Lift Pump Diagnostics & Troubleshooting Guide
Pre-1996 6.5 diesels only run the lift pump once oil pressure is built. The oil pressure sensor for these model years doubles as a switch, turning on the lift pump once oil pressure rises. The oil pressure sensor, or OPS is notorious for its frequent failures, which are do in part to the fact that the switches relatively fragile circuitry must carry the electrical load drawn by the lift pump. Detroit's engineers had good intentions when they devised this feature, but the results have been all but desirable. 1996 and later 6.5 diesels begin cycling the lift pump once the key is turned to the "run" position.
Symptoms of a failed or failing lift pump include:
• Hard start condition, excessive cranking
• Engine stutters under load, bucking or jerking during hard acceleration
• Poor fuel economy and overall performance
• No start condition (typically not a lift pump issue - check oil pressure sensor & PMD)
When replacing the lift pump, consider the fact that many owners find the 1992 to 1993 model year lift pumps to outlast and outperform the later pump. The lift pumps are cross compatible between the model years.
Lift Pump & Fuel System Diagnostics
Foreword - An engine can run with a weak or even failed lift pump. In fact, for pre-1996 model year engines, a no start condition is likely not a lift pump issue, but rather a failed oil pressure sensor/switch. The telltale of a complete lift pump failure is a bucking or jerking condition under load (usually acceleration), which indicates the injection pump is starving for fuel. The following steps will help troubleshoot all 6.5L diesel fuel system problems and narrow down the culprit. The procedures are slightly different for pre-1996 and 1996+ model year engines, primarily because pre-1996 engines must have oil pressure for the lift pump to receive power. 1996 and newer model year engines begin to cycle the lift pump as soon as the key is turned to the "run" position (i.e. the glow plug preheat is initiated).
In the following steps, cycling the lift pump refers to:
1992 - 1995 MY 6.5L Diesels - Cranking the engine over. The lift pump will only receive power once engine oil pressure builds. For these model years, the oil pressure sensor (OPS) doubles as a switch for the lift pump. There are ways around this, none of which we can recommend for liability reasons. Do not crawl under a vehicle when the engine is being cranked over. Perform applicable diagnostics from a safe position.
1996+ MY 6.5L Diesels - Cycling the key to the "run" position (as if initiating the glow plug preheat sequence). The lift pump will cycle on for a few seconds then stop. The lift pump should make an audible noise while it is pumping; when the sound goes away, turn the key to the "off" position and then re-initiate the sequence as necessary. You do not need to crank the engine over to activate the lift pump.
1) If an engine will not start, verify that the lift pump is receiving power. This is done by removing the electrical connector at the lift pump and measuring the voltage across the two terminals on the harness connector. If the lift pump does not receive power when it should be cycling (cranking for 1992 - 1995, key on for 1996+), there is an alternative problem. For 1992 to 1995 engines, the oil pressure sensor/switch is a likely cause. If the harness connector is being supplied a nominal 12 volts across the two terminals, the lift pump is receiving power as it should; continue troubleshooting, but the lift pump is unlikely to be the cause of the no start condition; verify battery voltage and consider that a PMD failure may have occurred. The 6.5L diesel also may be incredibly difficult, even impossible to start if there is a glow plug related problem (glow plug relay/controller and/or glow plugs themselves).
2) If an engine will start, but runs poorly and/or experiences fuel starvation symptoms that include, but are not limited to: misfire, hesitation, stalling, jerking/bucking sensation under load, or any combination of these conditions, a lift pump flow test is advised. Locate the bleed valve mounted behind the thermostat housing on the front of the engine. There should be a hose attached to the valve - locate the end of the hose (often tucked down into the engine valley for safe passage) and place it in a fuel safe container. Secure the container so that it will not tip while the engine is running and will not interfere with any moving components; you may wish to add a longer section of 1/4" fuel line so that you can place the container away from the engine. Start the engine, then open the t-handled bleed valve approximately 1/4 turn. If the engine immediately begins to stutter and/or stall, the lift pump needs to be replaced. If fuel flows slowly or inconsistently, replace the fuel filter, bleed the system, and perform the test again. The lift pump remains a likely culprit, but replacing the fuel filter before further diagnostics rules out a plugged and/or loaded fuel filter. If a steady stream of fuel flows into the container, the lift pump is functioning to some capacity, but a pressure test should be performed (see below).
3) Check the lift pump fuel pressure (procedures below). Pressure should be in the 3 to 7 psi range at idle. If fuel pressure is between 0 to 3 psi, replace the fuel filter, bleed the fuel system, then repeat the test (if you have not already replaced the filter). If pressure is out of spec, replace the lift pump. If pressure is within spec, the lift pump is unlikely a problem.
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• Locate the T-handle drain/bleed valve positioned directly behind the thermostat housing. The valve should have a section of hose attached to it from the factory, which is typically tucked into the engine valley. If there is no hose, attach a section of 1/4" fuel line.
• Place the end of the hose in a fuel safe container and start the engine. Open the drain/bleed valve ~1/4 turn (the valve does not need to be opened completely and is likely to leak if opened more than necessary).
