GMC 6.0 Ran fine until I changed plugs, wires, coils.

[RobertRN]

My 2004 GMC Yukon XL 2500 SLT w/6.0 was running fine. The idle sounded louder than normal but I felt no miss. Just seemed the idol rpm was too low. It has 180k miles on it so it was due for a tune up. So I spent a day (most of it was getting #8 done) replacing the plugs, wires, coils. It purred like a kitten. Of course the heater hose Y had to be replaced because I touched it. Also, the block temp sensor by #1 was replaced because I bumped it and it disintegrated. Today I jumped in it to go to the store and when it started, it felt and sounded like it was running on 4 cylinders. WTF? The check engine light started flashing, 3 blocks out. So I turned around. My code reader says....you guessed. P0300 and P0351 which is telling my brand new coil is missing. So I swapped with one of the old coils but still no change.

What is going on? It was fine and I broke it. Im freaking out because it's my only ride. ???

 

[Hey you]

Guess the old saying, if it ain't broke... applies here. I know what you have just been through, and you had high hopes for a better outcome.
Go back and check the coil connectors, I found several of mine had the plastic lock broken. So possibly one is not connected properly. Also plug connections too.
I also run the disabler for the 4 cylinder operation.
Hope you get back to normal soon.

 

[PPV_2018]

a tune up done right will never make you vehicle run worse, so lets consider a few things. . . . .

1.) coils packs are not a “maintenance” or “wear” item… do not change coil packs when doing a tune up. new plugs and wires are your friends yes, but new coil packs don’t always play nice. Only change coil pack(s) if they are malfunctioning. Otherwise leave em alone !

2.) Aside from 1.), recheck your wire connections.. sometimes you might hear the wire “click” when reinstalling on the pack or plug, but may not be all the way seated. You have to re check all 8 connections and you might find that one of them was not all the way “on”.. snug it tight and you should be good to roll..

3.) plugs gapped correctly? If one or more plug gaps are off it could cause some rough running. verify the plugs are gapped correctly.

4.) hopefully you used a quality brand for the plugs and wires. The rarest scenario here is possible that [one of] the wires may be defective, but i highly doubt so. More i suspect your issue lies in 2.) or 3.) or both.

If 2.) and 3.) check out and you still have problems, put all the original coil packs back in, and make sure the connections for those are good too .. your issues should go away.

 

[OR VietVet]

You said P300, multiple cylinders.. You started it next day and ran bad? You had probs with #8. You may have cracked some porcelains on plugs. Check them all, again. Y broke. Water went where? Temp sender broke. Access require any movement of previous tune work. Did you clean MAF and throttle bore?

P351 pointing at cylinder 1. Like has been said, suspect connector or wiring. Have a multiple cylinders misfire and also #1 code for coil. Sure sounds like you did something during your process. Strange that it ran fine right after. Do anything at all after the initial restart after parts replacement?

 

[RobertRN]

You said P300, multiple cylinders.. You started it next day and ran bad? You had probs with #8. You may have cracked some porcelains on plugs. Check them all, again. Y broke. Water went where? Temp sender broke. Access require any movement of previous tune work. Did you clean MAF and throttle bore?

P351 pointing at cylinder 1. Like has been said, suspect connector or wiring. Have a multiple cylinders misfire and also #1 code for coil. Sure sounds like you did something during your process. Strange that it ran fine right after. Do anything at all after the initial restart after parts replacement?

Hi,
When I changed the stuff, it started and ran like a Cheetah. Ran great for a couple days, after I changed the heater core Ys. It did of course leak on #8 and that caused roughness. But it straightened out. Was running great until yesterday. Here is what I did so far.

1. Checked all the plug wires. I bought some 8.0mm wires and when I would push them on I did not hear the "click". So if I did not I squeezed the connector slightly till I did.

2. I have several plug wire harnesses on the coils that, because she is a old lady, the securing clip broke so l made sure the plug was bottomed out in the coil and the wrapped it on. I made sure I had no blue showing on that plug harness when I pushed it on.

3. I put dielectric grease on all plug wires at the spark plug and the coil, also the wire harness to the coils.

4. I cleaned the air mass sensor with the special spray put it back on. Put dielectric grease on the electrical connector.

5. Cleaned the throttle body and put in new air filter.

6. I put the old coil back on #1 it makes no difference. Since #1 wire harness plug disintegrated when I pulled it out I have ordered a new harness plug. Comes tomorrow so I'm stuck till then. But will soldier a new harness in. I. Hoping that will do it.

7. I think today as you said I will put back all the old coils. They are an off brand but came highly recommended with over 2000 reviews. But got to get back to square one.

8. The new plugs are NGK #3186 this is what the parts store recommended???

This is where I am. I really thought this was going to be an easy update. That is my problem, I'm thinking again. Anything I'm missing? This truck, except for the occasional bad gas cap has NEVER had a check engine light, let alone flashing. I was horrified when that happened. I've owned it since new it had 12 miles on it then 21 years ago. Only things I have done until this year to the engine is oil, batteries, and tires, and 3 water pumps, starter, rear main seal that a friend did 4 years ago. This year I have put in 4 O2 sensors, 2 knock sensors, oil pressure sensor, block temp sensor, radiator and new metering tube in the air and recharged. Those were over a year ago and it has never failed me. It's a great truck I want her back on the road.

 

[OR VietVet]

SHOULD ALWAYS tell the complete story like you just did. You left out the part about the #1 coil connector breaking apart. IMO, especially anything ignition related and not caring about reviews that are easily made up, you should always stick with Genuine GM parts or AC Delco, instead of aftermarket parts.

