Lift pump woes . . . again

[DrRoth]

The lift pump on my 91. 0 is failing AGAIN!!!! I am tired of this problem and want to know more about the upgrade pump that I've heard about. I know that one is available from cummins, but I also heard that carter makes a piston style replacement. If so, which pump is better, which one is cheaper, and what other parts will I need for the install?? Oh yeah, where can i get one and approx. how much will it cost me??? I know that's a lot of questions but can anyone help??

 

[bgilbert]

Within my readers rigs are pics and part numbers of the low pressure Cummins piston style lift pump upgrade. Stick with Cummins parts. Although I believe the Cummins piston LP is made by Carter. You will need the other parts below for upgrade/conversion as well.



Cost, ~$150 for pump, $27 for new fuel line, $1. 00 for two gaskets, $5. 00 for a 'spacer', $1. 22 for two seals. =~<$200.

 

[JLEONARD]

Doc,

How long has the pump been lasting?

The stock style pumps are notorious for being reliable.

Jay

 

[Greenleaf]

You know it's failing because you did what to diagnose it?



-S

 

[DrRoth]

I have owned this truck since 1996 and have probably replaced the lift pump 5 times in about 160k miles. My truck quit running (not getting fuel) and I suspected the lift pump again so I removed it and manually attempted to pump it and it did nothing. On a new one that functions, the pumping action is evident. Are you hinting at that I may have something causing the pump to fail? I run the cleanest diesel I can find (sometimes i take what I can get on the road) and always try to use an additive such as power service in the winter time. I have already ordered the new pump from cummins, and plan to install it upon arrival. Any other suggestions/comments? Is the old style pump really notorious for longevity???? Most people I have talked to (in my area) have had similar problems with them. Most stations provide 50/50 no. 2/no. 1 diesel in winter, could this be a problem? Come to think of it that last two that failed were in winter . . . is this possible??

 

[Greenleaf]

Not likely due to winter mix fuel. It is a tad bit lubrified in terms of injection pump lubrication qualities, however it would not effect the lift pump enough to warrent concern.



You will want to test your fuel psi going to the injection pump and test the vacuum between the pump inlet and fuel tank.



There is a specification as to how much fuel the lift pump flows. You would have to test the pump to determine flow rate @ a given rpm.



Our lift pumps will rarely fail. They do wear out over time, however we are talking like 100,000 miles if even that. The piston pumps will fail more often than our diaghram pump will, and that ain't often.



-S

 

[DrRoth]

How do i test the flow rate? or the vacuum coming from the tank? Also you say that piston pumps are MORE likely to fail?? Should I be concerned about more problems with the new pump style? It was my understanding from reading other posts that the piston pump was better and will last longer??

 

[JLEONARD]

DrRoth, In the marine applications a vacuum gage would be installed at the filter... so I'm guessing we could install one in place of the bleed fitting on top of the filter. Any body care to comment on that??



Is it worth checking the hoses from the tank to the pump for kinks, etc??? Something that would cause a restriction??

Just a thought.

Jay

 

[Greenleaf]

A Cummins manual will give the details on pump flow specs. You have to crank the engine (for a specific time)and collect then measure the amount of fuel.



The vacuum gauge is placed at the inlet of the pump.



The piston pumps are rebuildable. They will fail just like anything mechanical can. In basic terms, the diaphram pump will last 200,000 and a piston pump will last 100,000. These are my observations. Someone will now argue this or prove me right.



-S

 

[bgilbert]

Since 96, how often have you changed your fuel filter? Fuel stations don't do nothing for anti-icing, they only garantee that the fuel when it comes out of the hose/nozzle its not jelly, but in liquid form. Couple a bad or dirty fuel filter with iced up or gelled fuel, hmm sounds like a fuel delivery problem. BTW where did you buy those old diapraghm lift pumps from, hopefully Cummins. ? Best thing for cold weather driving is try to keep the fuel tank as full as possible, don't wait til your low on fuel to get it. This will keep condensation down in the tank. Use diesel fuel additive every time you get fuel. Don't rely on the fuel station having treated fuel, they only dump the additive in there once temps reach say below 20*f. If you don't put your own additive in, you may add 20+gallons of untreated fuel at say 32*f, then encounter low temps and your fuel gonna gel up on ya. Run some anti-icing or water remover in your tanks, yes they make diesel fuel water removers. Next thing, when checking to see if you diapraghm lift pump works, when you prime the lever and no fuel comes out of the top of filter or bleed screw, bump the engine over a few times with the starter, this will get the camshaft aligned so that the plunger of the lift pump is riding on a lobe of the cam.

The only reason we were saying the piston LP was better is it will give more pressure and will keep up with fuel demand when one starts bombing the pump or turning up the fuel etc. Buy a fuel pressure gauge or take the truck to a diesel shop and have them test your fuel pressure.

On edit: the new LP you bought, did you buy another diapraghm or piston LP? If piston, you will need all the parts I listed for conversion.

