Coil voltage problem.

BSchrowang

Budget Builder
I have a 2010 crown civ police interceptor (i know cool right) and i just got it re assembled. And now i have a check engine light. So (i work at a mechanic shop) i put the obd reader on it and it says i have an ignition problem in cylinder 8. It usually means a bad coil. So i switched the coil hoping it would go away but no dice. So i replaced the spark plug and still same problem. So i took the handy harbor freight voltmeter and tested the input voltage to the coil. It has 2 wires ~ Constant 12v and spark control. The power wire is 2v lower than every other coil plug on the car. Can i run a cable from another 12v source to see if that solves the problem? I reallllly dont want to get a whole wiring harness, especially considering i just got this car put back together from bare block. Thanks

 
If it needs a constant 12v, then there shouldnt be any problems with hooking it up to a 12v power source.

Amplifiers dont care where their 12v comes from, I cant see a sparkplug being any different in that sense.

Never tried it, but sounds safe.

 
Well i looked at my problem again this morning.. Today the plug is reading the exact same voltage as all the others 12.4 car off and 14.03 running. So now i dont know what to do. Its still not firing on that cylinder at all. Its not misfiring. Its not firing at all. Ive tried new coil, new plug and tested voltage. What else could i do?

 
Update. I tried switching out fuel injectors for that cylinder and still nothing. So i put the obd reader on and loaded a cylinder contribution test. It measures the output of each cylinder individually. It showed 1-7 all good and 8 had no contrib. So i tried giving the motor some rev to about 1500-2000. And then cylinder 8 came on just as strong as the other 7. But when i let back off the throttle it dropped back to nothing. Any thoughts? Maybe a intake manifold gasket leak?

 
I would guess possibly a vacuum leak, you can spray fluid around areas that may be leaking and see if it raises rpm or idle as you spray.
Check your distributor cap for carbon build-up on that one cylinder's electrode. You said when you rev it, that it comes up and matches the other 7 at higher rpms.
Does the spark plug electrode show any signs of heat or carbon built up, compared to others? I would do a compression test on that cylinder for sure and compare to the others. Could have a sticking valve at low rpms or a bad spring. The ceramic on the plug for that cylinder is intact? Sometimes a crack can be hidden until it gets some dirt in it.
Trying to think of all the reasons that could cause this.
 
Screenshot_20210905-185252_Chrome.jpg
 
Also just for clarification, oem harness design put the ignition and fuel injectors feeding from +12vdc but there is a relay to turn on both lines and both are usually seperately fused.

Also most OEMS spliced the +12v lines for both injectors and coils. Run a temporary wire to #8 to +12vdc and start car, see if that works (the ECU controls dwell so no issue about overcharging coil). If it still does not work, that means the wiring to the ECU is bad OR the ECU #8 ign channel is bad.
 
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BSchrowang

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