(Scroll to bottom for a very short summary)
I've been battling all sorts of running issues with my Miata. It's a 1996 that has been boosted with a mix of parts. I have had it professionally compression and leakdown tested and everything is okay there. I now have the car back in my possession and am working to fix things myself.
The only issue I'm having is that the car is running rough, on what seems to be 3 of the 4 cylinders. Sometimes the car runs great, but most of the time I'm having issues. It comes and goes, which makes it hard to diagnose. It seems that I have spark, but no fuel. To test this I started the car and as it was misfiring, I pulled each injector plug. Each plug I pulled would make the misfire worse until I got to the 3rd cylinder's injector plug. When I pulled that plug, there was no change in the way the car ran.
I tested the 3rd injector with my ohm-meter and it showed a steady ~14 ohms. I tested the other injectors and got about the same numbers. Since I had a spare injector laying around (these are the 305cc Supra plug and play injectors), I decided to throw it in to see if it would fix the problem. I started it up and nothing changed, still running rough. So then I tested the individual injector plugs as the car was running and the voltage fluctuated around 9 volts for every cylinder except
the 3rd, which would read 0 volts. Now I knew that the 3rd injector wasn't firing because it wasn't getting any power to the injector plug.
I started to unravel the whole injector harness to look for any shorted or broken wires. I noticed that each injector plug had two wires leading to it. One was always a white wire with a red stripe. The other wire varied from injector to injector. The 1st one had a yellow wire, 2nd one had a yellow wire with black stripe, 3rd had a green wire with white stripe, and 4th had a green wire. Here is a picture. As highlighted in red, all the white wires were connected together in the middle, and then a single wire from the bunch was sent out to the plug of the injector harness. Each of the plugs other wires (yellow, yellow/black, green/white, green) went out to this plug as well.
To check if the wiring problem was within this injector harness, I went to the plug and checked it's female end of the plug (sending the power source). With the injector harness unplugged, I placed my volt-meter on the green/white (3rd injector plug) terminal and the white/red terminal. Then I had someone crank the ignition and checked for voltage, but nothing showed, only 0 volts. To verify the other injector plug was getting power, I moved my voltmeter to the solid green (4th injector plug) terminal and again the white/red terminal. When the engine was cranked, the voltage would again fluctuate around 9 volts. To me, this meant that the female end of injector harness plug wasn't sending power to the injector harness to let the injector fire.
Again, here is a picture to help describe where I placed my volt-meter. The yellow circle represents the solid green wire (4th cylinder). The green circle represents the green/black wire (3rd cylinder). The red circle represents the white/red wire from each injector.
I next went to check the ECU (I have stock ECU plus an AEM F/IC piggyback) to see if anything looked odd there. I unplugged the 3 harness connections to look for bad connections, found nothing.
Plugged things back together and gave the car another start. NOW THE ISSUE HAS MOVED TO THE 1ST CYLINDER! I did the same test I did earlier by pulling the plugs as the car ran, and when I pulled the 1st plug, there was no change in the way the car ran, but if I pulled the 3rd, it would almost die.
Fed up, I put everything away and called it quits for today.
3rd cylinder injector wasn't firing. Injector turns out to be good, the injector plug isn't getting power. Injector harness seems to be okay, but the plug that connects it to a bigger harness, isn't sending power to the injectors, therefore not firing certain injectors. After unplugging the ECU, the problem moved from the 3rd injector, to the 1st injector.
Where should I go from here? I believe from the things I've done so far that the problem lies in the wiring somewhere from the ECU to the female plug that connects to the injector harness. Or am I looking in the complete wrong spot? I don't know what else would cause this.