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HOW YOU CAN DIAGNOSE A LIFT PUMP
If your truck "Bucks" under hard load or towing this is an indication the engine is probably starving for fuel, due to low lift pump pressure. To test fuel pressure, install a fuel pressure gauge with a long hose on it after the fuel filter and before the injection pump. The long hose allows you to drive the truck and watch the gauge at the same time! Or..........if this is inconvenient for you come by, you can install our “Low Fuel Pressure Warning Kit” and it will diagnose low fuel pressure too. To diagnose lift pump performance click the ignition key to the start function quickly, so the engine doesn't start, and let go, leaving the key in the run position; the lift pump should run 25 seconds. If you don't hear the lift pump, check fuse number 9 in the fusebox on the left side of the dash, and if that is good, test for 12 volts going into the lift pump and if it doesn't run with 12 volts going into it, replace it. If you do hear it run and it doesn't make at least 5 PSI replace the fuel filter. If, after changing the filter, it DOES make at least 5 PSI go DRIVE the truck UNDER LOAD. If it doesn't make at least 5 PSI UNDER LOAD after changing the filter, or if you have to pressurize the fuel tank to bleed the system to get the truck to run, then change the lift pump. Please realize that revving it up proves NOTHING. If DRIVING UNDER LOAD the pressure drops below 5 PSI, replace the fuel filter (if you haven't already done so) and if that doesn't fix it, I am 99% sure you need a new lift pump. Another possible cause of low pressure is rusty fuel lines, sucking air. The way to test for air in the line is to replace a section of the fuel supply line with a clear polyethylene hose, between the lift pump and the injection pump, and run the engine to see if there are any bubbles, indicating air as the culprit. We proved on a dynomometer in 1998 that if you have 5 PSI, under load, you can make all the power available from a VP44, and 5 psi will provide plenty of return fuel for pump lubrication. We do not recommend running more than 12-15 PSI as that does NOT help the fuel system in any way. In fact higher pressures could diminish fuel delivery to the rotor and make the truck run worse at high RPM and possibly overheat and damage the fuel bypass solenoid. This diagnosis is only for the lift pump, but is necessary for the successful diagnosis of possible injection pump issues. If you don’t have enough lift pump pressure it will ONLY cause low power or bucking under load. All other driveability issues are caused by the computer on the top of the injection pump, or by fuel contamination and stuck internal parts in the injection pump. Replacement lift pumps that make more than 15 psi are in my mind overkill and not necessary. The FASS system is a dependable alternative and very popular, but not necessary. The "In Tank" unit sold exclusively by Dodge does NOT work as well as the old style on the side of the engine! Cummins still sells the exact replacement that mounts on the side of the engine and that is what we sell. Thanks for reading, Chip Fisher, owner of Blue Chip Diesel Performance
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