2018 Silverado 1500 - Cylinder 3 Misfire, Rough Idle less than 1000rpm - 188,000

Scotth82 · November 2

About a month ago, on a cool morning I noticed a rough idle develop. I didn't pay it much attention, thought it would clear up, but after a couple days it did not. No CEL at all. I run a scanner on it after letting it idle a while and it had a P303 Code for a Cylinder 3 Misfire Pending. I read it again a day later, no codes. I have read it multiple times and sometimes it shows a P303 Pending, but no CEL. Usually it goes away, but the rough idle remains. A little more diagnostic after seeing the P303, I Pulled the Wiring that feeds each coil pack off and noticed no change in the engine when removing cylinder 3 wiring harness. All other cylinder firing checks caused the engine to change, but not on 3. I checked for spark by removing the plug wire from the plug and the coil pack was providing spark because it was arching without the plug connected. I replaced the spark plug.(They had been changed at about 110,000) But I replaced that one plug, no change. I moved the plug wire, no change. I thought I would just change the coil pack, no change. So by now Im thinking injector. I remove the intake and decide to replace all 8 Injectors. They were filthy. Replaced the injectors to spec and put everything back together and still same issue no change. I did all I knew to do, so I took it to a reputable shop. They put a nice scanner that they can see live data and on idle only cylinder 3 misfiring, when accelerating the misfire counts go down, seem to go away. They did a compression test, and is has 210PSI. And it passes a Leak Down test. Value Cover was removed and all lifter/rocker arms moving. They inspect the cylinder with a scope/camera and didn't find anything odd. Their diagnosis is that the Cam may have a place worn on that cylinder and they recommended a cam and lifter replacement or they leaned heavily to replacing the engine. The rough idle is not a steady rough idle, it shakes the inside cab of the truck, kind of levels out and shakes again, and repeats. I am just wanting to see what some of you guys that are smarter than me have to say. I have researched this, know about the AFM issues, I lean toward that myself. I used a OBD AFM Delete Tool about 60K ago, so I have been running on all 8 for a while. And yes it rough idles, with or without the AFM Delete Tool. But as I lean toward engine repair or replacement, something tells me this may be more electrical or sensor related somewhere. I do understand that most sensors would affect all cylinders and not just the one. Please help!!! And thanks in advance!!!!

Scott

CamGTP · November 2

I'm kinda leaning towards the lobe on the camshaft is worn too.

I think you could check this by using a dial indicator on the top of the rocker arm. You could do it on cylinder 3 and then pick cylinder 5. If cylinder 3 doesn't has as much lift anymore, then that lobe has been damaged.

newdude · November 3

You've replaced the fuel and the spark, you've got air (good compression, etc.).

Assuming you don't have a lot of carbon build up on the intake ports, and you have good power and signal to the coil and injector, its camshaft time. The lifter(s) on that cylinder probably have the roller sticking or locking up on it and its slowly consuming the cam lobe.

We had a NY trooper 2018 Tahoe that had this happen. Non AFM cylinder, would misfire at idle, rev it up 2000-3000rpm and it was smooth. Ate enough of the lobe to idle bad but ran fine once you got it moving good.

Edited November 3 by newdude