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Cylinder #4 Misfire

3K views 16 replies 5 participants last post by  ap41563 
#1 ·
Ok, so I pulled a code yesterday because I noticed about a month ago the engine started having a rough ide and the engine was not smooth like it usually is.
Keep in mind this car has not been driven since the engine was rebuilt and the car was restored, minus going around the block 3 or 4 times.
Anyway it normally ran very quiet and smooth, I hooked up the Autel scanner and it shows a P0300 so I went into the live date tab section on the scanner while the engine was running and watched cylinders 1-8 on the bar graph live data and cylinder 4 was fluctuating. Since the coil for #4 cylinder/plug is shared with #1 how do I resolve this, the plugs are brand new ACDelco OEM plugs, the coils were not replaced after engine rebuild though. I noticed it started running rough when i ran it down to empty to burn all the old gas out of the car. I just put in a new OEM fuel filter and and 4 gallons of premium fuel since running it nearly empty but still notice the rougher than usual idle.
Could the EVAP line with little green cap have a leak, a vacuum leak somewhere causing the cylinder misfire, just wondering because first several times after rebuild there was no engine misfire codes. I cleared the codes after charging the battery because it was below 12 volts, there were 3 codes. P0231 P0300 And P0603. The P0300 in reference to #4 cylinder is the only one that stayed. How can i figure this out and fix it, what are possible causes besides , plug or plug wire, coil or vacuum leak. Plugs are new and the wires are too.
 
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#6 ·
I can try and see but the plug wires were brand new OEM wires when I installed them as were the plugs so unless something has completely fouled out the plug from running 3 hours or so since rebuild (have not been able to drive, waiting on new tags and plates) in my driveway and around the block 2 or 3 times, it should not be either but I will check. Thanks.
 
#3 ·
I believe this has been posted for you in the past - spark plug wire/coil pairing is important in a waste spark system. Make sure the coil base-to-ICM contacts are clean and that the ground cable is correctly terminated and bolted.

Each numbered coil post must be connected to the corresponding spark plug. Plug wires that are over a few years/50,000 miles old are also suspect. Check the wire boots and spark plugs for carbon tracking. Tracked plugs, coils, or wire boots cannot be cleaned. Replace only.

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#5 · (Edited by Moderator)
Plugs and wires are new OEM wires and plugs with a few hours of time on them since installed.

I believe this has been posted for you in the past - spark plug wire/coil pairing is important in a waste spark system. Make sure the coil base-to-ICM contacts are clean and that the ground cable is correctly terminated and bolted.

Each numbered coil post must be connected to the corresponding spark plug. Plug wires that are over a few years/50,000 miles old are also suspect. Check the wire boots and spark plugs for carbon tracking. Tracked plugs, coils, or wire boots cannot be cleaned. Replace only.

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Thanks Sub , I have never had this problem before though so not sure if this was shared with me before for any reason.
 
#10 ·
I think I will go ahead and replace the EVAP emission cannister harness, the plastic lines that go to the purge valve and has the green cap with a schrader valve , mine was broken into 2 pieces and super glued back together, did not seem to leak but it shows a small leak on scanner somewhere it said when I tried testing. This thing.https://www.ebay.com/itm/313613868120?ssPageName=STRK%3AMEBIDX%3AIT&fromMakeTrack=true
 
#13 ·
I wanted to bring this up about the rear head static ground strap, it is screwed into rear cylinder head with a 10mm bolt and the front is on top of Cam cover, I will post an image, The original ground strap was missing so I bought a negative battery ground wire from parts house and used it though it was a bit long, but I just wanted to mention it since Sub had mentioned something about the ground being terminated correctly, hopefully it will suffice.
 
#15 ·
Well here is an image of the #4 plug, looks wet with what looks like oil , probably mixed with the ARP thread anti seize that was used on plug threads after the engine rebuild . I do know the engine has not been broken in yet since rebuild, I know it needs a few WOT on the highway to seat the new piston rings which have not been done yet, As soon as my plates and tags come I will do the drive break in for the engine. I found a nick in the #4 plug wire, not enough to cause problems I do not believe, I wrapped it with electrical tape for now. I think most of what is on the spark plug thread is the thread anti seize because it was a dark thick substance, but the wet part could be oil from rings not being seated yet. I will update when I get wires switched, then I will switch the coil that is for #1 and #4 with another one to see if it is the coil.
 

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#17 ·
I had a misfire on cylinder number 4 after doing new plugs, boots and coils at 100k. I can tell you after about 100 miles I started getting a misfire on cyl number 4 as well. However, my check engine light was flashing under acceleration. I used all AC Delco parts and I could never prove that it was arcing around the boot but after switching out the boot with my old one. The problem never did resurface. That boot is actually still on the car 70k and 4 years later.
 
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