 | ||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||
|
Fuel Injector Operation ConfirmationPosted 12/13/2000By Doug Garriott
When an injector is off the car and in the injector lab, it is somewhat of a straightforward procedure to verify an injector's operation. I can visually check it for physical damage, signs of external leaking, or a clogged filter. Confirming that the injector has the capability to pulse and is within resistance specification are simple checks, too. Electrically loading the injector with a pulser for five seconds at a time and viewing the current waveform dynamically checks the injector coil winding's integrity with ease. On the flow bench, the injector can be checked under pressure for leakage, flow can be measured at different pulse frequencies and spray patterns can be observed from different angles. Some injectors have common failures that require testing to be done hot, with high side pressure specs and at a particular pulse rate. Simulation of these conditions can be achieved a lot easier and the tests are very conclusive when determining an injector's operation off the vehicle. But what procedure do you use to confirm injector operation when the injector's still in the vehicle? Do you have a procedure? Do you check all aspects of an injector's operation? Maybe the first question should be: How did you determine that the injectors are the problem? Can it be assumed that you did a thorough analysis of the:
All these systems need to be confirmed as operating properly before you can move on to the injectors. Checking an injector on the vehicle has changed through the years. Some of the tools and procedures from yesterday are not the tools and procedures of choice today. It is also important to have a good understanding of the type of fuel system and the PCM strategy for the system you are working on. So lets start with a good definition of what an injector is: It is an electrical-mechanical device that meters and atomizes fuel. From this definition, we base our diagnostic procedure. Electrical This is a two-part analysis. One is the electrical integrity of the injector and the other is the computer's ability to provide a pulse to the injector at the proper time. At one time, a resistance check of the injector was all that was done to confirm its electrical ability. This test seemed to be adequate and some techs still use it, but a one-time check of an injector's resistance is not always enough. This is due to the fact that resistance will change with an injector's temperature. I have seen thousands of injectors pass a resistance test at room temperature and fail when heat was added. Many techs use a noid light to prove a signal from the PCM. This test tells us nothing about supply voltage or injector pulse width. All a noid light will do is let you know that the injector driver in the PCM is going to ground. Is it a good ground? Keep in mind that most noid lights only have a resistance of 3.5 ohms and inserting one into the PCM control circuit changes the circuit's current characteristic. Today the digital storage oscilloscope (DSO) is a common tool. Many technicians look at an injector's voltage pattern to confirm supply voltage and good ground, inductive kick when the injector is turned off and a measurement of pulse width. Others use a low amp current probe with their DSO as the preferred method of obtaining a waveform. This allows the tech to confirm the PCM signal, injector pulse width, and injector circuit current usage. DSO patterns can also be helpful in diagnosing mechanical operation by showing the pintle hitting its opening and closing points.
Mechanical At one time a stethoscope might have been used to listen for a clicking noise coming from an injector. Many times, unless injectors were being pulsed individually, the vibration of one injector could be carried through the rail and heard at another injector. Here again, the injector waveform can provide this information with the observation of pintle bumps. Metering There are many different ways of measuring injector output, depending on make of vehicle. Some PCMs have a built-in test for injector balance that can be run through a scan tool or activated through the dash. This allows the injectors to be shut off individually and the difference in their cylinder's power contribution to be recorded. Keep in mind that a mechanical problem could cause a cylinder to fail this type of test. Probably the most common method is the use of a fuel pressure gauge and an injector pulser to determine injector contribution. Injector contribution is calculated by the difference of before pressure and after pulsing pressure readings. Although this test is widely used, there are no real concrete specs. Most specs say there should be no more than a 10kPa difference in drop per injector. This is approximately 1.5 psi; a small amount of difference if an injector only drops 5 or 6 psi total. Furthermore, there are no specs telling how much an injector should drop to begin with. General Motors has expanded on this test for its central sequential port injected engines. The new procedure uses a fuel chamber with spring-loaded diaphragm that connects to the fuel pressure connection valve and a dial indicator. When the injector is pulsed, the dial indicator measures the diaphragm movement, which correlates with the actual amount of fuel delivered. Hickok Inc. makes a product for Mac Tools called a Scanalizer. It is a multiport fuel injection analyzer that has the capability of testing fuel pump supply, fuel pressure, pressure regulator operation, injector electrical integrity and injector flow. An injector balance test using an exhaust analyzer (watching HC change) is another method, but most techs don't want to go through this procedure. Why? Because you must tap into the exhaust system in front of the catalytic converter to get an accurate reading of the hydrocarbon gases. This usually takes a little time to drill a test port and we know how hot the exhaust can be while we're trying to get this done. There are kits available for this test that have inserts to plug the exhaust when testing is done. You must also have a means of killing individual cylinders to perform this test. Atomization/Spray Pattern
Working with injectors daily and talking to technicians on the hotline about injector problems, I see or hear about all the strange conditions. I usually find that when a complete analysis is done, injector operation can be confirmed with no doubt left in the technician's mind as to whether the injector is working properly. When there is doubt, removing an injector and having it tested on a flow bench is always a safe bet. Injector service should be a growing part of all automotive repair shops. With the proper tools, procedures and knowledge, the task of taking on this work can be made easier.
|
|||||||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
| ||||||||||||||||||||||||||||||||||||||
This is an article within itself. I conclude that most techs have the theory that an injector sprays a conical cone shape spray. This theory is from looking at a throttle body injector spraying. A port injector may have this type of spray, but some, depending on nozzle design, may spray a straight stream. Some of the newer design injectors have an off-center fuel discharge hole. These are directional spray injectors and if you look at their pattern unaware of this, you just may condemn it. Anyway, how can atomization and pattern be seen when the injector is in the intake manifold? To make a comparison, the injectors will have to be removed.
