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Old 05-23-2016, 07:09 PM   #15
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My 8.1 had a similar problem. Replaced the wires and plugs, problem went away until I had the Banks Power exhaust installed. Failed smog test, so I took the MH to a local dyno shop. They found 2 cracked spark plugs that were causing the problem. I'm guessing the magician who installed the Banks was the culprit, but plugs are easy to damage when installed.
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Old 05-29-2016, 01:07 PM   #16
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It all comes back to the SAME issue. Missing under load (acceleration) is secondary circuit insulation breakdown. It matters not if the plugs, wires were replaced it's still breaking down under load.

Diagnosis is a process of accessing symptoms then going to the source of the issue. It's as simple as that. Follow the symptoms or don't and you'll be wasting time, money and a lot of time chasing your tail. It's your choice.

TeJay
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Old 05-29-2016, 07:30 PM   #17
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Fuel filter been changed recently? If not, that might be the cause.
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Old 05-30-2016, 06:50 AM   #18
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If you are confident in the quality of your plugs, plug gap, plug instalation and the quality of the plug wires and there instalation the only thing left is a weak coil but you have to find out wich one.
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Old 05-30-2016, 08:59 AM   #19
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The best I could come up with on his chassis is he does not have the COP design like the Ford V-10. The COP has a coil for each plug. That eliminates the spark plug wire as a possible problem.

If he has plug wires then he does not have the COP design and would have what is called a coil pack. When dealing with a misfire one can not make any assumptions.

It is my understanding when installing plugs into an aircraft if a plug is dropped it can not be placed into the engine. If something is fresh out of a box and assumed to be new and good that will/may come back to bite your butt.

I have been out of the repair business for 10 years and I know how fast the technology changes. Thirty years ago we used an oscilloscope to determine plug misfire. I know they have scanners of all sorts. What's even nicer is they are small enough to be attached and used while operating the engine down the road and under misfire conditions. Even some engines have the CPU capabilities of determining which plug may be misfiring.

The other possible solution is to buy a coil pack and just change it. That might eliminate the coil as failing under load.

His symptoms of misfiring under load are classic symptoms of a secondary insulation breakdown. That will be happening with those parts that are dealing with the high secondary coil or coil output voltage to the plugs.

Why does it happen only under load? Because under no or low load conditions the output voltage is not as high and the insulation keeping the voltage from jumping to another ground is sufficient. Under load the voltages go up considerably and the insulation keeping it in check fails so a new ground path is found and a misfire occurs.

That type of breakdown has been occurring in engines since the invention of the spark coil. That coil is nothing more than a step up transformer. Two coils of wire called primary and secondary kept separated by INSULATION. The 12 volts from our car battery is stepped up to to many, many thousands of volts.

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Old 05-30-2016, 09:13 AM   #20
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A number of years ago my wife's car developed a "miss". It was a V8 with a COP ignition system. There were no trouble codes in the computer.

After replacing the plugs and not fixing the problem, I bought one new coil and installed it on cylinder number 1. When that did not fix the problem, I took the old number 1 coil and installed it on cylinder 2 and so on. Cylinder 6 had the bad coil.
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Old 05-30-2016, 09:23 AM   #21
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The 8.1 has an indvidule coil for each cyl. and a short plug wire about 10" long. The OBD11 system will detect a misfire and locate the cyl if the misfire repeats enough.
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Old 05-30-2016, 12:08 PM   #22
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JoeSR,

That is precisely one way of diagnosing a defective component. In the field it's called, "To Cannibalize" Buy a known good part and swap until you find the defective one.

In some cases either I didn't have the tool to test something or they didn't make one. That was very common when we went through the big, big changes with all on board computers starting in 1980. That progression through what was new but only worked until the next upgrade and on and on.

I used to tell my students to wait as long as you can to buy new. The technology was changing so rapidly from year to year. What did we settle on? Almost all fuel delivery these days is EFI.

speedracer, Is there one plug wire for each spark plug??? I don't doubt you but it makes no sense to use the COP system then add a plug wire to each cylinder. The primary reason for going to the COP system was to eliminate the spark plug wires.

Those secondary plug wires fail often and that messed up the EPA guidelines which said that the vehicle had to run emission free for 50,000 miles. So they eliminated everything they could which needed replaced, adjusted (points) or if it had a high failure rate: Plug wires, coil wires, caps, and rotors.

TeJay
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Old 05-30-2016, 05:07 PM   #23
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Coil for each plug and plug wire about 10" long coil to plug.

OP has not responded in quite some time so not sure if he found the issue or is still trying to diagnose.
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Old 05-30-2016, 05:17 PM   #24
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If it has a wire for each plug then it had a coil pack or two and it is not a COP (Coil OVER Plug) design. The COP has low voltage wires running to each coil which is bolted to the top of the plug. The low voltage wires turn the coil on and off from the CPU or some computer designed for that. There are no high voltage wires in the COP design.

This system does not have a distributor cap and rotor. Each coil pack is controlled by a CPU. Most 8 cylinders had two coil packs with each coli controlling 4 of the plugs.

TeJay
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Old 05-30-2016, 07:43 PM   #25
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Tejay you are right it is not a cop system. There are eight coils bolted to the valve covers right above each spark plug with a short plug wire going to the plug.
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Old 05-31-2016, 06:38 AM   #26
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As far as I know, none of the GM V8's use the COP arrangement and mount the 8 coils on the valve covers. My Corvette uses this same arrangement as did my Silverado.
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Old 05-31-2016, 07:54 AM   #27
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So it looks like the OP, who as noted has not responded in some time, has either plug wires or coils (new) plugs failing under load. The next step(s) seems obvious.

TeJay
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Old 05-31-2016, 07:54 AM   #28
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Quote:
Originally Posted by vettenuts View Post
As far as I know, none of the GM V8's use the COP arrangement and mount the 8 coils on the valve covers. My Corvette uses this same arrangement as did my Silverado.
The reason for this is the design of the cyl. head has the spark plug too near the exhaust manifold and the heat would distroy the coil if placed directly on the plug. This also allows the use of larger more powerful and normaly more reliable coils.
It is the same design as the COP with the exception of the short plug wire.
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