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EFI Fuel Pump Prime Strategy

Detroit abandoned the carburetor for fuel injection on cars and trucks more than 30 years ago, and now the same thing is happening with gasoline-powered agricultural engines. You find electronic fuel injection (EFI) on everything from a lawn tractor to a utility vehicle to a welder engine. Even though each of these applications has its own unique features, the basic tenet of how it works is the same.

There are two types of EFI in use on small engines: throttle body and port. With the throttle-body system, the injector sits over a throttle plate and sprays fuel on top of it. This design is a combination carburetor and EFI. The other style has an injector for each cylinder that administers the fuel at the junction of the intake manifold runner and the intake port in the cylinder head.

Most – but not all – throttle-body systems operate on low fuel pressure (around 9 to 13 psi), while the port-injection designs employ fuel pressure of at least 45 psi. With either design, the only fuel storage is in the rail that the injector is connected to with a regulator controlling the pressure.

Since there is no float bowl to act as a reservoir, fuel needs to be delivered to the injector for the engine to start and run. The systems are designed to accomplish this with what the industry identifies as a two-second prime.

When the ignition is placed in the run position, the fuel pump is evoked via the electronic control unit (ECU) and a fuel pump relay for two seconds. If a tachometer signal from the distributor (or a crankshaft signal from a single-cylinder engine) is not received after the two seconds, the fuel pump is shut off. At this time, the fuel rail is charged up to the injector. 

To confirm the two-second prime, simply turn the ignition to run but do not go into crank. You will hear a buzz (the fuel pump running) and then a click when the relay opens and the pump shuts off two seconds later. If the ignition is then turned to crank, the fuel pump will turn back on via the relay once the ECU sees a tach signal that the engine is cranking. If the tach signal is ever lost, the fuel pump shuts off immediately as a safety precaution.

Many systems also include a fail-safe starting procedure that allows for the fuel pump to be evoked if the relay fails. This backup is usually wired into the oil pressure switch. If the two-second prime circuit fails and the engine is cranked over long enough for around 4 psi of oil pressure to be created, the fuel pump will turn on via the oil pressure switch, bypassing the fuel pump relay.

starting and diagnostics

The proper method to start any EFI engine in a car or a truck is to turn the ignition key to run and no farther. Wait for the fuel rail to prime, and when you hear the fuel pump relay shut off, continue into the crank mode. The proper sequence is run, wait, turn to crank. This will provide the quickest and most reliable starts regardless of how cold the weather is. If you bypass the two-second prime and go right into crank, the injectors are being pulsed when there is little to no fuel in the rail and the proper pressure then takes longer (more cranking time) to build.

When faced with a no-start condition or an extended crank cycle, your first diagnostic tool is your ear. Listen for the two-second prime. If you don’t hear it, then the fuel pump is not being activated and you need to see why. If you continue to crank the engine and either the oil pressure light goes out or the gauge begins to register and then it starts, most likely the fuel pump relay or circuit has an issue. 

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