An injector is the valve that lets a measured mass of fuel into one cylinder. When it sticks, leaks, wears or cokes, that cylinder runs rich, lean or late, and the others may be dragged with it if the leak steals rail pressure. Petrol port injectors, petrol direct injectors and diesel common-rail injectors fail in related ways and are tested differently. The pump that is supposed to keep the rail full is on fuel pump faults. The mixture and quantity logic is on fuelling faults.
Common petrol engine problems and common diesel engine problems both link here.
What “one cylinder” looks like
A single bad injector shows on one cylinder: a misfire count, a diesel quantity correction that has marched to one end of its range, a balance test that fails on that hole, or a plug that is wet or sooted only there. A pump, a filter or a pressure regulator shows on all of them. The exception is a diesel injector whose return flow is so large that the pump cannot keep the rail up. The engine then cranks without starting, or starts and stalls, which feels like a pump. Comparing the return from each injector is what separates them.
Pico’s account of common-rail back-leakage is the mechanism. The rail pressure fills the injector. When the solenoid or the piezo stack is energised, a small release of fuel into the return line drops the pressure on the control piston, and rail pressure lifts the nozzle. That return is normal. It lubricates and cools the injector. If the control seat or the nozzle leaks, the return rises, rail pressure falls, and the fuel that should have been injected is in the return hose instead. Or it dribbles into the cylinder.
Diesel common rail
Bosch’s solenoid-injector return test, as written into the Vauxhall Multijet procedure for that engine, compares the injectors with each other rather than with one universal volume. The engine is run, or cranked if it will not start, with a measuring container on each return. In that procedure the largest return may be up to three times the smallest. Above that, the high injector is the suspect. Bosch’s wording of the same rule, on the return-quantity device, lists the driving symptoms of an excessive return as a bad start, a no-start with the starter turning, the engine stalling, and a loss of power. The three-times rule is that procedure. Piezo injectors and other solenoid families have their own limits and their own adapters. Pico notes that manufacturers specify return volume against time. Use the figure for the injector that is fitted.
Third-generation Bosch piezo systems hold pressure in the return, about 10 bar in Bosch’s common-rail testing description, and the measuring device has to keep that pressure. Venting a piezo return into an open bottle, as if it were an older solenoid, is not the test.
Cylinder correction values support the story. They do not replace the return test or a bench test. A correction can move because compression is low, because a valve does not seal, or because the injector quantity is wrong. EDC15 diagnostic data shows the earlier form of the same comparison. A bench test, of the sort Bosch diesel test benches are built for, measures delivery and spray with the injector off the engine. That is the step when the on-car tests disagree, and it is the step that avoids replacing a full set because one cylinder was noisy.
Coding matters on injectors that are trimmed at the factory. The code tells the controller how that injector differs from the nominal one. Fitting a coded injector and leaving the previous code in the controller gives a quantity error on a good part. The code is on the injector. It is not a tune.
A nozzle that dribbles fills the cylinder after the injection event. The signs are diesel knock, black smoke, a wet cylinder, and fuel in the oil. That last one overlaps the oil dilution caused by an aborted particulate-filter regeneration. Rising oil level needs both possibilities. An injector leaking into the sump through a sealing washer, rather than past the rings, is a further diesel path and shows as fuel in the oil with a corresponding loss from the return or the leak-off circuit.
High-pressure joints stay pressurised after shutdown. The Multijet procedure tells the technician to wait at least 30 seconds after stopping the engine before opening the system. The vehicle procedure may demand longer. Diesel at rail pressure is an injection injury risk.
Petrol port injection
A port injector is a solenoid spraying into the intake port. It fails by not opening, by sticking open, by a spray that has collapsed into a jet, or by a pintle that is varnished. A stuck-open injector floods one cylinder, washes oil off the bore and can put petrol into the oil. A blocked injector leans one cylinder out. Because the fuel lands on the back of the inlet valve, a working port injector also keeps that valve clean. The misfire tests are a balance check, a listen or a current trace that shows the solenoid was actually driven, and swapping with a neighbour. A click only proves the pintle moved. It does not prove the spray or the volume. Haynes lists clogged or leaking injectors among the fuel causes of a misfire, next to a weak pump.
Petrol direct injection
A direct injector sprays into the chamber at the rail pressure the high-pressure pump has made. Bosch’s injectors meter and atomise that fuel in the chamber. The failure Professional Motor Mechanic describes at the tip is specific: fuel left in the nozzle after shutdown is cooked by residual heat, the deposits restrict delivery, the cylinder runs lean, and the tip runs hotter because the fuel that should have cooled it is no longer flowing. The same article is the source for the inlet-valve side of direct injection. The fuel never washes the valve, so oil mist from the crankcase ventilation bakes on. That carbon is not an injector fault, even though it misfires and looks like one. Cleaning it is on carbon deposits and cleaning.
A leaking direct injector shows as a long crank when hot, because the rail has bled down through the nozzle and the cylinder is wet, and as a misfire on that cylinder once running. Rail pressure that will not hold after shutdown, with the pump and the control valve proved, points at an injector or a pipe joint. Do not crack a direct-injection rail without the depressurising procedure. The pressures on current Bosch systems go up to 350 bar.
A usable order of tests
- Read misfire counters or diesel quantity corrections with the freeze frame. Note whether the fault is one cylinder or all of them, and whether it was cold, hot, idle or under load.
- Confirm rail pressure follows the setpoint under the condition that failed. If it does not, do the pump and filter checks, and on a diesel do the return comparison before condemning the pump.
- On a diesel, compare return quantities with the equipment and the limit for that injector. Include a cranking test if the engine will not start.
- On a petrol engine, swap or disable cylinders in a way the controller will report. A fault that moves with the injector is the injector. A fault that stays in the hole is spark, compression or, on direct injection, the valve and port deposits.
- Check the oil for fuel if the level has risen. Separate an injector leak from post-injection during a regeneration that did not finish.
- Code any injector that requires it, then clear the corrections and repeat the road test that produced the fault.
Sources
- Pico Technology, back-leakage testing. Why common-rail injectors return fuel, and how a leak appears in that return.
- Vauxhall Astra H procedure for Bosch solenoid common-rail injectors. The three-times comparison against the smallest measured return, the symptoms of an excessive return, and the wait before opening the system. This limit is that procedure.
- Bosch Mobility, gasoline direct injection. High-pressure injectors metering fuel in the chamber, at rail pressures up to 350 bar on the systems described.
- Professional Motor Mechanic, carbon in GDi engines. Cooked deposits in the injector tip, and inlet-valve deposits that fuel no longer washes.
- Haynes, fault code P0300. Clogged or leaking petrol injectors as a misfire cause.
- Bosch diesel test benches on this wiki, for off-engine injector quantity and spray checks.