Help - running rough, new plugs, wires, cap and rotor

Hey folks, got a mystery I'm hoping you all can help me solve
My truck started running rough and the check engine light would come on from time to time then go back off again. It started getting more frequent so I took it to Autozone one day while the light was on, they said it needed a typical tune-up, plugs, wires, ect.

So I bought platinum plugs, cheap wires, cap and rotor and installed them with the plugs gapped at .060 as they were supposed to be...or so I thought.
After the tune-up the light went away but the truck still ran a little rough and didn't have as much power as I thought it had before.

After a while the light came back like before, then one day it started blinking and the truck started running really rough and nearly stalling. I took it in and they said I had a misfire on cylinder six.

I checked the wires for resistance with an ohmmeter and one of them was not the same as the others so I took them back out and returned them. I also noticed that the plug had fouled. I removed the rest of them and they were ALL FOULED

I returned the wires for better Bosch wires and got a whole new set of plugs. Then I posted up on here and some great folks helped me figure out how to put the wires back on right and suggested that maybe all the plugs fouled because the plug gap or maybe the wrong heat range.

So I did a little research here and on GMT400 and a couple other GM sites and found a few posts that stated GM had changed the recommended plug gap from .06 to .04 because of misfire and pinging. Some say it's only on the iridium plugs, others say all of them. The old plugs look like they may have had an even bigger gap than .06.

I gapped the new plugs to .04 and put them in with the new wires and it sounds better but still really rough.
I have an after market cold air intake and a power programmer installed. Not sure if that affects anything. I did find and repair a couple of vacuum leaks too.

What do you guys think? Sorry for the looooong post. Any help is much appreciated

Yea I have heard a lot about these engines not running clean when your plug gap isnt spot on. But when your engine ran real rough and almost stall sounds like when my distributor went bad. Same thing happened, check engine light was blinking and engine ran like it was on two cylinders. but Im not sure what code it was throwing for that problem. I just know that I had everything placed and it ran like a top. (that was before I found this forum, now i do everything myself)

gulleyman said:

So I did a little research here and on GMT400 and a couple other GM sites and found a few posts that stated GM had changed the recommended plug gap from .06 to .04 because of misfire and pinging. Some say it's only on the iridium plugs, others say all of them.

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I've read the same posts before but can never find the exact GM document for it. Mine used to eat distributor caps like they were popcorn till I closed the gaps down though. People will argue you have to run .060 because that's what GM recommended when they were built.

Ignition System - New Spark Plugs/Gapping

Bulletin No.: 03-06-04-060A

Date: February 12, 2007

INFORMATION
Subject:
Information on New Spark Plugs and Gapping

Models:
2004 Buick Rainier
2002-2004 Cadillac Escalade, Escalade EXT
2003-2004 Cadillac Escalade ESV
2004 Cadillac CTS-V
1997-2004 Chevrolet Corvette
1998-2002 Chevrolet Camaro
1999-2004 Chevrolet Silverado
2000-2004 Chevrolet Suburban, Tahoe
2002-2004 Chevrolet Avalanche
2003-2004 Chevrolet Express, TrailBlazer
1999-2004 GMC Sierra
2000-2004 GMC Yukon, Yukon XL
2001-2004 GMC Yukon Denali, Yukon XL Denali
2002-2004 GMC Sierra Denali
2003-2004 GMC Envoy XL
1998-2002 Pontiac Firebird
2004 Pontiac GTO
2003-2004 HUMMER H2

with 4.8L, 5.3L, 5.7L or 6.0L V-8 Engine (VINs V, P, T, Z, G, S, N, U - RPOs LR4, LM4, LM7, L59, LS1, L56, LQ9, LQ4)
Attention: Please disregard the Set Gap Per Vehicle Specification statement on the side of an ACDelco(R) Iridium Spark Plug Box. The statement will be removed from the boxes in the future.

Supercede:

This bulletin is being revised to bring attention to the Set Gap Per Vehicle Specification on the ACDelco(R) Iridium Spark Plug Boxes. Please discard Corporate Bulletin Number 03-06-04-060 (Section 06 - Engine/Propulsion System).

A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed.

The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01 mm (0.040 in) when the spark plug is made.

The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.

---------- Post added at 12:22 AM ---------- Previous post was at 12:20 AM ----------

I would disable your programmer and retest with a scanner.

sunlitcomet said:

I would disable your programmer and retest with a scanner.

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Thanks for posting the bulletin, that's the one I saw alright! My truck doesn't seem to fit in there, it's a '97 Yukon GT, 2dr 4x4, with the Vortec engine.
I'll have to try resetting the programmer and scanning it again, maybe that will shed some light on things.
The strange thing is, the truck seems to run a little better with the plugs gaped at .04 (still really rough) and it fouled all the new plugs the first time I had them gaped at .06. The way it's running I don't doubt it will foul them all again this time. Seems like something else is not right, I'll try removing the power programming and trying the scan again.
Please keep the suggestions coming, great stuff here!

Well......I removed the power programming and had it scanned again. Same error code, misfire on cylinder 6. Took it to one mechanic I know to be good but pricey at times. He thought it may be a bad injector or something electrical, like a short between the computer and the injector or the computer itself, said it could take a looong time to diagnose.
I've never gotten into the injectors on any car and my other car took a poop this morning when the crank pulley fell off.
So I threw in the towel on my Yukon and dropped it off with another mechanic that's usually faster, cheaper and more honest, lol. Decided to try and do the crank pulley on the other car myself....sheez!
I'll post again and let you guys know how the story ends.

gulleyman said:

Well......I removed the power programming and had it scanned again. Same error code, misfire on cylinder 6. Took it to one mechanic I know to be good but pricey at times. He thought it may be a bad injector or something electrical, like a short between the computer and the injector or the computer itself, said it could take a looong time to diagnose.

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Is this code the same instant as your first scan or after you cleared active and history of codes and it came back?

Lets assume that if your plugs are fouled of fuel that you have a weak spot in your ignition but if you have eliminated that you may benefit from changing the fuel injector assy from the "Central SFI Fuel System with a new MFI (Multiport Flexible Injection) Fuel System". The shops around me just upgrade them because about about half of the customers come back later with the same problem after just cleaning them out. I have two injector that are failing their balance test so I going to convert the module to the new type which gives me all new injectors. Costing me about $400 diy. It is mostly blamed on bad quality gas causing deposits on the poppet nozles.

I think "sunlitcomet" is right, so does the mechanic lol.
Another member "Rebolution" had a similar issue, replaced the injector spider and still had the misfire though. He took his to the dealer and found out the distributor was bad. Someone else posted a link to a pretty good looking distributor on ebay for $60 shipped.

Sounds like I should have the mechanic check my distributor too since it's already at the shop and I can't just check it myself, then replace the injector spider if the distributor looks good. What do you guys think, sound like a good plan of action?

Also, where is the best place to pick up those new and improved injector spiders?

gulleyman said:

I think "sunlitcomet" is right, so does the mechanic lol.
Another member "Rebolution" had a similar issue, replaced the injector spider and still had the misfire though. He took his to the dealer and found out the distributor was bad. Someone else posted a link to a pretty good looking distributor on ebay for $60 shipped.

Sounds like I should have the mechanic check my distributor too since it's already at the shop and I can't just check it myself, then replace the injector spider if the distributor looks good. What do you guys think, sound like a good plan of action?

Also, where is the best place to pick up those new and improved injector spiders?

Click to expand...

+1 on the where to get the mpfi to replace the spiders?

And yes, have him check the Distributer. It shouldn't take him long, and if it's good then fine, you'll have peace of mind that it's not the problem. If it's bad then change it out.

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