Think I might have a misfire
 

I've got a 99 Sierra with a 4.8L. I've noticed for the past few weeks that when accelerating from a stop I'll get a light vibration. It does it in all gears, including on the highway at 70mph with the tcc locked.

It only seems to do it if I'm taking off fairly easy. If I give it a little more gas I don't notice it. It feels kind of like a tire that's out of balance but I feels like it loses a little power when it's happening.

The transmission shifts good and I don't believe it's related to that. It's just weird it's kind of a random thing. A little history, I just bought the truck and it has 49k miles. It spent a good bit of time sitting especially the past few years. The only thing I have done is replace the fuel pressure regulator.

I have not done plugs or anything else yet. There isn't a CEL but I am going to hook a code reader to it tomorrow to make sure there aren't any codes.

Any ideas?

I'd take a wild guess and say the plugs are original then, would never hurt to start with new plugs and wires anyway as they should be done.

I'm pretty sure they are. It's just weird to me that it runs fine 95% of the time. It's mostly just while accelerating up through the gears but once I'm at a steady speed it's usually fine.

I'm still going to do the plugs hopefully this week if the rain lets up.

Okay so I'm still not sure what I have going on. I don't know if it's a miss or what. The exhaust note doesn't seem to change any when this happens. One thing I forgot to mention is that sometimes if I am holding the throttle steady it feels like I let off a little and then pressed it again, but I am holding it still.

What does knock retard feel like? I noticed this morning on my way in that while on the interstate if I give it some gas I hear a faint ticking sound when its under a load in lockup. I'm wondering if I have a leak at the manifold that is triggering knock retard.

The rain let up and I just got home from work. Popped the hood and looked around and I do have a broken manifold bolt. It's the very back one on the drivers side, #8 I guess (my first GM).

I also crawled under the truck to check my u-joints. Everything seemed good except when I wiggle the carrier bearing it moves. I'll post a video shortly. I've never had a 2 piece drive shaft so I don't really know what normal is.

As far as the manifold, I planned on doing long tubes and an ORY at some point. I think if I'm gonna go through the trouble of removing old exhaust I'd rather put something better in its place. I'll do a search but if anybody feels like answering who has a good deal on then these days?

Play seems normal, when it goes really bad all the rubber with be almost gone.

They kinda suck to remove anyway.


But as for the "misfire", I'd still start with the small service. Clean the TB and MAF, do the plugs and maybe run some injector cleaner. Take her for a spin and see if anything is different.

It may seem like shot gunning parts but those things should be done regardless in my opinion.

I would definitely run a quality fuel injection cleaner through it. The injectors will gum up when the fuel sits for long periods of time. Like a month or more of sitting and not being run.

The play in the carrier is normal. The rubber shrinks. I replaced mine thinking it was bad and it ended up doing the same thing a few weeks later.