 

[RobertRN]

I got it fix, I had the #1 and #8 spark plug wires were not completely clicked on their coils but I think the big issue was; even though #3, #6 and #8 electrical connectors to the coils were clicked on with the clip when I took them off again, the plastic was so brittle the bottom of the connector has broke off inside the coil. So I ordered a set of new wiring harnesses and dug out the broken plastic from inside the coils, put some dielectric grease on the connectors and installed the new harness. Cleared the codes and now I have my cheetah back.

Can anyone tell me why the harness is tightly packed behind the mounting bracket for the coils? I have a feeling the wires may also have cracks in them from the heat under there. I did not take that bracket off and reroute the new harness there I kept it on top and away from any heat sources. Just seems like a GM idea but it not a good one. My dad, who passed 15 years ago, was a GM engineer and helped design a lot of GM engines and components. I would be asking him but can't. Is there any benefit having the wires under there?

 

[RobertRN]

One more thing, the wire harness plugs for the coils I ordered, even though Amazon said they fit...they were not even close. They would have been a cut wires and solder the new ones on. The correct wire harness with coil connectors came all wired up to plug into the main wire harness on the valve cover. Easy change over except digging out all the old plastic broken plug pieces out of the coils. Just saying Amazon is not good of the auto.

 

[89Suburban]

I got it fix, I had the #1 and #8 spark plug wires were not completely clicked on their coils but I think the big issue was; even though #3, #6 and #8 electrical connectors to the coils were clicked on with the clip when I took them off again, the plastic was so brittle the bottom of the connector has broke off inside the coil. So I ordered a set of new wiring harnesses and dug out the broken plastic from inside the coils, put some dielectric grease on the connectors and installed the new harness. Cleared the codes and now I have my cheetah back.

Can anyone tell me why the harness is tightly packed behind the mounting bracket for the coils? I have a feeling the wires may also have cracks in them from the heat under there. I did not take that bracket off and reroute the new harness there I kept it on top and away from any heat sources. Just seems like a GM idea but it not a good one. My dad, who passed 15 years ago, was a GM engineer and helped design a lot of GM engines and components. I would be asking him but can't. Is there any benefit having the wires under there?

As far as I know the harness should not be behind the bracket, that sounds strange.

LOL @ cheetah

 

[Fless]

Can anyone tell me why the harness is tightly packed behind the mounting bracket for the coils? I have a feeling the wires may also have cracks in them from the heat under there. I did not take that bracket off and reroute the new harness there I kept it on top and away from any heat sources. Just seems like a GM idea but it not a good one. My dad, who passed 15 years ago, was a GM engineer and helped design a lot of GM engines and components. I would be asking him but can't. Is there any benefit having the wires under there?

Pic of the bracket and the old harness position?

 

[OR VietVet]

Seems to have 2 different designs of coils and therefore different connectors?

 

[RobertRN]

Well, I can't take a pic of the harness behind the coil bracket it is long gone. It was falling apart from old age. During this 3 week stent, getting ready for summer and some other things were in need; I live in South Florida and my dash had completely rotted out to where a carpet cover could not fix it. I replaced all 6 HVAC actuators (4 in front, 2 in rear) they were either grinding or stuck, new blower this one was squeaking, new resister, fixed 3 chassis grounds that were looking shady, new dash cover and basel, and put a can of injector cleaner in this tank of gas, also due for oil change so I'm trying the Valvoline Restore and Protect. I also replaced the PCV hose connector it was looking like it was going to crack.

So since replacing the harness, wires, plugs, coolant temp sender, MAS sensor and cleaned out the carbon on the intake, it starts up runs ok down the road. The idle seems low but is showing 500 on the tach like always.

THE NEW ISSUE - The DIC instant MPG is very strange, going 50 mph on a flat road at operating temp, it is very erratic. Every time it changes it will be like 35, 12, 6, 28, 16, 2, 26, 10, 4, 38....if I am really focused into this I think I feel the engine sooth out at the higher numbers it could be just me.

I only have a cheap code reader and it is showing no codes. Any suggestions, sorry for being winded but wanted you to know all I have done. This truck has been amazing, I have owned since brand new and has never left me anywhere. Always starts, if the check engine light came on I would freak. It has never, except above but fixed that and cleared the codes.

If you think it needs a more intelligent code reader does anyone know someone trustworthy in the Fort Lauderdale area I can take it to.
Thank you
Bob

 

[RobertRN]

Here is what I am working on.
2004 GMC Yukon XL SLT 2500 with 6.0 has 179,000 miles only owner. I wrapped the whole thing last fall because Florida loves to eat clear coat and was looking pretty bad. Took a month but was well worth it.

 

[RobertRN]

Pic of the bracket and the old harness position?

Had to cut it out so no pic. Sorry. I left the new harness out front. I have never had that bracket off not sure why unless just a mistake during assembly in the factory.

 

[RobertRN]

New strange issue has surfaced, I have been driving it for two weeks now. Starts great, runs ok down the road, idle seems low though. Here is the weird thing; when at a constant speed on a flat road (I live in South Florida so not an issue). The DIC instant Econ jumps all over. It will be for example - 20, 12, 3, 14, 33, 23, 15, 4, 13, 19, 2, 17. There is no rhyme or reason, if I tune myself into this I think I feel the engine smooth out at higher numbers.

I thought it was the air compressor so I turned it off and no change. The compressor is getting due for replacement. I do notice when it kicks in and did not. I have not codes im stumped to what is causing such weird variance, any suggestions? I only have a cheap code reader. If I need an advanced read anybody know anyone trustworthy in Fort Lauderdale area. My cheetah is still limping a little and need her top notch. Please help.