5.2 Port Fuel Injection (PFI)
Key Takeaways
- Multi-port fuel injection places one injector per cylinder just upstream of the intake valve, giving more uniform distribution than single-point or throttle-body injection
- Saturated (high-impedance) injectors typically measure 11 to 17 ohms; peak-and-hold (low-impedance) injectors measure roughly 1 to 4 ohms and use a current-limited driver
- Typical PFI pulse widths run about 1.5 to 4 ms at idle and lengthen significantly under wide-open throttle and acceleration enrichment
- An injector balance test applies an identical pulse to each injector and measures the fuel-rail pressure drop; readings should match within about plus or minus 2 psi
- Carbon and varnish deposits on the injector tip distort the spray pattern long before flow rate drops enough to set a code
PFI Architecture
Port fuel injection (PFI) delivers fuel into the intake port, just upstream of the intake valve, where it mixes with incoming air before entering the combustion chamber. Rail pressure on a typical PFI system is 40 to 60 psi and is controlled by a mechanical pressure regulator or returnless electronic strategy.
Single-Point vs Multi-Port
| System | Location of injector(s) | Notes |
|---|---|---|
| Throttle body / single-point | One or two injectors above throttle | Legacy systems; uneven cylinder distribution |
| Multi-port | One injector per cylinder at the port | Industry standard for PFI; better distribution and emissions |
Most modern PFI applications are sequential multi-port: each injector is fired individually, timed to the intake stroke of its cylinder. Older or limp-home strategies may use grouped/bank firing, where injectors pulse in pairs or in a single bank simultaneously. Sequential firing gives the most precise mixture control and the best transient response.
Injector Drivers and Resistance
Fuel injectors are solenoid-operated valves. The PCM grounds the injector circuit to open the pintle, and the time the ground is held is the pulse width.
- Saturated drivers (high-impedance injectors) — The injector winding has high resistance (typically 11 to 17 ohms). Battery voltage is applied for the full pulse width and current self-limits.
- Peak-and-hold drivers (low-impedance injectors) — Resistance is low (about 1 to 4 ohms). The driver applies a high peak current to snap the pintle open, then reduces to a lower hold current to keep it open without overheating.
A quick resistance check with the harness disconnected is one of the first tests: a high-impedance injector reading 4 ohms is shorted; an open winding reads OL.
Pulse Width and Flow
Pulse width is the most direct view of how much fuel the PCM is commanding.
| Condition | Approximate Pulse Width |
|---|---|
| Cranking (cold) | 5 to 20 ms |
| Warm idle | 1.5 to 4 ms |
| Cruise | 2 to 5 ms |
| Wide-open throttle | 8 to 20+ ms |
Each injector has a rated flow in cubic centimeters per minute (cc/min) at a reference pressure. When replacing one injector, all should be checked or replaced as a matched set so the flow per pulse is consistent across cylinders.
Injector Balance Test
The injector balance test is the L1-relevant procedure for finding a weak or restricted injector without removing it.
- Connect a fuel pressure gauge to the rail.
- Disable the fuel pump so the rail will not refill.
- Use a scan tool or test box to apply an identical pulse (often around 500 ms) to one injector at a time.
- Record the pressure drop after the pulse.
- Compare drops across all cylinders.
All injectors should drop the rail pressure by the same amount within about plus or minus 2 psi. An injector that drops the pressure noticeably less is flow-restricted (clogged tip, deposits). One that drops noticeably more is leaking or has a worn pintle.
Cleaning vs Replacement
Deposits build on the injector tip from low-quality fuel, especially after hot soak, when residual fuel bakes onto the nozzle.
- In-rail chemical cleaning with a pressurized canister can restore mild deposits and spray pattern.
- Ultrasonic off-vehicle cleaning with flow testing is more thorough and produces measured flow data.
- Replacement is required when an injector has a damaged spray pattern, a stuck-open condition, or fails resistance limits.
On the L1, the correct answer often emphasizes diagnosing the cause of the deposit (poor fuel, no top-tier detergent, leaking injector after shutdown) so the repair does not have to be repeated.
During an injector balance test, four injectors drop fuel rail pressure by 9, 9, 4, and 10 psi respectively when pulsed identically. Which is the most likely conclusion about the injector that dropped 4 psi?
A technician measures an injector with the harness disconnected and reads 2.5 ohms. This injector is most likely: