Looking back into automotive history, it’s hard to believe that carburetors were still being installed on import vehicles as late as 1989! But carburetors require all sorts of external vacuum and electronic gimmickry to control the cold-starting, fast idle, hot idle and low-speed cruise functions. Electronic fuel injection has since replaced carburetors in all import applications because, unlike a mechanical carburetor, the computer-operated electronic fuel injection system requires only a few electronic inputs such as throttle position, coolant temperature, barometric pressure and intake air flow to provide accurate fuel metering to the engine.
FUEL INJECTOR CONFIGURATIONS
Early fuel injection systems were often throttle-body injection (TBI) configurations designed to be a bolt-on replacement for carburetors. Although TBI is less uncommon on imports than domestics, a few of the import TBIs that did appear were unusual because they incorporated one injector for part-throttle operation and another for full-throttle operation. Unlike multiport fuel injection, most TBI fuel injection units don’t require an intake manifold vacuum source to modulate fuel pressure because the fuel injection process occurs above the throttle plates at atmospheric pressure.
Unlike TBI fuel injection, multi-port fuel injection uses one injector per cylinder to introduce fuel into the intake port. Because intake manifold vacuum affects the volume of fuel delivered by a port fuel injector, most multi-port systems use fuel pressure regulators modulated by intake manifold vacuum.
Multi-port injection can also be timed to operate simultaneously, in “banks” of two or more injectors, or sequentially in the cylinder firing order. During initial cold cranking, they can be simultaneously fired once or twice for cold enrichment. Many early multi-port injectors were fired in banks, which meant that some V-6 engines might, for example, have their injectors fired in two banks of three, or in three banks of two. Some sequential fuel injection systems may revert to banked operation if, for example, the camshaft position sensor (CMP) fails to determine an engine’s firing order by locating the compression stroke on #1 cylinder.
FUEL INJECTION PRINCIPLES
First and foremost, let’s keep in mind that the fuel injector consists of a “pintle” valve held in the closed position by a coil spring. When the pintle valve is pulled open by an electromagnetic solenoid, fuel is sprayed into the intake port. Most fuel injection systems supply B+ voltage to the fuel injector when the ignition is turned on. In this system, the PCM may ground the injector circuit for a few thousandths of a second (milliseconds) in order to inject fuel into the intake port. As engine speed and load increases, the duration of the pulse in milliseconds increases to supply more fuel to the engine.
The injector system described above is an NPN or power-to-ground system. In relatively rare applications, the PCM may also be designed to pulse B+ power to a fuel injector that is already grounded. These systems are called PNP fuel injections. When observed with a lab scope, the PNP waveform is upside down in comparison to the more common NPN waveform.
Many throttle body fuel injection systems and a few port fuel injection systems hold the fuel injector open for an extended time by using a “peak-and-hold” strategy. To illustrate, about four amps might be required to pull the pintle valve open against the fuel pressure supplied by the fuel pump. When fuel begins to flow through the injector, only one ampere might be required to hold the pintle valve open. The peak-and-hold system displays a characteristic “two-peak” wave pattern on a lab scope because the injector driver actually deactivates the injector circuit twice during a single injection cycle. The Bosch peak-and-hold injector waveform also displays a unique waveform by rapidly cycling the injector on and off during the “hold” phase.
Because fuel injector diagnosis is a very lengthy topic and requires advanced-level skills in lab scope analysis, I’ll summarize some basic methods used to diagnose faulty fuel injectors. As for the most basic methods, TBI units can be checked visually for injector activation and spray pattern. Multi-port injectors can be tested by using a stethoscope to listen for their characteristic pintle click when activated by the PCM. All fuel injectors can be tested for specified resistance between their two-connector terminals.
On a more advanced level, a few PCMs will measure the current draw through the injector circuit and may store a diagnostic trouble code when the current draw is too high, too low or not present. Beginning with the introduction of OBD II diagnostics in 1996, the PCM may record a misfiring injector as a P0300 cylinder misfire trouble code. In either case, the starting point for diagnosing a suspected fuel injector problem on an OBD II system is to test for possible trouble codes with an applicable scan tool.
Partially clogged fuel injectors are probably the most difficult to diagnose. Clogged fuel injectors on OBD I on pre-1995 imports usually reveal themselves as cold driveability or sluggish acceleration complaints. Factors that aggravate fuel injector clogging are short-trip driving, excessive idling, low-speed driving conditions and poor fuel quality.
Clogging may be caused by dirt collecting in the “basket screen” located in the injector’s fuel inlet or, most commonly, by carbon and fuel residue collecting at the pintle valve itself. Either instance reduces fuel flow through the injector and causes the fuel to be sprayed into the intake port in an erratic, uneven pattern.
In order to detect clogged injectors, some engine management systems incorporate a scan tool-based injector test that measures a drop in fuel pressure when the PCM pulses a selected fuel injector. A similar test may be performed by using a fuel injector pulsing tool to manually trigger the fuel injector. Of course, the results of either test are invalidated if the fuel pump, pressure regulator or another fuel injector is leaking fuel pressure.
FUEL TRIM NUMBERS
Aside from sluggish throttle response and an occasional misfire condition, clogged injectors in 1996 and later OBD II-equipped imports may tend to increase fuel trim numbers. To better understand how fuel trim numbers are generated, the PCM compares the oxygen sensor or air/fuel sensor input voltage against a predetermined fuel injector pulse width value. If, for example, a longer than normal injector pulse width is required to maintain an average oxygen sensor voltage of 0.5 volts, then the PCM displays an increased fuel trim value in its datastream.
Of course, accurate fuel trim values are based on an accurately calibrated voltage input from the oxygen sensor. While most OBD II imports can detect relatively minor oxygen sensor calibration problems very quickly, most OBD I imports can’t. So, when diagnosing any fuel injection malfunction, it’s important to make sure that the oxygen sensor has been changed at recommended intervals or is accurate and responsive to fuel trim changes.
Other issues that may affect fuel trims are incorrectly calibrated air flow sensors, MAP sensors, low fuel pressures and vacuum leaks. Fuel trim numbers are best utilized on V-block configurations in which a clogged or shorted fuel injector will cause an increased fuel trim number for that particular cylinder bank.
SELLING FUEL INJECTOR SERVICE
Since multiport fuel injection originally appeared 30 years ago on import vehicles, it’s clear that many of these high-mileage systems are now failing to deliver full performance. Fuel injector service can range from adding a fuel injector cleaner to the fuel tank or by complete replacement of the fuel injectors themselves.
Generally speaking, using additives to clean fuel injectors is a better maintenance than repair strategy. In most cases, a modern fuel injector flushing machine provides the best in-vehicle results because most machines mechanically “pulse” the system in order to remove dirt from the basket screens and fuel rails.
Fuel injectors can be also be economically serviced by removing the injectors and having them inspected, cleaned and flow-tested in a professionally-equipped fuel injection repair shop. Last, some production models of fuel injectors are vulnerable to shorted windings.
The best long-term repair strategy in this case might be to install a complete set of new fuel injectors. Whatever service is performed, it’s important to always install a new fuel filter, and test the fuel pump pressure and volume before considering the fuel injection service complete.