I took the fuel injectors off of a wrecked Volvo 850 GLT 1996. I was testing them to see if they still worked by simply supplying voltage to them, and they do. I noticed that when I increase or decrease the voltage, the pulsing sound coming from them doesn't seem to change. Are fuel injectors designed to provide a constant flow of fuel no matter what the voltage? And the ECU just controls how long voltage is supplied to them? What I mean is, is the injector like a DC motor, speed increases with voltage, or is the "speed" constant?

  • An injector is not unlike a relay: it is a bi-state device, either open or closed. The fuel charge is determined by the injector design, fuel pressure and time of opening of the injector. Commented Sep 21, 2016 at 19:09

1 Answer 1


The injector has a plunger/pintle which is controlled by an electromagnet. The humming you heard is from that. The plunger/pintle has a spring which pushes back against the electromagnet so when the power is shut off, the plunger will close and stop the flow of fuel. This causes the hum. Here is an example of how an injector is built:

enter image description here

When the power is turned on, the electromagnet attracts the (in this picture it's labeled "magnet") plunger/pintle which opens the flow. The computer energizes the electromagnet for as long as needed, according to the fuel map to achieve the amount of fuel output. An electromagnet can energize with a small amount of current, but will be stronger with more. Fuel output is a factor of three different things. Fuel pressure, size of the injector (how much it can flow for a period of time, usually at a given pressure), and the length of time an injector is left open.

When testing old injectors, while testing the electromagnet is part of that, the bigger part is flow, flow consistency across all injectors, and checking for leakage. This is usually done on a fuel injector flow bench. This is specialized equipment.

  • Thanks for the picture and description. So the injector is not a variable device itself, meaning that there is only an on and off, there's no 50% or 25%. But what the computer controls is just how long the injector stays open, and it does this by knowing the fuel pressure, injector size and other aspects of driving. Do I have the overall idea correct? Commented Dec 21, 2014 at 5:01
  • @user7826 ... In a basic sense, yes. The only real adjustment is the amount of time the PCM allows it to stay open. Commented Dec 21, 2014 at 12:16
  • @user7826 ... Something else I want to help you understand is that the computer really doesn't know the size of the injectors, fuel pressure, etc. It has what's called a fuel map which is a grid of numbers the PCM can cross reference with the engine's parameters to know how long to keep the injector open. As long as everything is consistent, everything works out well. If you change one of the parameters, then you need to have the computer remapped so it will again put out the right amount of fuel for engine needs. Commented Dec 21, 2014 at 20:14
  • Thanks for the info. Where I can find the fuel maps for specific vehicles? Namely American, like Ford? And is it possible to change the parameters without having to go through the whole remapping process? Like if I wanted to customize my own fuel map for a vehicle, where/how can I do that? Commented Dec 27, 2014 at 6:34
  • @user7826 ... In order to customize your fuel map you have to have a tuning program to accomplish. There are a ton of different software options like SCT or HPTuners. Please note, this is not something which should be done lightly as you can really mess up your vehicle calibration if you don't know what you are doing. You cannot reasonably expect to change the parameters without going through the remapping process, at least not expecting your car to run correctly afterwards. Consider it a "together" process. Commented Dec 27, 2014 at 12:58

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .