Automotive Fuel Injector Testing Procedures

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Automotive Fuel Injector Testing

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By Mandy Concepcion

The fuel injector is the main actuator in a modern fuel injection systems. It is responsible for supplying the engine with fuel for combustion. In order to obtain the near perfect air/fuel ratio in today’s engines, the injector must meter and deliver a precise amount of fuel into the intake runners. The correct fuel flow and spray pattern can only be achieved, over a long period of time, through a well-maintained injector. In modern OBD II systems, injectors are closely associated with misfire code problems. There are many reasons why an injector could cause a misfire code. 

A shorted injector coil that draws too much current, a bad injector driver, an ECM that cuts pulsation to the injector due to an overheating problem to keep the engine cooler and clogged injectors are all possible conditions that will set those persistent misfire codes. In some cases, as in an overheating engine, the problem is not the injector itself but some other condition that causes the injector not to pulse and therefore create the misfire. The vast majority of fuel injectors are ground controlled. This means that of the two wires going to the injector one is held at steady 12 – 14 volts while the other lead is pulsed to ground by the ECM. This type of injector circuit is called negative trigger. There are, however, a few (European) manufacturers that have used positive or battery voltage trigger injector circuitry in the past. With positive injector trigger, the positive side is the one being triggered by the ECM.

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The component of the ECM that triggers the injector is called the driver. Injector drivers fall into two categories, the saturation and the peak-and-hold type driver. The injector driver itself is nothing more than a high current transistor and its main function is to switch the injector on and off.

A good number of late model ECMs are using the more advanced microprocessors, with 32 bit processor systems on most of them. These computer systems are capable of shutting down the injector driver in the event of a short circuit, a severe misfire, or an overheating engine. The Cadillac Northstar 4.6L engine was one of the first systems to employ such an ECM. The system shuts down the injectors intermittently in the event of an overheating engine in order for it to work cooler and therefore protect the head gaskets. It is important to determine if the misfire or lack of injector pulse is the result of an ECM strategy to save the engine from damage or an actual malfunction.

The 1st point (inj. Turn-on) shows the ECM driver pulling the battery voltage to ground. This action turns the injector ON. The 2nd point in the waveform or the space between the two vertical lines gives us the injector pulse duration. In this case about 4.5 mS. Then 3rd point is the injector turn-off. The vertical lines at the injector turn-on and turn-off points, should be clean and well defined. These lines show the condition of the ECM’s internal driver transistor. The 4th point is the inductive kick. This relatively high voltage spike, resulting form the collapsing magnetic field around the injector coil, is the main indicator of the general condition of the coil itself. The voltage usually ranges between 55 and 90 volts, with 65 volts being the norm. A low voltage inductive kick is a sure indication of an electrical problem. As said before shorted injector coil windings or any resistance in the injector circuit will show up in the voltage waveform as a low voltage inductive kick.

Fig: Detailed analysis of a saturation type fuel injector waveform.

As a side note, always remember that in some systems this inductive kick is clipped off. This is done through an internal ECM diode at around 30 to 45 volts and does not indicate a defective injector. In such systems, the upper part of the spike is squared-off or flat, typical of a diode clipping action. The 5th and last point of interest is the injector-pinttle-closing hump. This hump is not present in all injector waveforms and with practice a determination can be made as to which systems do show it.

The closing hump is an indicator of injector mechanical condition. If it is placed too far up the position shown in fig. 2, then it is a good sign of a dirty or clogged injector. If it is too far down, then the injector valve spring is weak. With some experience a fair and accurate determination can be made saving time and money.


The saturation driver is the more common of the two types. This driver transistor usually works together with a high impedance injector. High impedance injectors take their name because of their higher internal resistance (usually form 12 to 20 Ohms). These injectors are mostly used in multi-port injection systems. There are cases, however, in which low impedance injectors are used in multi-port applications, but such cases are rare.

The peak-and-hold driver is almost always connected to a low impedance injectors. This name is given because of their low internal resistance (usually from 1 to 5 Ohms). The low impedance injector is mostly used in TBI applications and generally requires a higher amount current to operate.

Fig: Wave analysis of a dual trace waveform. In this case, both current and voltage are shown. Current waveform points are represented by numbers while voltage waveform points by letters.

The peak-and-hold injector gets its name from its waveform characteristic. The actual current peaks at a certain level (4 to 6 Amps) so as to open the injector and then levels off at about 1 Amp to keep the injector open. Point (1 – A) is the injector turn-on. This is the point at which the ECM driver transistor grounds the injector coil. At this point the voltage goes low (grounded) and the current slopes up to about 4-6 Amps. The ECM does this to quickly open the injector. It takes a lot more current to force an injector to open (break the inertia) than to leave it open. This type of injector is used mainly in TBI applications, with it being bigger and heavier. The current needed to break the injector pinttle inertia is generally higher, hence the higher peak-current level. Point ( B ) is the injector peak duration. Injector peak times should never fall bellow 1.5 mS. Injectors with shorted windings will tend to peak much faster due to the low impedance of the windings. A range of 1.5 to 3 mS is normal. Point (2 – C ) is the injector peak current/inductive voltage kick. At this point the peak phase ends and the injector driver transistor goes into the hold phase of the injector pulse. Peak current range from 4 to 6 Amps, with voltage values of around 60 to 90 volts. As in the saturation type injector, a lower inductive kick is an indication of a problem with the injector circuit or coil windings. Point (3 – D) is the injector-hold time.

The peak time plus the hold time is the actual ECM commanded injector open time. Both duration times are taken into account. Normal hold current is around 1 Amp. Point (4 – E) is the injector turn-off and turn-off inductive kick. Voltage values here should be in the same range as point C with a straight vertical line indicating good turn off ability by the driver. At this point the injector pulse is over. Therefore, as said before, ECM commanded injector pulse duration is from point 1 – A to 4 – E. However, this is not the actual physical open-time, which is the time between the two dark cursors. The reason is that the injector just does not simply opens at the moment the ECM commands it to. It takes roughly ¾ of the peak current to fully open the injector and this corresponds to the first current hump (first cursor). Point F (second cursor) is the injector-closing hump. This hump can only be seen on the voltage waveform. Therefore, in order to make a determination as to whether the injector is clogged (misfiring cylinder) the actual physical open time has to be taken into account. With this in mind, a dual trace waveform capture of both current and voltage is the first step to a thorough and sound injector diagnostic. The reason is that the physical injector opening shows only on the current waveform while the closing of it shows only on the voltage waveform. Since today’s clamp-on Amp probes have come a long way, this is not much of a problem. A complete scope hook-up can be done in 5 minutes or less.


High impedance injectors in practical, real-life applications usually draw a current of around 950 mA to 1.2 Amps, while it is more common to see a low impedance injector draw as much as 6 Amps. Any automotive actuator is always affected by conditions that place excessive resistance in its particular circuit. A voltage feed relay with carbonized or eroded contacts, a rusted or deteriorated injector connector, damage at the injector wiring itself and an open injector driver at the ECM are all examples of excessive injector circuit resistance problems. The same can also be said about mechanical problems developing inside the injector itself. A clogged, binding or corroded injector pinttle will cause a severe misfire and engine performance will suffer.

 NOTE: This training blog is taken from our book "Diagnostic Strategies of Modern Automotive Systems". For further -how to test- instructions visit our book section on our website.







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