P0300 error after new plugs and wires

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jonp365

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So the wife’s 19 Tahoe (5.3l) went over 100k and having had it since under 50k, I knew it was time for spark plugs. I picked up AcDelco spark plugs and wires from Rock Auto. Replaced the plugs and wires Saturday, inspected everything, they came gapped at .040, wires all looked good and came with dielectric grease on the connectors. I just put a bit of antisieze on the plug threads, started by hand, and tightened slightly passed hand tight.

Started fine, but had the check engine light and it had a rough idle. My neighbor used to be a shop mechanic for a local power company, he now runs the shop, but still has all his stuff. We hooked it up to his Apollo-Dg scan tool and it showed a P0300 error, but now specific cylinder. He suggested rechecking the plugs and wires and if all was good there, new coils. Checked the plugs and wires, thought maybe I over tightened, but they were fine. Wires seated firmly.

Picked up new coils Sunday and installed them. Rough idle was immediately gone. No shuttering when accelerating, everything ran smoothly. Wife drove it on a 4 hour round trip Sunday afternoon with no issues other than what she described as a rough shift when slowing from freeway speed.

She came home, we hooked the scan tool up again (it didn’t clear the check engine light the last time) and got that cleared. Still ran fine. Took it for another test drive and the check engine light came back. Today after work, we hooked it up again,cleared the code, and monitored. All cylinders were working fine. No detected misfires. No sensor issues. Air mixture was fine. We checked each coil and they are working fine. I took it around the neighborhood again, no issues. Took it up to the corner to put gas in it and got it up to 70…no issues. Cruise at 55, fine. Accelerated out of cruise to 65…check engine light. Cruise and front collision systems deactivated (everything is connected to the computer so it wasn’t a surprise, they started working soon after). Came back, neighbor suggested disconnecting the battery for 15 minutes and driving it again. Did so, check engine light was gone, driving in the neighborhood was fine, got out on the main road and hit 60+….check engine light back on. Just to reiterate, there’s no actual rough idle or issues with acceleration now. If anything the RPM needle might move the slightest amount at idle, but it’s barely a blip. The only now is the check engine light and this weird hard downshift. Now other engine codes than the P0300.

I’ve read up on the P0300 error and there seem to be quite a few things that can cause it. My assumption (possibly wrong) is that it has to be related to the plugs/wires since that was when the issue first appeared. Neighbor is going to call his friend who is the lead tech at the local Chevy dealer to see what he says. He’s thinking a bad wire, but I just keep going back to it being the generic P0300 error which indicated random misfire. That said, none of the cylinder showed misfire at idle or even under load in the driveway. Anyone have any other ideas?
 

B-train

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Have you tried putting the OE wires back on? Are you confident that you didn't Crack a porcelain on one of the plugs (most likely #8)? Just things I'd think of or try in this situation since it became apparent since this one task was completed
 

OR VietVet

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Welcome to the forum from Oregon, if I did not do it when you joined.

You may not have misfires at all random cylinders but may have something that "effects" all cylinders. To gain access, what did you remove? Check all air snorkel connections. I have no idea if you removed that but leaning on it can unseat it. I own a GMT800. I am not completely familiar with your air routing. Maybe you unplugged the MAF and did not get it plugged in tightly when reattached the connector. Maybe should clean the MAF. Maybe clean the throttle bore. Maybe unhooked a vacuum hose for access and did not get reattached tight. Maybe.....
 

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If I understand what you said, you were able to look at individual cylinder misfire counts with the scanner. If that's not true, it's the first thing I'd do.

I'd also recommend a CASE relearn just to rule it out; random misfires can be caused by the crank position sensor (new or aged).
 
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jonp365

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If I understand what you said, you were able to look at individual cylinder misfire counts with the scanner. If that's not true, it's the first thing I'd do.

I'd also recommend a CASE relearn just to rule it out; random misfires can be caused by the crank position sensor (new or aged).
Yes. We saw the cylinder misfire counts in the history, so it was misfiring prior to the coils being installed. After we cleared the codes, we watched again and had zero current misfires. I will look into that and give it a try.
 
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jonp365

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Have you tried putting the OE wires back on? Are you confident that you didn't Crack a porcelain on one of the plugs (most likely #8)? Just things I'd think of or try in this situation since it became apparent since this one task was completed
I have not. I can dig them out of the trash (they're in the box the new ones came in so should be fine). I'm not confident of anything at this point :banghead:. All I know is I installed them all the same way, tightened the same amount. I took all the driver's side plugs out just to make sure and they were all fine (didn't feel like tearing into the passenger side).
 
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jonp365

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Welcome to the forum from Oregon, if I did not do it when you joined.

You may not have misfires at all random cylinders but may have something that "effects" all cylinders. To gain access, what did you remove? Check all air snorkel connections. I have no idea if you removed that but leaning on it can unseat it. I own a GMT800. I am not completely familiar with your air routing. Maybe you unplugged the MAF and did not get it plugged in tightly when reattached the connector. Maybe should clean the MAF. Maybe clean the throttle bore. Maybe unhooked a vacuum hose for access and did not get reattached tight. Maybe.....
Thanks! So driver's side was easy, just remove the oil cap and PCV tube to remove the plastic cover. All that has been replaced and triple checked. For the passenger side, I removed the wheel and wheel well liner. That gave me access to all the plugs. I didn't remove any of the air components, but I'm sure I leaned on them once or twice. I will double check all the connections there, after work. I didn't intentionally unplug the MAF, I don't see it coming out without intention, but I'll check that as well when I'm checking the piping. I'll add that all the engine stats seemed fine on the scan tool. Fingers crossed, I'd be happy if its just an unseated tube.
 
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jonp365

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Checked the air intake, everything is fine. The clamp on the airbox tube was loose, but the tube was still seated fully. I tightened it up. MAF looks clean, but I also don’t have any MAF cleaner so I’ll either borrow some from my neighbor or pick some up tomorrow. Nothing else is loose or disconnected. The only thing that isn’t on is the plastic cover over the coils, but that shouldn’t cause any issues. Hopefully my neighbor got some ideas from his guy. Otherwise I’m probably just going to take it to a shop and have it looked at. I’m passed the limits of my knowledge and I can’t have this thing sorta running since it’s the wife’s daily and she doesn’t like driving my 2500. Either way, I’ll keep updating.
 
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jonp365

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Neighbor talked to his guy, he said best bet is to bring it in. He seems to think it needs a relearn, even though it was just plugs and wires. Either way they’ll run a complete diag on it. Dropping it off tomorrow at 7:30. Will update with what they find.
 

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