Missfire/Missfire-like symptoms

Jfaulk · August 12, 2022

Good evening Ladies and Gents. I need some help troubleshooting my 2014 Sierra 5.3. 120K miles. Catch Can. Diablo 87 tune, AFM turned off. No other mods. Lately, I am getting what seems like a misfire or maybe what you'd call a "hiccup or jerking" at low RPM when accelerating but have no CEL and no codes. In the morning when its cool, seems to run fine. Later in the day or after driving for awhile, when accelerating from a stop or low speed, I have some vibration, reduced power, slow acceleration, and the occasional "hiccup" that I mentioned earlier. Once it gets up above maybe 15-20 mph, it seems to clear up and drives ok. Light acceleration seems to help reduce this as well. I feel like its important to reiterate that I don't have this when cool in the morning, or its so minimal that I don't notice it. Also, today our temps were about 25-30 degrees cooler than what we've had the last couple months and didn't have the problem. Could this be an AFM-related issue, plug/coil/wire issue, injector issue, fuel system issue, or something completely different? I'm willing and able to put in the work, but not sure where to start trouble shooting and would be prefer not to throw a bunch of unneeded replacement parts at it. While I understand it's not a great diagnostic tool and more for monitoring a few things, is there anything in the Torque app that would help me identify anything specific? Thanks in advance for any input.

Snowcamo · August 12, 2022

Pull the plug wires off the coils and clean the inner connection of the coils.

Free if you have sand paper.

Jfaulk · August 12, 2022

Thanks, Snowcamo. I'll try that tomorrow. Anyone else have input?

BlancoSilverado · August 12, 2022

I would think the intake manifold seals to the head but try the easier way first.

When it get hot and the issue comes, spray a little brake cleaner at the intake to head transition to see if the RPMs change.

Jfaulk · August 13, 2022

Good thought. Hadn't thought about a vacuum leak. Appreciate it.

300 Blackout · August 16, 2022

One of my aftermarket plug wires was getting cooked by my headers. As the wire heated up, electrical resistance increased and I’d get a similar symptom of a misfire with no codes as long as I was driving normal. It finally threw a code when I opened it up. Under normal driving, it conducted enough energy for ignition. Once it was under load, it couldn’t produce enough spark to ignite that quantity of fuel & pressure, so it missed repeatedly and threw a code. I replaced the wire and problem was solved.

Also, if you have ever brought it into the dealer for warranty work, recall, service, etc… there’s always the possibility they re-flashed your ECM to update it. If so, your AFM might be operating again and you’re perceiving that as the hiccup.

Jfaulk · August 27, 2022

Just wanted to report back with my findings...I checked for a vacuum leak around the intake manifold with carb cleaner and didn't find any leaks. Today, I pulled the coils, wiped them down, inspected, cleaned the electrode with steel wool, sprayed with electronic connector cleaner, blasted out with air and replaced. Pulled, inspected, and replaced all wires and plugs. I believe the #8 wire may have been the culprit. The plug boot was very dark, the ceramic was almost completely black near the the steel hex, and the plug electrode was pretty dark. The other 7 plugs and wires looked good. Runs better overall, smoother idle, and I have not experienced any of my previous symptoms. I'll continue to monitor, the more I drive this week. Appreciate all of the input.