Rough idle/ shuddering in idle.

[iboughtatahoe23]

I think it's possible ONE of them could be bad. So in the ECM programming, there's a threshold that has to be met for misfire counts on a given cylinder/coil pack before it will set a check engine light. I don't know what that threshold is, but I can tell you from recent personal experience you can have a misfire without it turning the light on.

The symptoms you describe in your first post are almost IDENTICAL to what I experienced after I dumped that motor in the suburban, save that my truck would also misfire while sitting at a red light. It was definitely more noticeable when in park, it would be easily missed in drive if I hadn't worked on cars for 20 years. But look, I thought I was trippin at first, because even when sitting in park it was barely there. I'm used to cars either running so terrible you KNOW something's up, or the engine controls are so sensitive that when a gnat farts near your mass airflow sensor it turns your dash into a christmas tree. I'm like, there's no way it's misfiring. It would set the light if it was. Maybe I didn't tighten the manifolds up enough, maybe this junkyard motor has a bad valve seat, maybe it's a bad mount, maybe I'm just getting old. So, I bought that vxdiag tool off Amazon because I didn't want to wait for a Tech2 clone to get here from China.

So what it showed was that on cylinder 4, I had low double digit misfire counts. I forget exactly how it shows it in the Tech2 emulator but it's a real time data output from each coil pack. I had 7 coil packs sitting at 0 misfires, and cyl 4 would get to like, 15 misfires, and drop back down to 0, go to 12, drop down etc. I could watch the tach just BARELY seesaw in time with what I was seeing on the tool. So I put a new coil pack on that cylinder and since then this has been the smoothest running motor I've ever had.

I do wanna be clear, this may not necessarily be your problem. You could have something wonky going on with the cylinder deactivation. Worn cam lobe. Bad gas. Previous posters have talked about running premium regardless of if you're tuned or not, and I agree with them. The 5.3L LC9 in my burb has a 9.5:1 compression ratio, which should see at least 90 octane. I use 93. You CAN run 87 in them, the ECM will just pull timing. Keep in mind it's not about "will it run better" but more "will it run optimally". Higher octane=resistance to detonation. As long as you're not filling up at the country gas station out in the middle of nowhere, odds are you're getting good quality gas. Running E85 vs premium is moot around my parts- taking into consideration the higher consumption with E85 I spend the same money. BUT E85 also has a higher octane rating than premium, something like 100 if I remember right.

Put your plugs and wires in and see how it runs then, and report back.

I should have mentioned I have an exhaut leak too

 

[iboughtatahoe23]

Ill
Keep running 93 and get them wires and plugs swapped out.

 

[iboughtatahoe23]

This is what I will be running from now on. If you say this compression rate calls for 90 and up

 

[iboughtatahoe23]

I will add some times it feels like when I need accerlation, at certain speeds it feels like acceleration is just not there I feel so slow. Like pedal almost Halfway and then it will start to get up. I would say I feel the fastest acceleration is in 2nd gear.
Idk it might just be a slow car and im
Not used to it.
If it was a misfire I feel I would have an engine light.

 

[iboughtatahoe23]

I will add some times it feels like when I need accerlation, at certain speeds it feels like acceleration is just not there I feel so slow. Like pedal almost Halfway and then it will start to get up. I would say I feel the fastest acceleration is in 2nd gear.
Idk it might just be a slow car and im
Not used to it.
If it was a misfire I feel I would have an engine light.

I feel like it might be so intermittent, if I do have a misfire that the ECU just doesn’t recognize it

 

[iboughtatahoe23]

So after driving a bit I stopped in park and the Rpm doesn’t seem to like drop. It stays at 600. There’s like hairline movements sometimes. But not enough to really see unless you really look at the tach

In reference to a possible misfire

 

[SNES Avalanche]

I will add some times it feels like when I need accerlation, at certain speeds it feels like acceleration is just not there I feel so slow. Like pedal almost Halfway and then it will start to get up. I would say I feel the fastest acceleration is in 2nd gear.
Idk it might just be a slow car and im
Not used to it.
If it was a misfire I feel I would have an engine light.

If the misfire is on the exhaust side lifters I don't believe it throws a CEL.

I've tuned my truck to a higher idle to reduce the rough idle but I'm fairly sure there's an issue with a lifter / the cam on my exhaust side which is why I never get a CEL

 

[wjburken]

Yes. There’s a coin type that looks like it has a keychain hole but it’s actually where you can bend the tip without touching the delicate parts. Don’t use the Slide part to gap, only to check the actual gap. Use the little hole to widen the gap.
View attachment 403334

For platinum plugs, I like to use a wire style gap indicator like shown below. I’ve read where the wire style is less likely to damage the electrode, but that may just be me.

 

[iboughtatahoe23]

Just got my plugs in, going to set them aside for my day off on Monday. As I don’t see myself doing this in 3 hours. I have a needle nose plier and basic tool set, but don’t have that “gapper” for the plugs.

In the meantime I’m watching videos.

 

[iboughtatahoe23]

So all I do, is go through each spark plug and wire, one at a time? Seems pretty simple, Besides slicing my hand up over on the 7 and 8 cylinder by the ac compressor.

Is there any way I can tell how new the wires are without pulling them? And the plugs?