Building an Electronic Fuel Injector Tester

This is a project I did several years ago on the dinofab website. It’s been so popular there and on my old YouTube channel that I decided to post it here on hack A Week.

I work as an automotive technician at my “day job”. Nearly all of the cars I work on have fuel injection systems. They all have several things in common: a control unit, a fuel pump, sensors and fuel injectors. Fuel injectors are made up of a coil of wire which opens a valve when energized. It’s a basically an electronic solenoid valve.

As time goes by these “valves” can wear out and become sluggish, leaky or just quit working. They’re pretty easy to test by simply applying 12 volts to the leads and listening for a click sound. But, that doesn’t tell the whole story. A better way to test them is to send a series of electrical pulses to them and see how much fuel they deliver. First a fuel pressure gauge is connected in line on the fuel rail, then the fuel pump is turned on momentarily to build pressure then turned off. The fuel injector gets a pulsed signal for a given duration and the pressure drop is noted. The process is repeated with the remaining injectors for comparison. Sometimes a few may have a different delivery rate which can be a great aid in troubleshooting a poor running engine.

These devices are available for $140 – $400 depending on their capabilities.
I decided that I could build one myself for very little money using the classic 555 timer chip and a few external components.

Below you’ll find a list of parts and some links to a few web pages that will help you understand the workings of the 555.
There’s also a build video which will show you how I put this together. This circuit will output a series of square wave pulses that are 8 mS on and 7 mS off for a duration of one second.

A word of caution about the N-channel MOSFET. Be very careful of the pin outs! These tend to vary depending on the manufacturer. Make sure you know the exact location of the gate, drain and source! Connecting one wrong will fry the 555 chip AND the transistor!
Be sure to add a protection diode between the source and drain even though most MOSFETs have them internally.

Notes on the schematic. I’ve been revising the schematic as I receive suggestions from people. Here and there I’ve added a few resistors and diodes that were not on the original design. Feel free to send any suggestions my way and I’ll include them. If you can’t find LEDs that are rated at 12 volts, be sure to add the proper resistor in series with your LED.

UPDATE: I’ve had a few emails asking if this will drive a Diesel injector. The answer is, NO. The MOSFET in this circuit is rated at 4 amps and Diesel injectors need 15 – 25 amps. Attempting to drive this kind of load will damage the MOSFET and possibly other components.
However, you CAN replace part # IRF510 MOSFET with a part # MTP75N03HDL Motorla MOSFET. The Motorola has a 75 amp rating.
Here is the datasheet:
Here is where to buy one:

I’ve designed another schematic below with two more switches and a 4.7 mF capacitor added.
SW1 is used to choose between a timed or continuous ouput.
SW2 is used to choose between a 1 second or 5 second ouput.
The only extra parts needed to build the circuit this way are:
1 – 4.7 mF capacitor and 2 – spdt switches.
I’ve updated the parts list to include these extra parts.

I encourage you to try building other circuits with the 555. It’s a very versatile chip with many applications.
Have fun and keep on making stuff! ( that was before I said “keep on hackin” )

Questions? Email me at:

Final draft of schematic as built:

Parts list:
1 – Radio Shack proto type circuit board
2 – 555 timer 8 pin DIP chips
3 – 100 ohm resistors – R1, R5, R9
1 – 470 ohm resistor – R6
3 – 100 K ohm resistors – R2, R3, R7
1 – 10 ohm resistors – R8
1 – Meg ohm resistor – R4
1 – 47 mF capacitor – C1
1 – 22 mF capacitor – C7
3 – 0.1 mF ceramic disc capacitor – C2, C3, C6
1 – 1 mF Tantalum capacitor – C4
1 – 4.7 mF capacitor – C5
1 – 1N4002 diode D1
1 – 1N4742 12v Zener diode – D2
1 – Red LED 12v rated
1 – Green LED 12v rated
1 – N-Channel MOSFET transistor – Q1
Radio shack part #276-2072
1 – Momentary contact switch – SW3
2 – spdt switches – SW1, SW2
2 – Aligator clips
A few feet of double strand wire
1 – Project enclosure box


Here’s the Express PCB layout file in zip format: fuel injector tester

ExpressPCB: This site offers a free download of a schematic and PCB design tool that’s easy to use. See screenshot above. They also offer fabrication of PCBs. Download the free software here: ExpressPCB

Rob Paisley’s 555 timer circuit page: This has a TON of great info on the 555 including a few timing calculators to aid in component selection.

Doctronics: Another great website with a lot of cool info and circuits for the 555.

Octopart: This site offers up a search engine similar to Google but it’s especially for electronic component part numbers.

About Dino

Self taught electronics and hardware hacker.
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