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Old 12-05-2022, 10:30 AM   #1
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Misfire periodic cyl 8 and 3

Good Morning here in California.

Engine runs good but I noticed when at idle a sporadic miss. I only have 20000 miles on my 8.1L motor home. Therefore I changed the spark plugs, as I learned here, AC Delco iridium, and the Wires, yes, Magnum.

I ran a compression test since I had the spark plugs removed. all great.
165 every cylinder.

I ran the codes, No codes.

I ran the functions on the Scanner and it showed still a sparatic miss in Cylinder 8 and a very sparatic miss in cylinder 3 at idle. I showed no miss when I accelerated; I ran up to 2500 rpm.

Thoughts.
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Old 12-05-2022, 10:39 AM   #2
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Could be the coil packs.
You sure the plug wires are seated in the coils and on the plugs ?

Mike in Colorado
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Old 12-05-2022, 12:48 PM   #3
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Yes, wires snapped in nicely. All are tight
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Old 12-05-2022, 01:10 PM   #4
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Faulty CPS or connector/wiring.
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Old 12-05-2022, 01:46 PM   #5
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Trek,
Just as an FYI, you can get a full set (8) of "ENA" coils on Amazon for $127.00 now, Or $75.00 each for Delfi's at Autozone and $377.00 per set of 8.

OR, Summit Racing has them for under $40.00 per coil and several to choose from.

Were it me, I'd replace the whole set.

Mike in Colorado
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Old 12-05-2022, 02:55 PM   #6
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Trek,
I just did a real deep dive on the ENA coils, and it looks like they get very good ratings from several sites.
I'd go for it with a full set of 8 @ less than a hundred thirty bucks, I don't think you could go wrong.
I've marked a set for me also on my watch list.

Mike in Colorado
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Old 12-05-2022, 06:06 PM   #7
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Before you run out and buy 8 coils you may not need you should have an injector test performed. This can be done with many of the different scan tools and a fuel pressure gauge. With the scan tool in injector test mode the ignition is turned on and the tool will tell you when to start the test. You fire each cylinder and watch for the drop in pressure, record it and go to the next cylinder, the fuel pump will run, stop then you fire the cylinder again watching the fuel pressure gauge. You do this for all 8 cylinders. If you have a weak injector you will see a variance in fuel pressure in the cylinder with the weak or plugged injector. Each time you fire a cylinder it is a programmed, timed event. Consequently the fuel pressure drop should be uniform across all 8 cylinders. If you have an injector or two out of sync. you know which injector(s) are at fault. A very dirty or bad injector will flow very little or no fuel. You would see a very small drop if any at all in pressure on the gauge when the cylinder is fired.
In the case of the OP I would check the suspect cylinders a few times if they passed the test the first time. It may be an intermittent problem.
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Old 12-05-2022, 06:33 PM   #8
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This is NOT SAFE but it is often the only way to find high voltage "leaks".

You will need an assistant, a flashlight and a misting bottle. Park vehicle in a very dark place, like a closed garage. Open the hood and with your flashlight visualize where the high voltage components are (cap, coil, wires). Turn out all lights. Have the assistant start the engine. Spray a ligh spray of water on the high voltage components and watch for light blue flashes.

Once you have identified one, it will probably have a nearly invisible carbon trace down to ground. Usually impossible to clean. Some kind of abrasive may work and then apply a thin film of silicon dielectric grease. This is now a maintenance item as the grease will have to be removed/clean and replaced ever 6-12 months.

On coil-on-plug, the carbon track is usually inside the boot or on the plug. Some COP have replaceable boots. Replace the spark plug at the same time and a thin film of silicon dielectric grease on the spark plug top and ceramic insulator.
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Old 12-05-2022, 07:49 PM   #9
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Quote:
Originally Posted by LETMGROW View Post
Before you run out and buy 8 coils you may not need you should have an injector test performed. This can be done with many of the different scan tools and a fuel pressure gauge. With the scan tool in injector test mode the ignition is turned on and the tool will tell you when to start the test. You fire each cylinder and watch for the drop in pressure, record it and go to the next cylinder, the fuel pump will run, stop then you fire the cylinder again watching the fuel pressure gauge. You do this for all 8 cylinders. If you have a weak injector you will see a variance in fuel pressure in the cylinder with the weak or plugged injector. Each time you fire a cylinder it is a programmed, timed event. Consequently the fuel pressure drop should be uniform across all 8 cylinders. If you have an injector or two out of sync. you know which injector(s) are at fault. A very dirty or bad injector will flow very little or no fuel. You would see a very small drop if any at all in pressure on the gauge when the cylinder is fired.

In the case of the OP I would check the suspect cylinders a few times if they passed the test the first time. It may be an intermittent problem.


That would be my next step as well. Also, a fuel pressure gauge will tell you quickly if an injector is leaking after which you need to identify which one.
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Old 12-05-2022, 08:37 PM   #10
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That would be my next step as well. Also, a fuel pressure gauge will tell you quickly if an injector is leaking after which you need to identify which one.

I've got another check while you are at it. Vacuum leaks, check the injector nose seals and intake gasket to head for vacuum leaks. Cracks in the gasket or injector nose seals can allow just enough air to cause an intermittent low speed misfire. Once the engine comes off idle, there is not enough leakage to effect the mixture to the point of lean misfire.
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Old 12-06-2022, 12:45 PM   #11
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Cylinder Misses

Cylinder 8 has perodic misses of 8 maximum this then returns to 0

Cylinder 3 has perodic misses 0 2 then returns to 0

All other cylinders show no misses.

These are very minimal misses that are very sparadic.

Engine has 20000 miles, almost nothing

I am wondering what fuel the prior owner used ???

the Generator has substantially more hours on it? may have been used mostly for guest home???

Possible carbon buildup on injectors and valves???

What about Sea Foam?
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Old 12-06-2022, 12:48 PM   #12
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X2 on the Sea Foam.
2 cans on a 1/4 tank of gas is what I would do.

Mike in Colorado
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Old 12-06-2022, 03:10 PM   #13
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When my tank gets down to 1/4 tank I will add sea foam.
thank you
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Old 12-07-2022, 06:55 PM   #14
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if they are not to difficult to get to swap the 8 and 3 coils with other coils.if the problem moves you have your problem if not move on to other options.
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