Engine missfire after changing plugs/wires

[bheston]

2000 Lumina, running fine but about time for a tuneup, so I changed plugs and wires. The next day it started missing, so I changed back to the old wires and plugs but that didn't fix it. Now it might run fine for 1-2 days (driving about 150 miles/day) and then it will start missing again. It usually happens after I have been driving highway speed for a while and I slow down to town speed. It will miss for the rest of the trip, and then might be fine the next morning.

The diagnostic computer shows 3 cylinders missing, one on each coil pack, so I can't believe I have a bad coil pack. I have run a couple bottles of injector cleaner through and that didn't seem to make a difference.

I have checked all 12V cabling that I can think of, battery, ground, and alternator cables are clean and tight. What else can I look for?

Thanks in advance for the help, I'm sure you guys can give me a few more things to look for.

[jeffcoslacker]

Are the ones the computer said were misfiring all on one bank of the engine, maybe the rear?

I've read some tech bulletins about problems from misrouting the plug wires and getting too close to sensor related wiring, causing an inductive signal in the harness that confuses the ECM...

I've seen people crack insulators on the rear bank plugs when they didn't roll the motor forward to make room to work...

[tblake]

also, the back three plug wires all run to the back inside of a split look wire case, if you have all three wires running parallel, they will induce voltage into one another, and cause a misfire condition much like you describe. What you have to do is twist the plug wires inside of the split loom, sort of a braiding thing. This breaks up the lines of flux, and will not allow them to induce voltage into another wire. What type of plug wires did you put on there? I hope they are ac-delco, and not crappy autolites or something. Also might wanna change the plugs too. I usually like to do this at the same time as wires.

[jeffcoslacker]

Originally Posted by tblake

if you have all three wires running parallel, they will induce voltage into one another, and cause a misfire condition much like you describe. What you have to do is twist the plug wires inside of the split loom, sort of a braiding thing. This breaks up the lines of flux, and will not allow them to induce voltage into another wire.

Holy Cow!!!

Why didn't I know this?

Nice one!

[tblake]

yeah, you didnt? Thats why your antenna wires are shielded with tin foil like stuff, it is grounded, so if electromotive interference (EMI) is induced into that antenna wire (or whatever wire), the shielding grabs the voltage, and grounds it so it never reaches the antenna center wire. Much is the same for your ignition wires. They have no shielding, and lines of flux cannot be stopped, they are always there no matter what. These are how magnets attract themselves. In an effort to stop induction into wires, we twist or braid them. This is why a lot of the low voltage sensor wires are often twisted together, speed sensors and such, to stop voltage from being induced into another wire and sending a false signal. Did I confuse you? I doubt it, but hope this helps you a littly bit bheston.

[bheston]

Well I braided the wires so they weren't running parallel with each other, and it has run great for two days now, so I think I got this one taken care of. Thanks for the great ideas guys, I don't know if I would have figured this one out on my own.

[RBStratman]

Great advise. Mine has recently started giving me the same trouble (98 with the 3.8 LTZ package). I changed the plugs and wires last summer, but recently my OBD II has been telling me of a misfire in Cylinder 2, so I replaced that plug and it ran fine for a day(180 miles a day) and then I get the same code for Cyllinder 1, runs good for a day and now is back to sluggish again. I'm going to replace the plugs and wires again this weekend as I've noticed some wear on the wire boots. I will buy better wires and plugs this time but will make sure to braid them as you've suggested as, I have a little electrical knowledge, so what you say makes great sense. Thanks. I didn't notice the wires being braided before I changed them, but will give it a shot this time.