Anatomy of a Lift Pump
Newcomers - and some older owners don't know exactly what a Lift Pump (LP) is, what it does, and what makes it tick - hopefully this will help towards a better understanding. Here's what a LP looks like:
This is approximately what the LP (Lift Pump) looks like as installed at the driver-side rear area of the Cummins in our trucks. 3 bolts attach the pump to a bracket on the engine, there is an in and out fuel line, and either a plug for power attachment to power the pump like this one, or on older styles, the wiring was attached to posts on the bottom of the motor. That style was eventually discontinued due to salt and other contaminents damaging connections and causing LP failures.
Below we see the internals of the actual pump section with the cover removed. You can see the vanes - this is a vane-type pump, and the rotor that holds them as they rotate inside the walls of the pump housing. This is the ONLY part of the pump that can be disassembled for any sort of cleaning or service.
Here below, is a closer look at the disassembled pump rotor, with the vanes laying in front - those vanes simply float in their respective slots, and press against the walls of the pump by the rotational force of the pump rotor. Occasionally, those vanes might become sticky and not float freely in their slots, reducing or stopping fuel flow.
Inside the pump housing can be seen the end of the pump motor shaft with the milled end that engages the matching slot of the nylon insert pressed into the pump rotor. Occasionally, the nylon shaft coupling will fail, and fuel flow will stop, even though the motor will still spin. No known means or replacements are available to replace a damaged one - so the pump usually must be replaced when this occurs.
Also seen, is the fine stainless steel mesh filter strip formed around the outside of the pump cavity to provide filtering of any coarser trash that might enter that housing<dead image link> http://img311.imageshack.us/img311/9895/lpump58yc.jpg
Below, we see the disassembled motor section on the left, next to the pump section on the right - as previously mentioned, the only way to disassemble the motor is to cut it open, which totally destroys it for further use. In the housing seen above the motor armature, on the left side, is the spring-loaded ball check valve used to regulate fuel pressure.
This is a VERY common failure point, when either the spring fails, or the ball beats the seat out of the housing, allowing fuel to merely circulate between the pump and motor, with none or little actually getting to the VP-44 injection pump. And as can be seen fuel actually does flow thru the motor as well as the pump inself in normal operation.<dead image link> http://img311.imageshack.us/img311/6483/lpump48qh.jpg
Here's a closeup shot down the inside of the motor, showing the bottom bearing and the green circuit board with electronics to eliminate static radiation in normal operation - also note the excellent condition of the brushes and other components. This pump had 110,000 miles on it when removed from service, and that only as a maintenance procedure, since it was still operating perfectly!<dead image link> http://img311.imageshack.us/img311/6058/lpump27hm.jpg
SO, there you have the anatomy of a LP, how it works, and some of the more common failure points!
Some additional issues as they apply to the stock LP. I have personally had *2* instances where the power plug to the LP became erratic, or lost connection completely, causing the engine to nearly die of fuel starvation - all that was needed to correct the situation, was to re-seat the power plug, and normal LP operation was restored.
A second area to keep watch on, is the push-on type hose connections to/from the LP - they have been noted for air leakage, especially after a LP has been changed and the original factory seal broken. Added to this, is the occasional tendency for fuel line splits, often right down inside the fuel tank where they can't be seen - those breaks usually reveal themselves when poor truck operation regularly occurs when the fuel level gets down to a point below where the line is split and leaking.
And finally, one of the best upgrades to the stock LP. is the addition of what is referred to as a "Pusher" pump - usually a Carter 4600 pump nearly identical in appearance to the stock LP - which is also a Carter item.
The added pusher pump enhances and assists operation of the stock LP, and is especially beneficial when power mods to the Cummins are added, and require a more abundant and reliable fuel flow.