 

[DrRoth]

The fuel filter has been replaced regularly (about every other oil change or every 10k miles). I bought a piston pump, from cummins (even though it's not a cummins part??) this time. The diaphragm pumps I have put on before were from either napa or oreilly's. With napa, I had two bad (out of the box bad) pumps in a row. The last one was from oreilly's and lasted about a year (maybe 30k max). When I would use the pumps manually to pump fuel to prime the line i would always seem to get decent fuel flow with minimal effort, so i know, beyond the shadow of a reasonable doubt, that I at least don't have a severe blockage. Also would there be some way to pressurize the tank with a few psi of compressed air in order to check to see if I have decent fuel flow from the line at the engine? Also does the fuel tank have a breather of some sort? I have noticed that sometimes when i remove the filler cap that I hear a hiss of a slight vacuum. Could this be contributing as well?

Oh yeah and I do have all the parts needed for conversion.

 

[bgilbert]

And you bought the piston lift pump Cummins part number 3936320? Very important that you did and not just any plain ole piston lift pump. BTW, I think your problem was from the non-Cummins lift pumps. Another cheap good thing to do for fuel delivery, is remove the two banjo bolts at the fuel filter housing, and drill the holes in them out bigger, and smooth so you will increase fuel delivery or flow. -GL's idea.

 

[DrRoth]

Yes I bought the part number 3936320. I have also ordered the replacement banjo bolts from genos' garage to put on with the bigger holes. Thank you.

 

[JLEONARD]

The vacuum gauge is placed at the inlet of the pump.

Yes, you're correct. . my bad. Mixing my applications again!

lso would there be some way to pressurize the tank with a few psi of compressed air in order to check to see if I have decent fuel flow from the line at the engine?

Yes I have done this to find a fuel leak... you can use an air gun and a rag to seal it at the tank fill... . don't use much pressure. . in other words let it leak past the rag. . you only need a few psi in the tank.

 

[DKasper]

Could there be some restriction between the tank and the lift pump that is stressing the capabilities of the lift pump? I can't imagine 5 pumps being bad/wearing out in that period of time . . .

 

[k2diesel]

Could there be some restriction between the tank and the lift pump that is stressing the capabilities of the lift pump? I can't imagine 5 pumps being bad/wearing out in that period of time . . .

Funny you should ask. I've been suspecting fuel supply restriction for some time now on my 91. 5. I've been getting intermittent blue gray smoke as well as poorer mileage, and an idle speed that varies +/- 200 rpm depending on vehicle inclination (truck pointing uphill or down). Replaced fuel filter twice (cut them open both times-clean) and the lift pump. Then, as per the shop manual, I tested for fuel restriction by installing a vaccuum gauge before the lift pump. The manual states : " maximum allowable restriction is 3. 75 inches hg. Do Not operate engine if restriction is above this amount, or damage to the injection pump may occur". Well, whaddya know, climbing a hill under load my gauge showed app. 10 inches hg! And when backing downhill and applying the brakes, it would read 8, while my tach fell 200 rpm! And when heading downhill and applying brakes, tach would gain 200 rpm and restriction was at 0-1! So I'm saying to myself, self, it's time to yank that bed off, get in that tank, and see what the heck is going on. So, I fire up my trusty 843 bobcat, (diesel, of course) and in about 30 minutes I've got the bed unbolted and off. Got the fuel pickup assembly out, and sure enough, the inner filter screen was pretty much plugged up with glomation, oh yah and there was also an old fuel additive cap-foil floating around in there too ! Has anybody ever taken out their fuel pickup assembly and really LOOKED at it? What a convoluted mess of a design. No sir, I don't like it. It has a plastic reservoir cylinder~4" in diameter that is screwed to 2 slotted steel brackets to allow for adjustment, the screws had loosened up and it was sliding up and down around an inch and a half. It has an outer screen at the bottom with a float and a foot valve. It appears that under normal operation, the float is up, allowing fuel to flow from the outer screen, up through the foot valve and into the pickup tube. But when the tank is almost empty, the float falls to the bottom of the screen cage closing the foot valve and causing the fuel to now be drawn from the smaller inner fuel screen within the plastic cylinder/reservoir. But guess what? The float gets saturated in time, and sinks, stays sunk, causing the fuel to always be drawn through the smaller, more prone to clogging, inner screen. And by the way, that hard plastic, spiral, fuel pickup tube? It's probably less than 1/4" inside diameter at its smallest point. The guys have been talking a bit lately about fuel supply restriction and electric pumps and what not. I'm thinking of adding an electric pump too. But take the time and check your fuel pickup assembly, you may be amazed at what you find. GOOD LUCK! K2

 

[JLEONARD]

K2,

Good find and good report.

 

[DrRoth]

Hey thanks for all the help guys, I put the new pump on and all is going well for now. I checked ( what I could) for a restriction, and finding nothing, I wonder if I may have a similar problem as described above. However, how hard is the assembly in the tank to get out? My flatbed would take hours (maybe days!) to remove (three toolboxes, tools, welder, air compressor, etc. ), I have more above tank room than with a bed, can I still do what I need to do?

 

[mudcat]

You could drop your tank down instead of removing the bed. I have done this on a second gen and after looking at mine and hearing K2Diesels description i believe they are the same design. Be it a poor one. It isn't that hard to drop the fuel tank and remove the pickup/ sending unit.

 

[DrRoth]

Thanks, I'll try dropping the tank.



I think I might have a leak in the fuel line somewhere, but I'm not sure. I can't find any fuel, but I think I'm getting air in the system. When the truck sits for more than a couple of hours (overnight), it doesn't like to start. Before, it would start on the first or second crank, but now it cranks more and misses at first. Do I have an air leak or just some leftover air in the system (or something else)? Any recommendations?