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Engine running only on rear cylinders

735 views 12 replies 5 participants last post by  Mr. Completely 
#1 · (Edited)
In the process of putting a 83 zx motor in my 77 z, don't know if that makes a difference or not.

Every time I start my engine up it only runs on the rear 3 cylinders I know the spark and timing are all good on this motor. I know the early z's have kind of a spray and pray setup when it comes to fuel injection and doesn't matter what injector is plugged into where so I flipped the rear 3 injector plugs onto the front and the front cylinders started firing where as the rear was now not. I pulled the ECU plug out and checked voltage to and from the injectors and all was good. I also checked trigger wheel input and grounds just in case, all was good.

So my question now is what else is there to check, Is there something perhaps I'm missing? I don't recall the old transistor ignition system and the fact that it is replaced by the ZX's matchbox distributor have any effect on the injection system. Clearly I missed something or perhaps my ECU is dead???
 
#2 ·
Use the FSM or fuel injection bible. You'll see in the diagram that the positive side of the injector is hot all of the time. From the injectors, the wiring goes down to the dropping resistors and down to the ECU where it gets grounded by the ECU.

So you can use the manual to see what pins the dropping resistors go to on the ECU. Once you find that out, you can unplug the cable from the ECU and put the key in ON. You should see voltage to ground on the cable where the dropping resistors are wire to. Test both sets of resistors. If you see voltage to ground at both, the problem is probably in your ECU. If you only see voltage to ground on the back set, you probably have a wiring issue. If you don't see voltage to ground on either, you have a technique issue.
 
#4 ·
Well, you could try grounding the dropping resistors quickly and see if you can hear the injectors click.
 
#5 ·
Grounded and could hear all individual injectors open. Checked voltage again at injector connectors and the good side has 12v on both pins but the bad side has 12v on one side and around a 1v on the other and the bad injectors have 9v going to the dropping resistor. Sorry if this is basic stuff, the electrical side of things is new to me. On the To-Do list to learn. Thanks
 
#6 ·
You verified the injectors would fire. That removes another variable. I'm not an expert on the L-Jetronic, but it seems like you confirmed an ECU issue.
 
#8 ·
#10 ·
I see where my test description lacked a little detail. That's what I get for describing a test when I don't have the FSM available.
I forgot which side was grounded.
Rectangle Font Parallel Schematic Slope

From the positive of the battery cable, the current path goes through the fusible link and down to the dropping resistors. It gets split out to the injectors and goes over to the ECU to get grounded.
With the key in ON, you should see voltage to ground at the pins indicated above. I believe you'll need to ground pin 5 on the ECU connector to get voltage to the injectors with the ECU disconnected.

Note: It's also handy to have a noid light to test the injector firing.
 
#12 ·
I've heard that the front and rear banks on the z are split up inside the ECU so you occasionally hear problems like this where one half goes dead. I've opened up the ECU and couldn't see any obvious damage, this seems to be common. Perhaps I'll update once the new ECU's come. I ordered a factory unit as well as a "K" ECU which seems to come from boshe directly or were dealership replacement units, information is kinda lacking with those.
 
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