Understanding EFI fuel pump prime strategy…October 8, 2020
Give me two seconds!
Detroit abandoned the carburetor for fuel injection more than thirty years ago. Now, the same thing is happening with gasoline-powered agricultural engines.
You will find electronic fuel injection on everything from a lawn tractor to an irrigation pump engine.
Do not be dismayed. Even though each application has its own unique features, the basic tenet is the same, using a two-second prime signal.
Whereas a carburetor has a float bowl along with a needle and seat to control fuel flow and act as a reservoir, an EFI system does not.
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 of 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 cylinder head’s intake port.
Most but not all throttle-body systems operate on low fuel pressure (around 9 to 13 psi) while the port injection designs employ at least 45 psi fuel pressure.
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 engine’s injector to start and run.
The systems are designed to accomplish this with a theory 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 tach signal from the distributor (or a crankshaft signal from a single-cylinder engine) is not received after two seconds, the fuel pump is shut off. 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 be energized 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 failsafe starting procedure that allows the fuel pump to be evoked if the relay fails.
The backup is usually wired into the oil pressure switch. Suppose the two-second prime circuit fails. If the engine is cranked over long enough for a predetermined amount of oil pressure to be created, the fuel pump will turn on via the oil pressure switch, bypassing the fuel pump relay.
Proper starting and diagnostics
The proper method to start any EFI engine in a car, truck, or other use, 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.
It is RUN, wait, turn to CRANK. This will provide the quickest and most reliable start regardless of how cold the weather is.
Suppose you bypass the two-second prime and go right into CRANK. In that case, the injectors are being pulsed when there is little to no fuel in the rail, the operating pressure then takes longer (more cranking time) to build.
When faced with a no-start condition or extended crank cycle, your first diagnostic tool is your ears.
Listen for the two-second prime. If you do not hear it, then the fuel pump is not being evoked, 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 the engine starts, a good rule is the fuel pump relay or circuit has an issue.