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83 z24 misfire


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I've read a thread here regarding unplugging the coils one at a time to identify a problem with a poorly running engine.


I have a periodic miss either at idle or as I roll through the rpms. Compression check (driver's side of block) shows 125/125/125/123. 


When I pull the top coil engine dies. When I pull the lower coil nothing happens, engine runs the same. I've got 12+volts at the lower coil, put a new one in also, no change. Did the spark plug check on the coil wire and only had spark on the top coil. 


New plugs, wires, cap, etc. Stock carb. 


Curious what I can check next to find the miss? 

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Loss of the exhaust spark is almost always the fuse. Replace the first fuse on the far left of the fuse box. This is the power supply for the exhaust coil. You will notice an immediate increase in power and better mileage. With only the one side firing the burn time is much longer so the engine will act like the timing is too retarded.


Compression is very low.

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Weird. The intake coil is powered directly from the battery through a fusible link to the ignition switch. The exhaust is powered from the ignition switch also but through that fuse.


The intake coil should have 12v on it directly from the ignition switch. As this wire that powers the coil, also powers the ignition module in the distributor, and it's working, there must be 12v on the coil positive terminal. Can you check that the intake coil has power on the positive terminal???

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12.08V at the pos terminal on intake coil. Thin red wire going to neg terminal and black wire with small red dash going to pos terminal.


Same voltage on exhaust coil. Brown wire going to pos terminal and white wire with blue stripe going to neg terminal. Exhaust coil wires are heavier, maybe 12 awg. 


12.38 v at battery.


Which coil would be the Intake coil? The one on this truck that has the lighter wires is closest to the grill (it was connected to the intake on the dist.). The coil with the heavy wires is closest to the fender (it was connected to the exhaust on the dist.).


I switched the coil wires and it started and is warming up. I'll drive it and see if there is any changes.





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The coil powered by the fuse is the exhaust side. Anyone could have switched the high tension wires by mistake. No big deal it will run exactly the same.


If you have power at both coil negative terminals, do you have spark out of both??? If one is not sparking then the coil may be bad. Swap the wiring on the coil and if the problem switches to the other set of plugs then the coil is bad.

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Both coils have power at negative terminals. 


No spark coming from the coil with the heavier gauge wire. Replaced this coil with a new one. Same, 12v at neg terminal, no spark 


Other coil has same voltage and engine runs the same regardless of which position the coil wire is in the dist. 


Fuse still ok, engine dies if removed.

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OK the module in the distributor has mistakenly been wired to the coil powered from the fuse you mention. That's not correct but it will work. This means the coils have had the wiring messed with and put back on wrong.



There is a black plastic 3 terminal block probably attached to the rad? with wires to the distributor on one side and wires to the coils on the other? Can you post a picture of the wiring? I think it's just wired wrong.

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I believe this is the block you mentioned. https://photos.app.goo.gl/qGNHaBERRGk9X2EC7


I see where on the side going to the distributor the wires have been sliced poolry. (Blue, red and thin black with white stripe) I'll put butt connectors on them. 


This shows the wires to coils 




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 Yes. They are all different size studs and lugs, hard to mix up. However, if they have been cut and actually spliced together better take the tape off and take a look.





The coil on the right  looks correct.


The White/Blue and Brown wires go the the intake coil. Looks correct. If you follow the Brown back it should go to the terminal block, the wire it connects (shown Blue) should go to the B (battery?) terminal below. On either side is E (exhaust) and I (intake). Both the E and I go to the negative terminals of the coils.



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"If you follow the Brown back it should go to the terminal block, the wire it connects (shown Blue) should go to the B (battery?) terminal below. On either side is E (exhaust) and I (intake). Both the E and I go to the negative terminals of the coils"


Brown connects to a black/white then to the "B" terminal at dist. 


Both the "E" and "I" terminals connect to their corresponding negative terminals on the coils. 


Very minimal evidence that the intake terminals inside the dist. cap have been working. Replaced with new cap <1k ago. The exhaust terminals inside the cap appear to doing the work, as we know ...


Just changed out both coils with new.


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Because you were reading voltage through the coils on the negative side they can be assumed to be conducting and ok.



If I and E wires trace back to the coil negatives and both coils have power on the + terminals then I'm afraid that it should fire.


Swap the E and I wires on the distributor module. If the problem swaps to the other coil then the module is, then, most likely at fault. You took the electrical tape off those spliced wires????? A loose connection would cause this.

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If one side of plug was never firing the engine will be timed for the shorter burn time of 5 degrees. With single plug firing it should be 10-12 degrees so 5 degrees will act very retarded and lack power and gas mileage. I think you will be pleasantly pleased when you get this running on all 8 plugs. 

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I just swapped the E and I wires on the terminal block. Like you said, it just moved the problem to the other coil. 


I kinda wondered if it would, but as a result, the tach started working. 


Is it reasonable to assume the ICM?


Or since it's obvious that this rig has been messed with, is there anything on the ignition switch/fuse block side that could have this fouled up?

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3 hours ago, NorCal720xx said:

Also pulled the exhaust coil fuse and the engine stayed running...


It will run because it's now powered directly from the ignition switch as it should be. Tach is also on that coil.



Hold on the module change.

Got an idea. Find the 4th wire on the distributor harness and unplug it. See if the spark comes back. It's the separate white? wire with it's own plug below...




The fourth wire is a special circuit using a low vacuum switch that detects heavy load/full throttle and causes the ignition module to shut the exhaust side plugs off to reduce engine noise. If that is activated there won't be exhaust side plugs. Unplugging should deactivate this.

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The coil that is not firing.... Can you perform a continuity check on the wire between the coil negative terminal and the E or I terminal (I forget which) on the module. You'll have to disconnect the module end. If this wire is broken a new module won't fix this. If it isn't broken then a new module is in order.


Soooooo close!

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