Trouble shooter

Motor,  Jul 2003  by Seyfert, Karl

• E-mail
• Print
• Link

Air, always an enemy of diesel fuel systems, has several new places to hide in Ford's Powerstroke engines. Tracking it down may try your patience.

No Air Apparent

Our shop is working on a 2001 Ford F-250 Super Duty, with a 7.3L Powerstroke engine. The engine lost power and stalled while the customer was driving. After considerable difficulty, he was able to restart it, only to have it stall again. The engine would not restart after that. We have verified that the electric fuel pump runs and is supplying fuel. We also replaced the fuel filter (primed it first), but the engine still would not start. At one point I suspected that the injection pressure regulator (IPR) valve might be to blame, but now I'm leaning toward the fuel shutoff valve. Am I in the right area, or is there something else I should, be looking for?

Ron Harrison

Pine Key, FL

Well, Ron, aside from a few basic facts, your letter didn't give me much to go on. Perhaps it would be best to begin with an explanation of how the Powerstroke's diesel injection system works, then finish with possible causes for your no-start difficulties and some suggested diagnostic tests.

Older diesel engines were equipped with hydraulic injection systems that used an injection pump to pressurize the fuel and actuate the injectors. The amount of fuel injected depended on the mechanical operation of the injection pump governor, which adjusted volume based on engine load and rpm. On these designs, any air that's allowed to enter the fuel lines can affect the injectors, possibly preventing them from operating and keeping the engine from starting.

The Powerstroke diesel is similar to those earlier designs, but with some key differences. It supplies fuel to the injectors through fuel rails inside the cylinder heads, which reduces the number of exposed lines. Instead of using an injection pump to fully pressurize the fuel, the Powerstroke relies on pressurized engine oil. When the control unit determines that a cylinder should receive fuel, it signals the injector driver module (IDM), which sends a 110-volt pulse-width modulated signal to the injector solenoid atop the injector. The solenoid opens a poppet valve, which allows high-pressure engine oil to flow into the intensifier piston. This oil forces the intensifier piston downward, pressurizing the fuel already inside the injector. When the pressurized fuel inside the injector reaches approximately 21,000 psi, the injector pintle moves off its seat and fuel is injected into the cylinder through the injector nozzle.

The control unit manages the injector solenoid on-time, measured in milliseconds. Fuel is injected as long as the poppet valve remains open and high-pressure oil is flowing into the injector. The control unit also regulates the pressure of the oil reaching the injectors by adjusting the duty cycle of the injection pressure regulator (IPR) valve.

The cavity at the top of the amplifier piston is seven times the size of the fuel cavity at the bottom. This hydraulic advantage further multiplies the strength of computer-controlled oil pressure to pressurize the fuel inside the injector. So a maximum oil pressure of 3000 psi produces an injected fuel pressure of 21,000 psi.

The very high oil system pressures require a very strong return spring, to ensure that the injector poppet valve closes promptly when the injector solenoid is deactivated. When the poppet valve closes, spring pressure returns the injector to its closed state. The engine oil is exhausted into the valve cover area and eventually returns to the sump. The high operating pressures also explain why the injector solenoid needs 110 volts and 8 amps to operate. The necessary power is supplied by the injector driver module.

On older diesels, any air in the fuel could cause a no-start or other drive-ability problems. The compressible air in the system diminishes the fuel pressure at the injectors, possibly keeping them from opening, or allowing them to open only part way. Since the Powerstroke system relies on pressurized engine oil to pressurize the fuel and inject it into the cylinders, air in the oil is possibly a greater concern than oil in the fuel. These engines require a special antifoaming oil additive to prevent oil aeration. The additive is included in oil with an API service rating of CF-4 or CG-4. However, it becomes depleted over time, so regular oil changes are critical. If your customer has gone 3000 to 5000 miles (depending on how the vehicle is used), an oil change would be a good place to start. Consult the vehicle owner's manual for specific oil change interval and grade recommendations.

Air may also be getting into the fuel supply system. To quickly check for the presence of air in the system, press down on the pressure tap Shrader valve immediately after one of your starting attempts. Straight fuel (no air) should be expelled. You may also need to temporarily install a piece of clear tubing in the fuel return path to the tank. Any air that's found its way into the system will be clearly visible (no pun intended).