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Old 04-18-2016, 05:02 PM   #1
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Default School me on injector function

Ok, not on the actual function of the injectors themselves, but the process of the injectors firing in a wasted spark system.

So my Miata is still a wasted spark system in that there are only two coils and each coil energizes on every rotation. So the coil that fires spark plugs 1&3 energizes and fires both spark plugs simultaneously. Cylinder 1 is on the intake stroke and so we have our air, our fuel, and our spark - combustion. In cylinder 3 we have an exhaust stroke which is expending gases but we still get a spark plug fire that is "wasted" because there is no compression and no fuel.

Here's my question. How is there no fuel?

Car is a 91 with a factory injector harness. There is a red/white wire that is common to all 4 injectors. Then a yellow wire that is the trigger from the ecm that fires both injectors 1&3 and a yellow/black wire that is the trigger for 2&4. With only one wire sending a voltage trigger to injectors 1&3 at the same time, how does only the proper injector fire? It's my understanding that the aren't both firing. I would think if they were, the paired cylinder not on the compression stroke would be getting fuel washdown past the rings. Is the CAS signal somehow tied in and inturupting the injector not needed?

How does this work?
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Old 04-18-2016, 05:09 PM   #2
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The coils are paired 1/4 and 2/3. The injectors are paired 1/3 and 2/4.
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Old 04-18-2016, 05:11 PM   #3
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I think I see what you're saying. The wasted spark is going to fire when the intake valve is closed, and the exhaust valve is at the end of it's stroke. So, the exhaust has been exhausted, there's no new fuel, and no compression. The first injector fire will happen while the intake valve is closed, which is why it doesn't ignite, then the second injector fires, the intake valve is opened, and all the fuel from both injection events enters the combustion chamber.

Main answer to main question: there is fuel, it just isn't in the combustion chamber.
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Old 04-18-2016, 05:21 PM   #4
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Injectors are a switched ground, not a switched power. So you have a constant 12v at each injector, and then the yellow and black wires are pulled to ground to fire the injector.
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Old 04-18-2016, 05:38 PM   #5
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Quote:
Originally Posted by acedeuce802 View Post
Main answer to main question: there is fuel, it just isn't in the combustion chamber.
Basically, yes.

In most batch-fire 4 cyl configurations, each pair of injectors fires twice per engine cycle, for 50% of the required duration. Somewhere I have scope traces which show the relationship of the injector squirts to the intake cycles.

But injector timing isn't even all that important here. You could implement a wasted-spark system in a carbureted engine, or one with TBI or continuous injection just as easily. Because none of these designs inject fuel directly into the combustion chamber, they do it upstream of the intake valve.

So, consider the four cycle process. For a 4 cyl engine, the #1 and #4 pistons move up and down at the same time, as do #2 and #3. We'll look at just the 1/4 pair here.

Starting at 20 BTDC, piston #1 is approaching the top of the compression cycle, and #4 is approaching the top of the exhaust cycle. Both plugs fire. In #1, the plug ignites the compressed mixture, and in #4, it just flashes across whatever small amount of exhaust gas still remains in the cylinder. There's no burnable mixture in #4, because it's just getting done with the exhaust cycle and the inlet valves haven't yet started to admit fresh air and fuel.

Now, the engine runs through a full crank revolution, and we're at 20 BTDC again. #1 and #4 are both approaching TDC, only this time, #4 is on the compression cycle (having just inhaled a fresh charge of fuel and air) and #1 is on the exhaust cycle from the previous combustion event. Both plugs fire again. This time, #4 ignites and #1 does nothing.


With that background, it doesn't matter one bit what the injector timing is. You could have all four injectors spraying continuously (as many 70s / 80s German vehicles did), but none of that fuel is getting into the combustion chamber of the cylinder on the exhaust cycle until *after* the "wasted" spark event happens and the intake valves open.
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Old 04-18-2016, 05:50 PM   #6
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Old 04-18-2016, 06:02 PM   #7
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Ok, so if injectors 1&3 both fire together, how in tuner Studios test mode can I fire injectors one at a time? And don't say I can't, because I have.
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Old 04-18-2016, 06:03 PM   #8
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you are confusing spark w/ fuel
you can run seq fuel, all 94+ cars are wired to do it
spark on the other hand...
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Old 04-18-2016, 06:04 PM   #9
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There are a few ways to hook it up. I'm not sure what your ECU setup is.

But usually it is literally 1 transistor for 2 injectors. Which means you can't fire them independently.
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Old 04-18-2016, 06:30 PM   #10
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Quote:
Originally Posted by guttedmiata View Post
Ok, so if injectors 1&3 both fire together, how in tuner Studios test mode can I fire injectors one at a time? And don't say I can't, because I have.
If your car really does have the factory '91 injector harness, and it has not been modified, what you are saying is physically impossible. You cannot, and you have not. The injectors are wired to the same driver. It is physically impossible to trigger only one of them. If you have actually triggered a single injector without triggering its paired mate, then your car is wired sequentially.

Last edited by Savington; 04-18-2016 at 06:42 PM.
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Old 04-18-2016, 06:34 PM   #11
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read my post above: iirc all 94+ cars are wired for seq fuel, you just enable it with ms and voila.
not so with spark where you have to physically wire in 2 extra outputs to fire a single plug
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Old 04-18-2016, 06:43 PM   #12
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Ok, so obviously I need help figuring out an issue that I'm not smart enough to figure out. I started a different thread under the MS heading, but that turned into emails between me and Matt at DIY.

