Persistent P0308 (#8 Cyl misfire)

[tlamot01]

So I'm turning to my fellow GMC Yukon owners to help me solve this problem that just won't go away.

Per recommendation of many posts, I replaced the stock vortec 5.7L SPFI injector spider with a MPFI. I can't tell any difference between the two packs because the truch still misfires on the 8th cylinder. It is really noticeable in the lower RPM range and goes away on hard acceleration.
I pulled the spark plug out and it looks a bit brown, but otherwise dry and a clear mark on the electrode where the spark is making good contact.
I've replaced the coil, cap, rotor and injectors. The only things left are plugs and wires. Since taking out the #8 spark plug was nearly impossible due to aftermarket headers, I'm inclined to just replace the AC Delco's with NGK's.
I have a fairly expensive Fluke and checked the #8 wire's resistance. It measured at .768K ohms. Every google search I could find on testing wires indicates that the resitance should be between 10-15K ohms per foot. I thought my reading was strange so I decided to test all the wires on that bank. The #2 wire shows 1.397K ohnms; matching up with the near double length of the #8 wire. A similar reading was obtained from the #4 wire, which was also nearly double the length of the #8 wire.
Is there a reason why my wires are reading such low resitance. Even though I'm not convinced the wires are defective?
What else should I be checking. I've already dumped so much into getting the SFPI to MFPI conversion and coil/cap/rotor replacement.
My brother seems to think I may have a clogged cat or bad o2 sensor, which isn't too unreasonable since I'm pushing 150K on the odometer. Just hoping to get some advice before putting another paycheck into my Yukon.

Any suggestions would be greatly appreciated.

-TIM

 

[SunlitComet]

do not use ngk plugs. Acdelco or Autolite only. A cat gone bad would effect more then one cylinder. From the service manual:

VISUAL INSPECTION

1. Inspect the routing of the wires. Improper routing can cause crossfiring.
2. Inspect each wire for any signs of cracks or splits in the wire.
3. Inspect each boot for signs of tears, piercing, arc through, or carbon tracking. If the boot needs to be replaced, twist it a half-turn in either direction to break the seal before pulling on the boot to remove the wire.

SPARK PLUG WIRE RESISTANCE TEST

1. Disconnect both ends of each wire. Make sure the wire terminals are clean.
2. Set ohmmeter on the high scale and connect it to each end of the wire being tested. Twist the wire gently while watching ohmmeter.
3. If ohmmeter reads above 30,000 ohms (no matter how long the wire is), or fluctuates from infinity any value, replace the wire.
4. If the resistance of any wire is not within the following ranges, replace the wire being tested.

• • 0 to 381 mm (0 to 15 in.) wire - 3,000 to 10,000 ohms.
  • 381 mm to 635 mm (15 in. to 25 in.) wire - 4,000 to 15,000 ohms.
  • 635 mm to 889 mm (25 in. to 35 in.) wire - 6,000 to 20,000 ohms.
  • Longer wire - should measure about 5,000 to 10,000 ohms per foot.
• recheck and report back here.

 

[LVNeptune]

Did you ever have the intake manifold gasket replaced? Run a compression test on that cylinder and see what comes back. I just had mine replaced and the 8th cylinder was showing a misfire as well before hand. After it works great. No codes

Edit: Didn't fully read. Couldn't hurt though.

 

[Vinniety]

Does the code come up when its cold then go away when it warms up? Does the truck run okay even though the check engine light is on? Does it feel like it has a miss or not?

I have the same issue except with cylinder 6 and i changed plugs, wires, coil and still no good results. I read the intake gasket causes this during cold weather which is my problem more when cold the check engine light comes on. I decide to tighten the bolts on the intake just a little to see if i get lucky and wow did i. It lasted almost a year until it started again. maybe that can be your issue it seems that you changed everything else related to that cylinder. As other stated if it was a CAT issue it would cause different codes.

 

[tlamot01]

Put new AC Delco plugs in and the situation is slightly improved. Wires are on the way from the Advance Auto Parts warehouse. The idle is now much stronger, but the misfire is still there at low throttle (between 0-20 mph and when I let off the gas at freeway speeds). Hard acceleration shows no symptoms of misfire. I'm hoping replacement of the wires fixes this as there is just nothing left that I haven't replaced...

 

[Vinniety]

Did you ever clean your intake? Go get a can of seafom intake cleaner and spray it up. Also maybe do a seafoam treatment through brake booster it does wonders,,,,trust me!!!

 

[SunlitComet]

it still sounds like your spark delivery is week but nut totally broken. Almost like the insulation in the ignition cabling is breaking down.

And DO NOT use seafoam thru the brake booster line. follow the link below on its proper use on vortec engines.
http://www.seafoamsales.com/cleanin...n-vortec-and-other-exotic-intake-engines.html

 

[SunlitComet]

route wire like so.

 

[tlamot01]

@ SunlitComet
You sir are truly a wealth of information. I will continue to share my results after proper wire routing is accomplished.

 

[tlamot01]

After much consideration of cost, I decided to have the truck looked at by the local diagnostics shop. Luckily, their $80 diag fee would convert into labor and or parts if work was necessary. In this case, work was necessary. They found my distributor housing cracked and seperated in multiple areas. The tension holder was hanging on by a sliver and the lower part of the shaft housing was completely separated. Thankfully, the gear kept anything from falling into the crankcase.
So, they just happened to have a spare distributor housing and they replaced it and computer set the timing to the correct vortec setting.
So far so good. No repeat P1345 or random misfire codes.
Total cost of repair was $270.