#3 cylinder mis-fire - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums
 
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post #1 of 11 (permalink) Old 05-25-2019, 09:03 PM Thread Starter
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#3 cylinder mis-fire

engine was missing at 180,000, so a cap & rotor seemed in order. water pump was also bad and a new one was installed along with a complete optispark from a well known vendor. the engine ran rough and inspection with a timing light showed #3 firing erratically; changed plug wire & plug and checked compression (which is 150psi) but erratic firing still present. removed distributor and checked cap which had zero resistance for #3. with further inspection, a scorch mark was seen on the plastic insulator plate. the mark went through and the metal plate was scorched on one side. it seemed that the plastic plate may have had a crack in the area where the plate drops. I installed a new plastic plate and new cap & rotor and the engine was still rough. Then i took one of my old optisparks and installed the new cap & rotor while keeping the OEM insulator plates. The car then ran great for about a mile. inspection revealed that, once again, only #3 was firing erratically. have not opened up the dizzy but i am expecting to see burn marks in the vicinity of #3 electrode....again
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post #2 of 11 (permalink) Old 05-25-2019, 11:11 PM
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Swap an injector to see if #3 is leaking.


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post #3 of 11 (permalink) Old 05-26-2019, 10:20 AM Thread Starter
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injector was OK and also swapped computer, ICM and plug & plug wire. going to get new spark plugs and double check. At idle one can see the normal flash of about 5/ sec except #3 which has a blurry rapid flash. I think that the insulator plate has again been compromised......it should be noted that #3 terminal is pretty much directly above the optispark module. what was supposed to be a simple tune-up has become a royal nightmare!.......although i can now probably do this repair with my eyes closed..
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post #4 of 11 (permalink) Old 05-26-2019, 11:00 AM
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I feel your pain buddy

97 Trans Am - being modified for DD/Track Car use
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post #5 of 11 (permalink) Old 05-29-2019, 08:13 PM
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Quote:
Originally Posted by rmestate View Post
injector was OK and also swapped computer, ICM and plug & plug wire. going to get new spark plugs and double check. At idle one can see the normal flash of about 5/ sec except #3 which has a blurry rapid flash. I think that the insulator plate has again been compromised......it should be noted that #3 terminal is pretty much directly above the optispark module. what was supposed to be a simple tune-up has become a royal nightmare!.......although i can now probably do this repair with my eyes closed..
Fuel, Air, Spark...
Fuel;
You’re sure the #3 injector is fine, how about the injector connector plug, and the wires where they come out of the loom ?
Do you have the proper voltages at the injector connector or the tool to test the injector plug for broken or loose wires ?
Air;
Are you sure the ‘O’ rings on the #3 injector are sealing properly (or need replacing) and may be letting air past messing with the F/A ratio ?
Spark;
Hmmm... it’s so easy to blame any optispark new or old, but with it being isolated to one cylinder, it’s hard to put the blame there, unless like you said, it ran good for a minute then started missing again after you changed the plate and rotor.
Is there any chance you’re experiencing cross wire arcing ?
Have you checked the routing of plug wire #3 for grounding to the engine, frame, or other plug wire and done a water spray test with engine running looking for arcs in the dark ?
You seem to know more about troubleshooting ignitions and distributors than I do, but sometimes we overlook simple steps in diagnosing issues so I hope you don’t think I’m being redundant in my thoughts on the topic.
Good luck, I hope you get it figured out.
Get a new plug AND plug wire that you check and know are good.

