9.3 Electronic Fuel Injection Circuits, Fuel Pump & Injector Testing
Key Takeaways
- High-impedance saturated injectors typically measure 12-16 ohms, while low-impedance peak-and-hold injectors measure about 1.5-3.0 ohms.
- Most port fuel injection systems run 250-400 kPa (about 2.5-4.0 bar) of rail pressure, and gasoline direct injection adds a second stage of 50-350 bar.
- The ECU usually commands the fuel pump for about two seconds at key-on to prime the rail, then cuts it until a crankshaft signal appears.
- A restricted fuel filter holds normal pressure at idle and collapses under load, so an idle-only pressure test routinely misses it.
- Never open a gasoline direct injection high-pressure line while the engine is running or immediately after shutdown, because residual pressure can inject fuel through skin.
What the Competency Standard Requires
Repair Electronic Fuel Injection System (EFI) sits inside the Repair Fuel and Emission Control System standard. Its performance criteria require you to check the EFI system with a scan tool, verify fuel pump pressure with a pressure tester, check injector resistance with a multimeter, and replace a clogged or contaminated fuel filter. The auto electrician owns the electrical half of this system: the pump circuit, the relay, the injector drivers and the sensor inputs that set pulse width.
The Fuel Pump Circuit
A typical port-injection fuel pump circuit contains:
- Fuel pump relay - a standard ISO relay using DIN 72552 terminals. Terminal 30 is fed from a high-current fuse, terminal 87 feeds the pump, terminal 86 receives switched ignition, and terminal 85 is switched to earth by a low-side driver inside the ECU or body control module.
- Inertia or impact cut-off switch - fitted on many vehicles in series with the pump feed and tripping on a collision. A tripped switch is a genuine no-start cause and simply needs resetting once the reason is confirmed.
- Fuel pump module in the tank, containing the pump, the strainer, the pressure regulator on returnless systems and the level sender.
- Dedicated earth point, often on the body under the rear seat or in the boot, which is a frequent corrosion site.
Prime behaviour. On most vehicles the ECU energises the pump for roughly two seconds at key-on, then switches it off until it sees a crankshaft position signal. Listening for that two-second prime from the fuel tank is the fastest first check on a no-start, and its absence points straight at the relay, the ECU driver, the inertia switch, the fuse or the pump.
Electrical test sequence on a dead pump
| Step | Test | Interpretation |
|---|---|---|
| 1 | Voltage at relay terminal 30, key off | No voltage means the high-current fuse or feed is open |
| 2 | Voltage at relay terminal 86, key on | No voltage means the ignition feed or its fuse is open |
| 3 | Voltage drop from terminal 85 to battery negative during the prime window | More than about 0.2 V means the ECU low-side driver or its earth is faulty |
| 4 | Voltage at the pump connector during the prime window | Voltage present but no pump running means the pump or its earth has failed |
| 5 | Voltage drop on the pump feed and earth under running load | More than about 0.5 V total means circuit resistance is starving the pump |
| 6 | Pump current draw with a clamp meter | Typically 4-8 A; a high reading suggests a dragging pump, a low reading suggests high circuit resistance |
Circuit resistance is the quiet killer here. A pump receiving only 10 V instead of 13.5 V spins slower and delivers less volume, producing a symptom that looks exactly like a failing pump, which is why the voltage-drop test at step 5 must be completed before condemning the pump.
Fuel Pressure Testing
The competency standard requires a fuel pressure tester, not an estimate. Typical specifications:
| System | Pressure | Note |
|---|---|---|
| Port injection with return line and vacuum-referenced regulator | 250-300 kPa (2.5-3.0 bar) at idle, rising 50-70 kPa when the vacuum hose is removed | The regulator holds a constant pressure differential across the injector |
| Returnless port injection with in-tank regulator | 350-400 kPa (3.5-4.0 bar), constant | No vacuum reference; the ECU compensates in software |
| Gasoline direct injection, low-pressure stage | 400-600 kPa | Feeds the mechanical high-pressure pump |
| Gasoline direct injection, high-pressure stage | 50-350 bar, load dependent | Read only with a scan tool, never with a workshop gauge |
Three readings matter: running pressure, pressure rise when the regulator vacuum is removed on a return system, and residual pressure after shutdown. A rail that bleeds down quickly after key-off points to a leaking injector, a leaking regulator or a failed pump non-return valve, and explains long cranking after the vehicle has stood.
