mystifying misfire - Team Camaro Tech
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post #1 of 11 (permalink) Old Jul 20th, 14, 05:17 PM Thread Starter
Mike
 
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mystifying misfire

Hi all,

I recently installed a BBC in my Camaro and it ran great for a couple of months. Then one day I noticed it was running a little rough and as I drove home it suddenly and noticeably got worse with lazy throttle response at low rpm. Since then, I've determined the #4 and #5 cylinders aren't firing (using the old water on the header tube trick). I've been racking my brain trying to find the problem but to no avail. I pulled the valve covers and verified the valves operate (visual, did not measure movement but looked like the other valves), swapped spark plug wires and spark plugs, verified spark using the timing light and grounding the plug on the engine block, checked timing, and checked cylinder compression.

I'm at a loss as to what to do next. Am I missing something obvious? Does anyone have any thoughts?

Thanks in advance,
Mike
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post #2 of 11 (permalink) Old Jul 20th, 14, 05:33 PM
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Re: mystifying misfire

Is it possible the plugs are loosing their ground on those two cylinders?


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post #3 of 11 (permalink) Old Jul 20th, 14, 05:47 PM
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Re: mystifying misfire

Do a compression test on those two cylinders that aren't firing and then test the other two on that bank that are firing and see if/how much of a difference you have between the good ones and the bad ones.

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post #4 of 11 (permalink) Old Jul 20th, 14, 06:20 PM
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Re: mystifying misfire

Have you pulled those 2 plugs to look at them?

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post #5 of 11 (permalink) Old Jul 21st, 14, 01:39 PM
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Re: mystifying misfire

Was the compression on cylinders #4 and #5 the same as the rest? What condition are those two spark plugs in. Use a multimeter and check the ohms on those two wires. Post the compression readings, ohms reading. Look under the cap very carefully from the center button to the #4 & #5 post for cracks. You can also ohm the cap too. If you try and do not get a reading scrap the post with sandpaper and try again. Post all your readings. Most likely you need a new cap and rotor.
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post #6 of 11 (permalink) Old Jul 21st, 14, 02:20 PM
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Re: mystifying misfire

Pull a HT/ plug, see if have a healthy spark?
Re check the HT lead order.
Cracked cap and/ or rotor
If u have not wiped it yet, look real close and u can sometimes see where the spark has been tracing across.
Quick check compession.. thumb over the spark plug hole... If u have compression mixture and spark it will fire, even by the thumb test.

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post #7 of 11 (permalink) Old Jul 23rd, 14, 03:56 PM Thread Starter
Mike
 
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Re: mystifying misfire

Compression is good on all cylinders ranging from 165 - 175.
I haven't checked the resistance on the wires but I did swap wires with a different cylinder and saw no change. Plugs look good too but swapped those as well. Likewise, spark looks good and strong when grounded on the engine block.

I haven't checked the distributor though. I'll do that next and post results.

Thank you all for the input.
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post #8 of 11 (permalink) Old Jul 23rd, 14, 07:38 PM
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Re: mystifying misfire

i had something similar recently. checked wires with timing light and found the dead one.
origin of problem: the cap/rotor.
if it was running well, it has to be something simple. or should i say start with the simple

edit: oops never mind. just read your post above

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post #9 of 11 (permalink) Old Jul 23rd, 14, 08:43 PM
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Re: mystifying misfire

Turn out the lights, let your eyes adjust, run it in the dark. See if you can see an arc on the cap, wires or block by those cylinders. Watch fingers on fan.

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post #10 of 11 (permalink) Old Jul 23rd, 14, 11:34 PM
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Re: mystifying misfire

Quote:
Watch fingers on fan.
Hmm the lights are out, and why put fingers on the fan?
Or did I get that a bit wrong lol

Its like when someone walks under something low "watch your head.."

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post #11 of 11 (permalink) Old Jul 25th, 14, 11:10 AM
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Re: mystifying misfire

Ask me if putting fingers in the fan hurts. Didn't happen with the light out even.

1969 Camaro 383
2.02/1.63 & 234/244 Vortec
TH350 / 12 bolt 3.73
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