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Old 05-27-2011, 11:57 PM   #1
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Default Injector(s) not firing. Wiring issue? (Pics inside)

(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.

General summary:
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.
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Old 05-28-2011, 12:51 AM   #2
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Check the ground. (You say come and goes) sounds like a loose cable making ground, contact when you hit a bump etc.

Use a noid light, you can make one from dashboard lightbulb, some soldering and of course two wires.

Instead of going by the voltage, check by light. In no way (IMO) it is something from the ECU to the injector harness. Maybe the ground from that specific injector is loose etc.

Stop messing around with the ECU harness, unless you have been doing some MS stuff, then thats another story. What is all that cut wires?

Since when did the problems start?

What was installed / done before problems arose?

* By yourself a book, grab some coffee (not beer) and learn to read the diagrams.

The Ground is shared by all four injectors and with the distributor (at least on all Escort GT/ LX), it could be that specific ground, but it is so strange. Unless the car overheated or if at some point the engine was removed and pulled on the harness, and still it is rare.
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Old 05-28-2011, 12:58 AM   #3
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Quote:
Originally Posted by mazpr View Post
Check the ground. (You say come and goes) sounds like a loose cable making ground, contact when you hit a bump etc.

Use a noid light, you can make one from dashboard lightbulb, some soldering and of course two wires.

Instead of going by the voltage, check by light. In no way (IMO) it is something from the ECU to the injector harness. Maybe the ground from that specific injector is loose etc.

Stop messing around with the ECU harness, unless you have been doing some MS stuff, then thats another story. What is all that cut wires?

Since when did the problems start?

What was installed / done before problems arose?

* By yourself a book, grab some coffee (not beer) and learn to read the diagrams.

The Ground is shared by all four injectors and with the distributor (at least on all Escort GT/ LX), it could be that specific ground, but it is so strange. Unless the car overheated or if at some point the engine was removed and pulled on the harness, and still it is rare.
Any idea where the ground for this would be? I'll have to check some local stores for that light you speak of. It doesn't seem to come and go as I hit a bump. Normally it will either run smooth, or start off rough right away. A few times I let it idle for a while and then it switched over to smooth and stayed there. It's never ran smooth and then all the sudden ran rough after driving it.

I'm just stumped at the fact that it is switching from injector to injector, but never shuts off two injectors. Also, I bought the car already boosted, so that's why the ECU harness had been cut up. It's a mess, but I've done my best to clean up some wiring from the PO already. It hasn't been fun.

I don't think the engine was ever pulled or wires have melted. The problems all started when I came home for winter break during college. I started toying with the car and the sound/alarm system, and then noticed it was running funny. I don't think what I did had anything to do with it though. And now that I'm home from school I've had more time to look at it and begin to work on it.
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Old 05-28-2011, 01:34 PM   #4
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From what I understand from your posts, and definitely the post from mazpr, I think you both are thinking about the injector wires incorrectly.

The injectors always get +12v whenever the ignition (key) is on. That is the white/red wire that is common to all of the injectors.

The ECU sends a ground signal to the injector to turn it on. There is no single ground wire that is common to all of the injectors.

To find the problem in the wiring harness, I would check for continuity. You'll need two things:
1. A digital multimeter that has a setting for continuity (hopefully one with a buzzer).
2. A friend/assistant.

Flip your digital volt meter to test for continuity.

Firmly connect all of the harnesses from the ECU all the way to the injectors. Unplug the ECU and unplug the injectors.

Find the 4 pins on the ECU that go to each of the 4 injectors.

Put one lead from the volt meter onto one of the injector signal wires on the end of the injector harness, and the other probe in the corresponding tab on the ECU side of the harness. Your multimeter should start buzzing, indicating continuity.

While holding the probes in place, and starting at the injector connector, move, twist, push, and pull the wires & connectors. Do one section at a time, moving methodically from end to end of the harness. Don't be afraid to put a little force on it. When you get to a connector, push & pull on it and the wires going in and out of each end.

if you have a wiring problem, at some point, you'll lose continuity.

I once had a similar problem, and it drove me crazy. In my case, it turns out that a couple of the tabs inside of the injector harness' 8-pin connector had loosened a bit, and were not making reliable contact with the pins from the other side of the connector.

I would take a close look at that boomslang harness and any wire that passes through it or is tapped to it. If you have any vampire taps, inspect them closely.

When you've found and fixed the problem, ziptie & secure the entire wiring harness every few inches. These problems are often caused by vibration and wires moving around. Stop the movement, prevent the problem.
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