In general terms at least - lately the engines I've been working on are diesels and I've never had to troubleshoot fuel injection problems on gasoline/petrol engines. So, I'm wondering, how many variations are there on fuel injection systems – big ones which would require you to think about them differently when doing diagnosis or troubleshooting?

The reading I've done so far has me thinking that the injectors themselves and their control systems are pretty much a constant and that once you know the principles they apply to pretty much any car and/or engine management system. Is that right, or are there some distinct variants?

  • Is this a question limited to injector variants or does it include fueling system variants as well?
    – Zaid
    Dec 24, 2016 at 20:16
  • I was thinking injector variants – but really what I'm trying to do is to help somebody who's working on an unfamiliar vehicle to do basic diagnosis and troubleshooting.
    – dlu
    Dec 24, 2016 at 20:18
  • 2
    There is a large difference between port injection and cylinder direct injection. Dec 24, 2016 at 21:01
  • @FredWilson - Is CDI the diesel-like post intake valve injection? Is basic/initial troubleshooting different with CDI injectors than with port injectors (are those the right names)?
    – dlu
    Dec 24, 2016 at 21:18
  • 1
    @dlu Yes CDI is similar to diesel with a very high pressure injector in the cylinder. It is commonly know as GDI (gasoline direct injection). Basic troubleshooting is very different. Port fuel was the standard up until about 10 years ago when GDI started to become common. Dec 24, 2016 at 22:24

1 Answer 1


From the point of view of someone working on the injection system injectors are all the same with two minor variations:

  • Switched hot (the ECU modulates the injector by controlling the +12 V supply to the injector)
  • Switched ground (the ECU modulates the injector by controlling the injector's connection to ground)

Either way, what is happening is an electrically controlled valve in the injector is being commanded to open to supply fuel each time the piston reaches the top of the compression stroke just before the spark plug fires.

The amount of fuel injected is controlled by how long the control signal is applied for or the duty cycle of the controlling pulse.

So, let's put this knowledge to use for some basic troubleshooting:

  • A basic check for a working injector can be done by listening to the injector with a mechanic's stethoscope or by holding the tip of a long screwdriver to the injector and placing your ear on the handle to listen for the distinctive clicks of the injector opening and closing. The clicks should be regular and vary with engine speed. Since the engine will be running when you do this be super careful to keep hair, jewelry, and loose clothing well away from any belts or other moving parts.

  • If you have a basic DMM with an Ohm (resistance) scale you can also check by disconnecting the wiring harness and measuring the resistance between the injector terminals. For measuring resistance the polarity of the injector doesn't matter. Each model of injector will have specs for normal resistance, but as a rule of thumb a reading between 10Ω and 15Ω is likely to be in the ball park. Anything very close to zero (< 1Ω) or very high (> 100Ω) is definitely suspect. Another check is to measure the values on all of your injectors – they should be roughly similar within say ±2Ω of each other.

  • You may want to read this page and add to your answer. There's a lot more to know about fuel injectors and injection systems than you have here. Dec 24, 2016 at 20:52

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