Help, engine stumbles at hi speeds,

[Steve Graham]

My truck will accelerate good through 2nd and 3rd gear to red line. 4th is good till the RPM's gets near 2000 rpm, then it stumbles, power falls off and will not go past 2000 rpm. In 5th it stumbles and will only hold speed.



So far have changed fuel and air filters and found no change and checked the aneroid for problems, it looks good. The injection pump is stock. The lift pump was replaced at 80K, now have 153K miles on the truck

I pulled the fuel line at the lift pump, lowered it to the ground and fuel comes out at a good flow, no apparent restrictions. I hate to throw parts at, like another new lift pump?

Any suggestion?



Steve

 

[Greenleaf]

That's why a (good) diesel mech. makes good money. They don't throw parts.



You will have to put your troubleshooting hat on... ... ... .



Install a fuel pressure gauge and a fuel restriction gauge and test run the vehicle under the exact same paramiters.



Or, throw parts.



GL

 

[cerberusiam]

Like Scott said, throw parts or try a little cogitation.



Any smoke from the exhaust when it falls on its face? What color?

 

[bgilbert]

An old boost gauge, an aftermarket drilled and tapped fuel banjo bolt, and boost tubing kit is all you need to check fuel pressure. Sounds like the lift pump isn't doing its job. Did you buy a Cummins LP when you replaced it? FWIW, $200 can have you setup in a more reliable healthy piston lift pump.

 

[Steve Graham]

Just got back from the Dodge Dealership looking for a "O" ring kit for the quikdisconects that are on both ends of the fuel line. Parts guy didn't even try looking any thing up, said you half to but the whole fuel line, wonderful.



Cerberusiam, no smoke form the engine to speak of.



The lift pump was from NAPA. It started leaking while I was on a trip, not near any Cummins shop.



There is that bleed on the banjo bolt going into the fuel filter, will make a adapter for that hole or get another banjo bolt and build a custom bolt that will hold two banjo bolts.



Thanks guys.

Steve

 

[tugboatphil]

Steve,



Along with a pump replacement, I got the higher flow banjo bolts from Geno's Garage.



I forgot what your truck looked like so I went to your gallery. I remembered the olive paint. I love it!!

 

[Steve Graham]

results of a little cogitation

And I think I have the solution! Since this (generally) only happens when I give it large amounts of fuel and the fact that it will cruise. Through 2nd and 3rd gears the engine is not being taxed very much, never much on the EGT, so I figure the problem must be the lift pump!

I have a pump on will-call at Cummins SW in PHX and will pick it up in the morning.



Steve

 

[Steve Graham]

Success!

A $100 well spent.

Another clue that it was the lift pump, I could operate the lever and cover the output port with my thumb and failed to feel any pressure build up.



Steve,

 

[BushWakr - RIP]

That may not always tell the tale... if the lift pump is sitting on the cam lobe that drives it, you'll have no effect pumping the priming lever... . just an FYI. .



pb...

 

[bgilbert]

$100??? Wow, I thought they ran about $50 . Didn't wanna spend an extra $100 and go for the more reliable piston lift pump?

 

[Steve Graham]

BushWakr: forget to mention that the pump test was done with the pump on the bench not attached to the engine.



One thing that that I remember in hindsight that should have flagged me was when I primed the fuel filter after changing it the priming lever never stopped pumping. It would never build up enough pressure where the diaphragm would not pump anymore. That should be a warning to everyone.



I did a destructive investigation on the lift pump (tore it apart). The intake valve had failed. These resemble a thumb tack made of "rubber" or some flexible material. The stem holds the valve in place and disk is the valve. The valve part had broken from the stem. I guess that the fuel in the line from the tank gave it enough resistance to allow the pump to work and pump fuel up to the injection pump but not enough resistance to go fast.



Steve

 

[Steve Graham]

I don't know if a piston pump would be any more reliable if it used the same valves.



Steve



#ad

 

[JLEONARD]

$100??? Wow, I thought they ran about $50

I paid around $75 for mine thru NAPA with my son's discount.

 

[Greenleaf]

The piston pump is entirely different. They (Cummins) will have many different flow rates per the application. Run with the OEM pump for some time untill you require more fuel than it will supply and upgrade to the mechanical piston or an electric model pump.



-S

 

[bgilbert]

I paid around $75 for mine thru NAPA with my son's discount.

Well I'm starting to feel as though that $200 I spent on my piston LP upgrade wasn't too bad after all.

 

[BushWakr - RIP]

I'd agree with Scott... the piston pump moves alot of fuel at higher pressure... at least more than most need.

If you can regulate your pressure a bit you'd do ok... . I don't recall off hand what the numbers are but I'm sure someone can jump in here with that info.



I know that when I replaced my stock lift pump with a "new stocker" my pressures came up pretty good.

On a flow bench we tested a VE by varying the supply pressure from roughly 6 or 7psi to as much as 18psi and didn't see any significant difference... . to be fair though... . that test was done with a "specified test orifice"... not the larger ones that would translate to the larger injectors we run so it's your call...



pb... .

 

[BushWakr - RIP]

I'd agree with Scott... the piston pump moves alot of fuel at higher pressure... at least more than most need.

If you can regulate your pressure a bit you'd do ok... . I don't recall off hand what the numbers are but I'm sure someone can jump in here with that info.



I know that when I replaced my stock lift pump with a "new stocker" my pressures came up pretty good.

On a flow bench we tested a VE by varying the supply pressure from roughly 6 or 7psi to as much as 18psi and didn't see any significant difference... . to be fair though... . that test was done with a "specified test orifice"... not the larger ones that would translate to the larger injectors we run so it's your call...



pb... .