Last edited by SoCalChlln; 05-29-2019 at 08:16 PM.
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post #6 of 11 (permalink) Old 05-30-2019, 10:42 AM Thread Starter
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the original cap & rotor were really bad. replaced entire dizzy with new unit from vendor with mitsi module. engine continued to miss and spark check revealed only #3 was erratic. checked distributor and found that insulator plate was burned through at #3 and rotor had blisters on the bottom. purchased new cap & rotor with new insulator. continued to miss so plug wire and plug were swapped and compression checked at 150psi. still missing at #3 so spare opti with the new cap & rotor installed. ran really well for about a mile then missing resumed. installed original opti with serviceable C&R and engine still missed at #3. checked injector and it was not leaking and was heard to be functioning, but swapped it anyhow. no matter what combination of parts used, the engine continues to miss at #3. i have scanned the ecm and there are no codes and sensors appear to be responding normally but knock sensor goes reading goes up and sometimes retards the timing. last year i foolishly used valvoline syn-blend which immediately resulted in an oil leak at the rear of the manifold so that was repaired. i am forced to believe that either #3 has some poor compression or that there is a leak at the intake port and air/oil is being introduced to the cylinder. my current solution is to get out my other wagon because it is time to go fishing....for actual fish.
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post #7 of 11 (permalink) Old 05-30-2019, 12:28 PM
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I’m sorry to hear that you’ve been getting run through the chasing birdies mill...
Sounds like you’ve addressed all the possible ignition problems, and having swapped the injector with no positive results, did you check the injector wiring harness at the connector that plugs into the injector itself for bad wires or contacts ?
(23 year old wiring) insulation hardens, causing the wires to break at the weak points when bent or manipulated.
Arcing a distributor plate happens when there’s nowhere for the spark to go. That’s why you’re not supposed to turn the motor over with a plug wire not properly attached or grounded by holding the plug against a good ground surface when checking spark.
I was thinking the plug wire could be broken and resistance is too high so it’s jumping a gap and sparking in the distributor. But you’ve changed the plug wire, was it with one you KNOW is good and has been tested.
It ran good for a minute after replacing the cap and rotor then burnt the plate again ?
Sounds like spark with nowhere to go...
Beyond that, you may be dealing with a valve train issue, broken spring, bent valve not closing all the way, something mechanical specific to cylinder # 3.
Bad lifters you can generally hear, but a worn out roller bearing or a cam lobe worn down could cause the missing problems you’ve described. But that doesn’t explain the arcing... you could be suffering a dual problem which makes it very difficult to diagnose.
Oh ya, and don’t assume that because a part is new, that it’s working like it’s supposed to, I just went through that myself with a brand new ACDelco EVAP canister purge valve. I kept assuming since it was new it couldn’t be the problem.
Wrong...
I wish I knew more about how the pcm controls ignition, fuel and spark systems, or the test procedures and the proper values testing should produce.
Have you tried posting this on the ISSF forums. More guys with LT1’s over there.
https://www.impalassforum.com

Last edited by SoCalChlln; 05-30-2019 at 12:48 PM.
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post #8 of 11 (permalink) Old 05-30-2019, 08:37 PM
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II believe it's time to put an oscilloscope on the opti low & high resolution signal wires to see if optical sensors or the pick up wheel is your problem. You can get a pc based scope on ebay, amazon,etc for around $70. Every lt1 owner should have an oscilloscope.

The scope will show if low & high resolution signals are the correct square wave. It will also let you know if signal drops out or changes form while engine is running. The pick up wheel could be mis installed or have the low resolution cut outs in the wrong place.
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post #9 of 11 (permalink) Old 05-31-2019, 01:42 PM
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I was just thinking of how the grounding system works on the LT1’s and be aware that they are very finicky when it comes to the engines grounding strap that affects all the engines electrical systems.
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post #10 of 11 (permalink) Old 06-03-2019, 09:23 PM Thread Starter
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The burned insulator happened only once; perhaps there was already a crack when installed. with three different optisparks and three different cap & rotors the result was always the same. likewise with three different plug wires and plugs. compression on #3 showed ~150psi but no leak down check yet. i am suspicious that a manifold removal to fix an oil leak at the back resulted in an intake leak at the port for #3. have to consider the false positive that the toedtli crappy cap was basically still working and the real issue is #3 cylinder....thanks
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post #11 of 11 (permalink) Old 06-04-2019, 11:55 AM
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Quote:
Originally Posted by rmestate View Post
The burned insulator happened only once; perhaps there was already a crack when installed. with three different optisparks and three different cap & rotors the result was always the same. likewise with three different plug wires and plugs. compression on #3 showed ~150psi but no leak down check yet. i am suspicious that a manifold removal to fix an oil leak at the back resulted in an intake leak at the port for #3. have to consider the false positive that the toedtli crappy cap was basically still working and the real issue is #3 cylinder....thanks
An intake leak is easy to find using carb cleaner, or starting fluid. You don’t have to disassemble it to check.
Start the car and while it’s idling, use the plastic spray tube to accurately spray some carb cleaner/starting fluid where you suspect the leak to be, and listen for a change in the idle either up or down.
Change in RPM’s = intake leak.
No change, look somewhere else, (valve springs, cracked valve, etc).
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