• If the engine stalls, the lift pump needs to be replaced. If the engine continues to run and little to no fuel is pumped into the container, there is likely a restriction in the fuel line or the lift pump is weak. If the engine continues to run and a continuous stream of fluid exits the hose, perform the pressure check below.
• Install a fuel pressure gauge to the drain/bleed valve hose.
• Start the engine, then crack open the drain/bleed valve and read the fuel pressure. Fuel pressure should read between 3 and 7 psi at idle. Anything lower than 3 psi may indicate that the lift pump is not performing properly and should be replaced.
• If fuel pressure is within spec, the problem you're experiencing is not likely the lift pump.
Lift Pump Replacement Procedures
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• Disconnect both negative battery cables.
• The lift pump is located horizontally inside the frame rail on the driver side of the vehicle, roughly halfway between the engine compartment and fuel tank (exact lateral position will vary based on truck configuration/wheelbase).
• Place a suitable drain pan on the ground beneath the lift pump and rest a shop towel/rag on the inner frame rail below the pump. It is highly recommended that latex gloves are worn during these procedures.
• Unplug the lift pump electrical connector and position it aside.
• Remove the (2) bolts securing the lift pump bracket in place with a 13mm socket. Disconnect the electrical harness and return fuel lines from the bracket, then slide the bracket towards the rear of the vehicle off of the lift pump.
• Assuming you're installing an OEM ACDelco lift pump (as you should be, unless you've opted for a superior aftermarket alternative), the nuts on the lift pump inlet/outlet require a 19mm wrench while the fittings on the fuel line side require a 5/8" wrench - flare nut wrenches are preferred to prevent stripping. Note that the hex nuts on the pump are fixed and therefore only used to hold the fuel pump in place while the fittings on the fuel line side are loosened/tightened.
• Loosen, but do not remove completely the compression fittings at the pump inlet/outlet. If you loosen the fittings to the point that fuel begins to leak, you've loosened it too much. The goal is to loosen the fittings such that they can be removed by hand in subsequent steps.
•The trick to replacing the lift pump without taking a bath in diesel fuel is to use generic, flexible vacuum line caps with roughly a 3/8" I.D. They are available at any auto parts store for a few dollars and can be used to plug the fuel lines while you are both removing and installing the lift pumps. The alternative requires impeccable speed and coordination to quickly remove the old and install the new pump; it also tends to result in an oily mess.
• Remove the male fitting from the lift pump outlet, maneuver the fuel line forward towards the front of the vehicle (the outlet line moves generously and can be rotated slightly towards the passenger side), and plug the outlet line with your finger. Next, install a vacuum line cap over the fuel outlet line fitting. The tip of the fuel line is flared, so you will need to work the cap over the flare.
• Once the outlet fitting is plugged, remove the lift pump from the fuel inlet line, then plug the line with a finger until you are able to cap the line in the same fashion as the previous step.
• Install the replacement lift pump in the opposite fashion by removing the vacuum cap and tightening down the compression fitting. I prefer to install the pump at the inlet first, followed by the outlet. However, it is not imperative which side is installed/removed first and the outcome will be the same regardless - I just find that maneuvering the outlet line is easier without the pump installed on it, so I install the pump on the inlet first.
The lift pump is directional and labeled at the outlet; do not install backwards.
• Securely tighten the inlet and outlet fittings, then reinstall the lift pump/return line bracket and reconnect the electrical connector.
• Performed correctly, the drain pan will have seemed irrelevant and you're rag should be merely damp with diesel fuel. If you're covered in diesel fuel and have succeeded in creating an oily mess, don't be discouraged - some things get easier with experience, and that's why we recommend placing a drain pan beneath the pump!
• To bleed the fuel system of air, first install a section of 5/16" fuel line to the bleed valve located on the top of the fuel filter cannister and the opposite end inside a fuel safe container.
1996+ Model Year Engines: Cycle the key to the "run" position and leave it there until the lift pump stops cycling (you will hear a whining noise from the pump while it is operating). Once the pump shuts off, turn the key to the "off" position, then back to the "run" position. Continue this process until a steady stream of fuel comes out the line, then close the bleed valve on the fuel filter housing and remove the hose.
1992 - 1995 Model Year Engines: The fuel pump will not cycle unless there is oil pressure, therefore you will have to crank the engine to bleed the air out. With the bleed valve on the fuel filter open, crank the engine over for no more than 10 seconds, then position the key to the "off" position and allow the starter to cool for 60 seconds. When not cranking the engine, close the bleed valve on the fuel filter to prevent air from re-entering. Repeat this process until a steady stream of fuel flows out of the fuel line, then close the bleed valve on the fuel filter housing and remove the hose. Do not neglect to allow the starter to cool between cranking cycles. It is not uncommon for the engine to start running after 1 to 2 cycles, while some engines will take several attempts in order to bleed the fuel lines. If the engine starts, you can let it run until all air is bled from the fuel system and then close the bleed valve.
All Engines: Once air has been bled completely through the fuel filter housing, start the engine and purge the remaining air from the T-handle valve located behind the thermostat housing. Attach a hose to the valve first if there is not one and place the end of the hose in a fuel safe container (similar to the diagnostic process described above). Once a steady stream of fuel is observed and no air bubbles are purging through it, close the bleed valve. Note that there are several alternatives to these fuel system bleeding procedures.