Chevy and GM trucks 1992-1999 (with 6.5L engine)

94-00 GM 6.5L Troubleshooting

 

The 6.5 GM uses a pump mounted driver (PMD) which is an electronic device mounted to the injection pump. The PMD tends to get overheated and is prone to failure if not relocated to a cooler location with a PMD relocation kit. This is a common problem with all 6.5L GM's even if not converted to SVO, conversion to SVO may exacerbate this problem do to the heating of the fuel. It is strongly recommended that the PMD is relocated prior to SVO conversion. Access to the PMD is difficult, it is easiest to purchase the PMD relocation kit with a new PMD module and just leave the old PMD on the injection pump.

 

The fuel selector valves can be mounted to the fire wall near the center of engine to keep hose lengths short. The hose connections to diesel filter and injection pump can be made at the rear of the engine, the top portion of the intake manifold needs to be removed for access , but entire intake manifold itself does not need to be removed. The inlet hose to the injection pump is 1/4”, 1/4” hose will slide over the 5/16” hose barb or an adaptor can be made with two hose barbs screwed together.

The fuel supply and return from diesel tank are located behind the engine by fire wall. They are a steel hard line which connect to rubber hose, access is difficult and on some models the wiring harness may need to be unfastened and moved slightly to gain access to the lines.

 

Problem: Lacks power, runs rough, stalls on vegetable oil and/or diesel.

*Before attempting to troubleshoot this or any other fuel system problem, verify vegetable oil filter and diesel filter are not clogged.

 

*Verify all plumbing connections are correct and match manual exactly.

 

*Verify wiring is correct: Use a test light to observe that both A and B valves energize when on veg, Only B valve energizes on purge and no valves energize when on diesel. Also confirm that valves are grounded securely..

You can also test operation by listening/feeling the valves, they should produce an audible click when turned ON or OFF.

 

 

Cause:

 

 Air leak: To find an airleak, a clear section of fuel line can be installed in various locations throughout the fuel system to determine the source of the leak. Run the engine to observe any air bubbles in the clear line, if necessary, move the section of clear line to other locations to determine the exact location of the leak. A vacuum/pressure gun (Mityvac or cylindrical type suction gun) can be handy to apply pressure or vacuum to fuel system to help find the source of an air leak. If any fuel leaks are noticed, repair this first as it is probably the source of the air.

 

            1) Diesel or vegetable oil filter seal damaged or not tightened securely

 

            2) Leaky hose connection(s).

 

3) Verify solonoid valve stems are tight. These are the parts in the center of valve which screw into the body. If the stems requires tightening, a spanner wrench is the appropriate tool. If no spanner wrench is available, you can lock two nuts together at the top of the stem and then tighten with wrench. Do not use pliars to tighten stem as it is easily dented and will cause valve to jam.

 

            4) Verify pex line is submerged and secured to bottom of tank. Pex line should reach near the  center of tank so that fuel is less likely to slosh away.

 

            5) Verify drain screw on filter is secure

 

            6) Verify filter is tightened securely and seal is intact.

 

 

Restrictions/excessive vacuum: To check for restriction, use a vacuum gauge to monitor vacuum while engine is running, preferably under load. Install the vacuum gauge at the inlet of electric lift pump, if vacuum exceeds 14”, a high vacuum condition exists and can cause poor performance, air leaks and decreased filter life. A vacuum gun (mityvac) or suction gun can be used to apply vacuum or pressure to help diagnose restrictions and to clear any obstructions.

 

1) Clogged filter, change filter and inspect contents. Poor filter life can usually be attributed to inadequete filtration, poor oil quality or the presence of algea contamination in fuel system.

 

2) Tank not venting, verify vent in fuel cap is not plugged. To test for this leave cap loose or remove a screw from hatch plate. Also verify diesel cap is venting, the diesel cap must vent well since when purging, the volume of fuel displaced from tank is increased due to return fuel being sent to veg loop. This can cause increased vacuum in diesel tank at shutdown and may cause fuel to be drawn back to tank (loss of prime).

 

            3) Debris in tank obstructing pex line or covering inlet of pex line, inspect tank and pex line for debris.

 

            4) Kinked fuel hose or pex line, excessively long lengths of fuel hose or pex line.

           

            5) Pex line distorted/crushed by over tightening of compression fitting.

 

            6) Obstruction in valve, verify that valve flows freely and seals in both positions.

           

7) Electric fuel pump non functional or wired incorrectly, test fuel pump and wiring. There have been problems reported with the GM factory fuel pump power circuit, causing no power to fuel pump

 

 

Problem: Vegetable oil tank filling with diesel:

 

            Cause:

           

            1) Left on purge cycle

 

             2) B (return valve) stuck in ON position when on diesel, sending returned diesel to veg tank

 

            3) A (supply valve) not energizing when switched to veg or has poor ground, causing diesel to return to veg tank

 

 

           

Problem: Diesel tank filling with vegetable oil

 

            Cause:

 

 

            1) Not purging long enough or frequently switching between fuels.

 

            2) B (return) valve not energizing or has poor ground, stuck in N.O. Position

 

            3) A (supply) valve stuck in ON position

 

           

 

Problem: Will not start after conversion

 

            Cause:

 

  1) Fuel system not primed thoroughly. The diesel filter should be filled with diesel, the veg loop should be primed using a vacuum or suction gun. To prime the veg side, turn ign switch to on (not start) position and switch to veg (energize A valve), connect the vacuum gun to Com valve A and draw fuel from tank, through filter, to the com port on A valve (or inlet to lift pump). It may take a few a minutes to accomplish this, make sure fuel is atleast 70F as priming will be more difficult if vegetable oil is too cool . After priming veg side, switch back to diesel and perform the same priming procedure. If vehicle still will not start, check for air leaks.

 

Problem: Filter and/or tank not heating well, little or no coolant flow

 

            Cause:

           

            1) Coolant hoses air locked, try filling hoses with coolant and then driving vehicle for approximately 20 minutes. Monitor coolant level and add as necessary, as air is displaced, coolant level will drop.

 

            2) Coolant hoses kinked or routed excessively high.

           

            3) Coolant hoses not plumbed correctly. Supply coolant must be plumbed into heater supply hose from Cyl head, return coolant hose must return to water pump.

 

           

Problem: Low vegetable oil temps.

 

            Cause:

 

            1) Verify meter is accurate, double check with thermometer or another meter. Verify that meter is wired correctly and is grounded well.

           

            2) Incorrect vegetable oil filter installed and does not fit in coil securely, causing poor heat transfer. If a crossover filter is used, use a strip of aluminum flashing or foil tape around the filter so that it fits in the coil securely.

           

            3) Low coolant temps. Verify coolant is approximately 190F, if running to cool, replace thermostat. In cold weather, MBZ's may require that the radiator is blocked enough to maintain heat.

 

  4) Filter or hoses exposed to direct wind, insulate as necessary and avoid excessive hose length.