A restricted fuel filter produces normal pressure at idle and a pressure collapse under load. Testing only at idle is the most common way to miss it. Replace the filter at the manufacturer's interval; contaminated fuel is a real risk in dusty operating conditions, and that is exactly why the competency lists filter replacement as a performance criterion.
Injector Drive Types and Resistance Testing
| Type | Resistance | Drive method | Notes |
|---|---|---|---|
| High impedance (saturated) | 12-16 ohms | The driver grounds the injector for the whole pulse width | Most modern port injection |
| Low impedance (peak and hold) | 1.5-3.0 ohms | High opening current, then reduced holding current | Older performance and some diesel-derived systems; never drive one from a saturated driver, which will overheat |
| Gasoline direct injection | About 1-2 ohms, driven by a 50-90 V boost circuit | Boosted voltage generated inside the ECU | Boost voltages are hazardous, so back-probe only with approved leads |
Measure injector resistance at the injector connector with the harness disconnected and compare all cylinders. A spread of more than about 1 ohm across a matched set is significant even when each reading sits inside the nominal band. An open injector reads OL and never fires; a shorted injector can destroy the ECU driver.
Testing the drive signal
- A noid light plugged into the harness connector flashes when the ECU is pulsing. It proves the drive circuit only; a steady glow or no flash both indicate a circuit fault.
- Scan-tool live data shows injector pulse width, typically 2-4 milliseconds at warm idle on a port-injected petrol engine, rising with load.
- Bidirectional injector control lets you cut one injector at a time and observe the resulting drop in engine speed. A cylinder producing no speed drop is already not contributing.
Injector balance test
With a pressure gauge on the rail and the pump prevented from running, pulse each injector for an identical time and record the pressure drop:
- All injectors should drop rail pressure by a similar amount.
- A smaller drop indicates a restricted or partially blocked injector.
- A larger drop indicates a leaking or stuck-open injector.
- A spread beyond roughly 10 kPa between cylinders warrants cleaning or replacement.
Inputs That Set Injector Pulse Width
The injector is only the output. When the mixture is wrong, the fault is usually upstream.
| Input | Effect on fuelling when it fails |
|---|---|
| Mass air flow or manifold absolute pressure sensor | Scales the base fuel calculation directly; contamination usually under-reports airflow and creates a lean condition |
| Engine coolant temperature sensor | A sensor stuck reporting cold causes permanent enrichment, black smoke and poor economy |
| Intake air temperature sensor | Applies a density correction trim |
| Throttle position sensor | Sets acceleration enrichment; a dead spot on the track causes a stumble on tip-in |
| Oxygen or air-fuel ratio sensor | Provides closed-loop correction, covered fully in section 7.3 |
| Crankshaft and camshaft position sensors | Set injection timing and sequence; loss of camshaft phasing forces batch fire |
| Battery voltage | Injector opening time lengthens at low voltage, so the ECU applies a voltage-correction offset. A weak charging system genuinely changes fuelling |
Safety Rules for Fuel System Work
These are assessed in the practical and are not negotiable.
- Relieve fuel pressure before opening any line, using the manufacturer's procedure, which is typically removing the pump relay or fuse and running the engine until it stalls.
- Never open a gasoline direct injection high-pressure line while the engine is running or immediately after shutdown. Pressures up to 350 bar can inject fuel through skin and cause a life-threatening injection injury that needs immediate surgical treatment.
- Remove ignition sources. No smoking, no grinding and no unsealed drop lights. Keep a suitable fire extinguisher within reach.
- Catch fuel in an approved container and use absorbent material. Fuel must never enter workshop drains.
- Disconnect the battery negative terminal before disturbing fuel pump wiring in the tank area, and ventilate the workspace before working near an opened tank.
- Replace single-use clamps and O-rings. A reused quick-connect O-ring is a fire risk.
- Wear eye protection and nitrile gloves. Petrol strips natural oils from skin, and repeated contact causes dermatitis.
An engine cranks but does not start. With the ignition turned to the ON position the technician hears no two-second prime from the fuel tank area. Which set of checks should be performed first, and why?
A technician measures four port fuel injectors and records 13.8, 14.1, 13.9 and 2.4 ohms. What does the 2.4 ohm reading most likely indicate?
A vehicle idles and revs normally in the workshop but loses power and hesitates badly when driven under load up a gradient. Fuel rail pressure measured at idle is within specification. What is the most appropriate next test?