Misfires after new plugs and wires

[GrayKnight]

I have a 1999 Yukon with a 5.7L V8, 230K miles on the odometer. It's been running fine, perhaps a little tired but still gets me down the road.

With winter coming on I decided to do some routine maintenance, including new AC-Delco plugs and wires. I carefully gapped the plugs to .060, as indicated on the decal in the engine compartment. (Out of curiosity, I checked the gaps on the plugs I took out and they were all at .062 or .063, so I'm pretty sure the gap isn't the issue.) After changing the plugs and wires, one at a time to make sure no wires got crossed, I started the engine expecting the same smooth idle as before the change. Instead, the engine ran rough. When I tried to drive it around the block the Check Engine light came on, and then started flashing. I made it back to the driveway and dumped the error code, which was P0300 indicating multiple cylinder misfires. I cleared the code, started the engine, waited until the Check Engine light came on again, and dumped the code. Same error.

I didn't do anything else except clean and recharge the K&N air filter.

Does anybody have any ideas as to what might have happened? I'm reasonably competent with mechanical issues but electrical stuff baffles me. Why would an engine that was running smoothly start missing in multiple cylinders after I installed new plugs and wires??? All suggestions gratefully appreciated.

 

[ezdaar]

Tighten the gap to 45, see if missfore goes away. if it gets better but doesnt remove it comletely, tighten to 35.

 

[GrayKnight]

Thanks, I'll try it. FWIW, the plugs were all pregapped to .035 out of the box. Forgive my ignorance, but are plugs pregapped for specific applications at the factory?

 

[ezdaar]

Yup.

But, with a good high output ignition you can open the gap, sometimes significantly.

Tighten them back up and see what happens.

 

[wrager]

Hope you didn't over-oil that filter...

 

[livingez_123]

Hope you didn't over-oil that filter...

This was my first thought.

 

[SunlitComet]

if you got plugs that were pre-gapped to anything other then 0.060" you have the wrong parts. Unless you want to modify your ignition system use only delco or delphi ignition parts or if you wish just for the plugs you can use delco 41-933 and also Autolite XP605 plugs. They are pre-gapped to 0.060". D not let anyone tell you otherwise that a different bigger or smaller gap is okay when putting them into the engine. They are absolutely wrong. you should be using platinum type plugs which are always gapped at the factory.

---------- Post added at 05:15 PM ---------- Previous post was at 05:14 PM ----------

If needed have someone with the right scanner look at your misfire history so as to pinpoint the specific cylinders giving you a hard time.

 

[GrayKnight]

This was my first thought.

OK, if I over-oiled wouldn't I get a MAF sensor code? Also I forgot to mention that I started the engine for a minute to see if there were any issues before reinstalling the air intake box and filter and it ran rough then, so I don't think too much oil is the problem, or at least not all of it.

And, please, folks -- I do appreciate your comments and advice very much and maybe I overstated my level of expertise, but "I hope you didn't over oil that filter" without any further information doesn't help much. Maybe "Over oiling your filter could be the problem and here's how to tell if you did..."? Thanks again for your help.

 

[SunlitComet]

you could get a "performance" related code if the maf was contaminated enough. Usually at that point the pcm will lock the maf sensor out of the system and the engine will run in a speed density mode. http://www.carcraft.com/techarticles/electronic_fuel_injection/.

 

[GrayKnight]

OK, it's running again. Thanks to SunlitComet for thee pointer to the MAF sensor. After I figured out how to clean it (and it *really* needed cleaning) the old Yukon is running better than in a long time. Thanks to all who took the time to respond.