)

[Robyn]

Once the thing died it had probably sucked in a bunch of air and then that would make it tough to start even on the level.

The last time DaHoooley did this stunt I quit out on the highway after I romped it hard. I crested a grade and it quit entirely, but I was going on down and the engine stayed hooked up due to the road speed. Took about a half mile before the fire came back on. Was fine after that.

The next day it stumbled in town and thats when I replaced the lift pump and other goodies. The thing stumbled again later and I filled the tank and poof, problem gone.

Here about amonth ago or so, the rig started stumbling after a long decel and then back on the throttle. Was the fuel filter. I run a 2 mic Racor and it was full of crap.

Fresh filter and all is well.

These little critters like good clean fuel and plenty of it.

The DS4 system flows a huge volume of fuel through the system to cool and lubricate the IP and to keep the PMD cool (That was the plan anyway)

This sytem can flow near 35 gallons an hour maybe a tad more according to the info I got through the standayne shop here locally

I sized my Racor for a flow of 40 gallons per hour through put on the filter.

The DB2 pumps dont shove near that much fuel through the system.

The nice part of the high flow volume is that it will keep the fuel warm in the winter and the IP relatively cool in the winter.


Missy

[DaveBr]

I had a DB2 pump that was worn out and it wouldn't start after pulling hard. I would have to wait for it to cool down or pour cold water on it before it would start. Folks here say add oil to your fuel and see if your truck runs better. The higher viscosity fuel makes it harder to bypass the IP plungers.

[moondoggie]

Good Day!

"Once the thing died it had probably sucked in a bunch of air and then that would make it tough to start even on the level." It wouldn't start on the level after I verified lift pump flow (but not pressure). I opened the brass valve, put 'er in some gear other than park or drive, turned the key to "crank" (this only works on 95s), & got what I thought was adequate fuel flow without air bubbles. I actually did this twice. It is, of course, possible that all the fuel in the filter etc. flowed backwards into the tank, such that I was seeing flow at the brass valve, but not enough got to the engine in the time I cranked for it to start. I doubt this, because when I tested, the flow was immediate; if it had run back into the tank, I'd think it might take awhile for me to see flow. But, hey - this stuff busts my chops most of the time - I'm open to believe anything. :confused:

The mechanic at the Chevy garage said he got the same thing: flow, but very low pressure (I didn't record what pressure he said he got).

As I said previously, I'm going to intentionally run 'er down to 5 gallons or so in the tank before refilling, & keep an eye on things for awhile. If I've also got trouble with pinhole(s) and/or plugged sock, etc., I'll jump on it & report here.

The troubleshooting help is wonderful, but I had hoped for thoughts on my premise. I'm under 1/3 tank, the truck runs perfectly since the mechanic replaced the lift pump...of course, the day ain't over yet, is it? ;) There's plenty of time for another gremlin to rear it's ugly head.

Blessings!

(signature in initial post)

[DmaxMaverick]

No one's mentioned it yet, so I will.......OPS. They can, and do, fail over time, intermittent, low current, and can/do kill fuel pumps. Poor wiring/connections/grounds can do the same.

Oh, yeah...One other thing.....CHANGE THE FILTER, silly man. That should have been step 1 (and you know that), especially since you KNOW it's dirty and due. Don't do anything else until you have a fresh filter in, and confirm you don't have a bowl full of junk. Don't suspect a "weak" or otherwise failing IP until everything upstream has been eliminated. Of all the components, the IP is the least prone to fail (although they get blamed/replaced most often).

[DaveBr]

You could go back to the boat ramp and see if you can make it happen again. If it doesn't then you have solved the issue.

[moondoggie]

Good Day!

"You could go back to the boat ramp and see if you can make it happen again." Actually, I'm going to. Well, not to the boat ramp, but a hill where I can roll down flat & get 'er restarted again. I'm letting the tank get real low first. I'll post results here. Unfortunately, we don't drive around all that much anymore.

I just wondered if this situation was telling me the IP's getting weak.

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