L9h 6.2 miss at idle no check engine light

[Silver62]

I have a 2012 slt Sierra with the 6.2 i just got it about 2 months ago with only 70,000 miles. I didn’t notice it went i first got it, but now when idling it will be smooth for a few seconds then feel like a little stumble or slight miss for a second and it will do this every few seconds the whole Time it idles, it seems to get worse when the Rpm are going back down to around 700-800. And it also has a rough start after sitting over night and after that It will be fine for the rest of the day. When first starting it will high idle for about a second then kinda die and stumble then resume high idle and gradually go back down to an idle. The previous owner had the fuel system cleaned not long ago with the bg system and I’ve tried different gas stations using 93 octane to see if it got any better and also changed plugs and wires. I’ve also brought it to the dealer to see if they can find anything wrong and they hooked up theirs Tec 2 and said everything was normal and the 6.2 motors just have a rougher idle which i find hard to believe. I also have a 2003 Tahoe with a 5.3 with 270k that idles smooth which is why i find it hard to believe that the 6.2 just has a rougher idle 

[Bad Bowtie2]

Pull some spark plugs & look at them. 

 

Replace with A/C Delco iridium plus if they look like the originals. 

 

Inspect the wire boots while you are there too. If they appear cracked, worn, or too soft, buy some MSD, Summit Racing, etc 8mm coil wires. 

 

Clean the mass airflow sensor with CRC brand mass airflow sensor spray cleaner. Do the same to the throttle body & "butterfly". Same thing CRC brand spray cleaner- Just use a clean shop rag to wipe the carbon out. Youtube has some great videos!!

 

Drive & report back to us...

 

 

[Silver62]

On 9/23/2019 at 3:02 PM, Bad Bowtie2 said:

Pull some spark plugs & look at them. 

 

Replace with A/C Delco iridium plus if they look like the originals. 

 

Inspect the wire boots while you are there too. If they appear cracked, worn, or too soft, buy some MSD, Summit Racing, etc 8mm coil wires. 

 

Clean the mass airflow sensor with CRC brand mass airflow sensor spray cleaner. Do the same to the throttle body & "butterfly". Same thing CRC brand spray cleaner- Just use a clean shop rag to wipe the carbon out. Youtube has some great videos!!

 

Drive & report back to us...

 

 

Well i got new plugs and wires and the old plugs didn’t look abnormal when i took them out, also cleaned the Mass air flow and throttle body, problem is still there. I think their may have been a tiny bit of improvement on the idle but still nowhere near smooth. Still comparing the idle to my 270k mile 5.3 and it’s nowhere near as smooth 

[Gagliano7]

Could also be one of your coils. I had a coil that would work great at part throttle and wot but not at idle. The way i found it was using a temp gun i checked every exhaust primary and found one that was a lot cooler than the others and that was the bad coil. 

[Silver62]

15 hours ago, Gagliano7 said:

Could also be one of your coils. I had a coil that would work great at part throttle and wot but not at idle. The way i found it was using a temp gun i checked every exhaust primary and found one that was a lot cooler than the others and that was the bad coil. 

Thanks for the suggestion, do you remember how much of a difference in temp there was when checking the primary’s?

[Bad Bowtie2]

What brand of spark plugs & wires did you install??

[Silver62]

11 hours ago, Bad Bowtie2 said:

What brand of spark plugs & wires did you install??

Ac Delco I’m pretty sure they were the iridium I’ll double check when i get home and the wires were ultima select they didn’t have delco in stock 

[Gagliano7]

15 hours ago, Silver62 said:

Thanks for the suggestion, do you remember how much of a difference in temp there was when checking the primary’s?

They all vary but the bad one will be quite a bit lower around 100 degrees or more.

[Silver62]

Well update on the problem the truck it sat for about a week without being started and i got into and fired it up and no stumble or hesitation on startup now I’m not sure if it’s the colder weather sending more fuel to it or cleaning the mass air and throttle body worked. but the day after cleaning them it started and it had the hesitation. But for whatever reason waiting a week to start it has done the trick and the last 3 mornings it hasn’t done it. But the idle is still funky will be smooth for a few seconds then have a stumble or two and will go back to being smooth then stumble again the whole time it idles checked primary’s and coils with temp gun and replaced the only coil that had a temp difference from the others which was about 50-60 degrees colder than the others and the coil was about 25 degrees colder than the others

[Bad Bowtie2]

I would call around to some GOOD shops in your area- Even a GM dealer or two.

 

Ask them if they sale BG products. They make a good top end fuel induction cleaner. 

 

It definitely can't hurt & may solve your problem as well. If nothing else, it is needed maintenance with these fuels around now.