So here is the other thread for the background. https://www.miataturbo.net/megasquir...-signal-88467/

and here is the conversation with Matt:

Matt: At this point I am not 100% sure if it is an internal issue, but it's hard for me to think of anything else. The only other thing I can think of trying is reloading the firmware and loading a map from before the problem happened - let me know if that results in any changes.

Me:This is so frustrating and strange. So here is how it went.

Loaded an old tune. No difference. Injectors 1&3 would still test fire but not 2&4.

Then I loaded the current firmware. Loading it went fine. It then took 3 attempts to get TS to connect and read anything from the MS. Then before I could even get test mode set up, everything locked up. Actually had to shut down the computer. It then took 2 attempts to get TS to acknowledge the MS. I then tested inj 1 and then everything froze up again. Resetting again, everything started working correctly. Here's the weird results of the testing though. Only inj 1 will test fire. So now inj 3 does not test fire either and 2&4 still won't fire. Here's the question that raises though.

How can individual injectors be test fired in the first place? There are only 3 wires going to the injector harness. A common red/white that goes to all 4 injectors for power, a yellow that triggers both 1&3, and a yellow/black that triggers both 2&4. So if I choose to test fire inj 1 it is sending a signal to the harness through the yellow wire which should also then be sending the same voltage signal to inj3, right? I'm confused how the injectors could possibly be fire individually in test mode in the first place, yet that's what they have done in the past.

Matt:I am starting to suspect a problem with the injector wires or the injectors themselves if injector 1 will fire but injector 3 will not. There's nothing in the ECU that can cause this.


So in the past, I've been able to choose individual injectors in test mode and fire them one at a time. A very distinct individual injector by feel and sound. yet, you guys are saying it's not possible.

What would sequential wiring look like? I can tell you I am using the factory plug from the main wiring harness with the 3 injector wires. Not really possible for someone to have done anything that makes it sequential past harness plug I'm assuming? That would require independent wiring, no?

Again, all wires show continuity. I'm also still not getting a voltage change signal off the yellow/black #2&4 from the ECU to the plug connector - but continuity is fine.

What other testing can/should I be doing?
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Old 04-18-2016, 06:45 PM   #13
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You wouldn't have 3 injector wires, you would have 5.
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Old 04-18-2016, 06:46 PM   #14
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Quote:
Originally Posted by 18psi View Post
read my post above: iirc all 94+ cars are wired for seq fuel, you just enable it with ms and voila.
not so with spark where you have to physically wire in 2 extra outputs to fire a single plug
So this car, although a 91, is a piece meal. I think it's the 91 injector harness, but not 1000% sure. How do I tell the difference?

Also, tried every injector setting in TS and it didn't change any results in test mode.
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Old 04-18-2016, 06:54 PM   #15
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Quote:
Originally Posted by aidandj View Post
You wouldn't have 3 injector wires, you would have 5.
So I just went down to make sure without a doubt I don't state something wrong.

Each injector has 2 wires. A red/white - power, and then either a yellow or yellow/blk - trigger.

There are 6 total wires to the plug connection on the ECU side.

There are 6 total wires on the injector side as well. 3 going to injectors obviously, one to ground, one to a sensor on the water housing on the back of the head, and the last one just terminates - cut and capped.
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Old 04-18-2016, 07:43 PM   #16
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If you can check continuity between injectors it should tell you. All four will be continuous on the red/white wire. If it's a 90-93 then 1-4 and 2-3 will show continuity. That's yellow and yellow/black wires.
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Old 04-18-2016, 08:35 PM   #17
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Quote:
Originally Posted by guttedmiata View Post
There are 6 total wires to the plug connection on the ECU side.

There are 6 total wires on the injector side as well. 3 going to injectors obviously, one to ground, one to a sensor on the water housing on the back of the head, and the last one just terminates - cut and capped.
If the cut wire was black with a blue stripe, it was for the water temperature sensor which feeds the dashboard gauge. The blue/white and black/green wires are for the water temp sensor that feeds the ECU.

That connector, for reference, is called X-12 in the factory schematics.

Here's what the wiring looks like on a stock 1.6L injector harness (except CA 93):









Quote:
Originally Posted by guttedmiata View Post
What would sequential wiring look like? I can tell you I am using the factory plug from the main wiring harness with the 3 injector wires.
You'd have at least two other wires coming out of the injector harness, beyond what you've described.


Quote:
Originally Posted by guttedmiata View Post
Not really possible for someone to have done anything that makes it sequential past harness plug I'm assuming?
No.

Converting an engine from batch to sequential injection requires that wires be added to the injector harness itself, and also continued on down to the ECU. The injector harness MUST be modified.
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Old 04-18-2016, 10:44 PM   #18
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Thank you everybody for your input. I have reconfirmed some things I thought I knew but was second guessing. I also learned some new things. I also learned that if I thought I had been firing individual injectors that I'm crazy. Not something new. Been called crazy a time or two before.

But mostly it caused me to spend another hour in the garage tonight running tests again. I determined that #3 is indeed firing again as it was before the firmware update. Not sure if it was temporarily stuck or what. Also determined that all wiring and connections are good. Also determined that there is still no trigger being sent to the yellow/black wire from the ecu and that there must be a bad injector driver. It's basically the only possibility at this point.
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