Rough Idle

bam2880 · July 11, 2011

i have a 2008 gmc sierra 5.3 vortec with AFM and it idles kind of rough and when i put the hood up it kind of shakes a little bit but nothing major what could it be

JustinB · March 15, 2012

My '08 does the exact same thing as well. I started noticing it shortly after I purchased my truck. I noticed it because it shakes the truck. I can even feel it in the door panels.

I did a lot of searching and reading on the subject, and it seems like a fairly common issue with these trucks. I actually started getting a CEL (P0121). I cleaned the throttle body and while it made a difference in the rpm fluctuation, it didn't help the stumbling/stuttering/shaking issue. Before the TB cleaning, the rpms would fluctuate like the engine had a slight misfire or hesitation. It appears to idle smoothly now by looking at the gauge, but sometimes the stumbling/shaking issue can feel pretty bad.

I swear it almost feels like bad motor mounts. However, I haven't gotten a chance to take a look at them yet. I previously owned an Audi A4, and around 86k miles, my front motor mount was worn completely out which caused almost the same exact sensation in the vehicle.

I've removed and cleaned my throttle body, removed and cleaned my MAF, checked all my vacuum hoses for leaks, checked around my intake tubing, box, etc., for leaks, and have came up with nothing to cure the problem.

I did read that some people had success with replacing the grommet on the brake booster. One guy even said that just removing and re-seating the brake booster vacuum line cured his problem. Maybe that could help you?

I'll eventually end up taking mine to the dealership and getting them to inspect it. My parents have always owned GM vehicles and I've owned a few myself, and never had a problem like this. You think when you pay that much for a truck that it would at least idle somewhat stable. The truck runs great when accelerating and such, but when I stop at a red light, it feels like the engine is stuttering almost to the point that I subconsciously want to give it gas to smooth it out. Maybe we should all start writing letters to GM about it, or calling and complaining?

Dozbubba · March 15, 2012

I've got a 5.3 2011 and haven't had this problem.

tgprince · December 11, 2011

i have a similar problem with my 2006 sierra. its not throwing any codes but at idle it fluctuates 500-700 rpms. also when i let off the break and let it coast forward it it jerks the truck, not terrible but enough to be annoying. i tried new plugs and wires but no luck. once i get on the gas the truck runs smooth and strong. it only does it when the truck is warmed up and the rpms drop. any ideas?

lonestar angler · March 15, 2012

Same here with the rough idle. Worried I will be wasting my time at the dealership without an engine light on.

spock · July 15, 2011

Join the club. My 5.3 runs great but the idle in park has some flutter to it that shakes the antenna. RPMs are steady. My 93 C1500 w/305 idled so smoothly you might not think it was running.

semper3318 · July 14, 2011

I have the same problem. I recently bought a 2010 Silverado with the 5.3 and it idles rough sporadically. It's very annoying. I got rid of a 1999 S10 with 200,000 miles that puured like a kitten. Now I have to deal with a $30k truck I've wanted for years unable to idle constant. Going to dealer this weekend for warranty request, from what I've seen online GM is dodging addressing any problem with the rough idle.

i have a 2008 gmc sierra 5.3 vortec with AFM and it idles kind of rough and when i put the hood up it kind of shakes a little bit but nothing major what could it be

jro909 · July 19, 2011

you got a cold air intake or any after market mods?

strickster1 · March 26, 2017

Hey everyone. Please bare with me, in new at navigating around this site and I hope I got this in the right place. I have a 2015 silverado with 32500 miles. I took it to the dealer a couple of weeks ago because I heard what I thought to be very slight spark knock while accelerating...very faint and it might not of even been spark knock and also one evening it was very slow to turn over and crank up.. Only one time. Other than this , it is fine and has ran and idled as smooth as glass , regardless of whether its in park or drive and regardless of whether the ac is on or off. Now, I got it back and the next day coming home from work, I came to a stop and the rpm dropped to around 400 and it made a slight shake and like it was about to get to low to run and it jumped back to around 500 rpm. Now it will sometimes drop to about 400 when I come to a stop and sometimes it won't and sometimes when it dips to 400 it will be slightly rough and go back to about 500. I also know that when I got it back from dealership I asked what they did and they said they couldn't find anything wrong and that they did all tests on battery and all readings were good but they also said they RESET THE PCM OR BCM OR BOTH. Please help me.

semper3318 · July 19, 2011

Nope, stock as can be. It does it pretty regularly, it's not horrible but enough you can feel it in the cab. I can sit and watch the tach fluctuate. It will sit at about 500 RPM then bounce up to 600-700 RPM then back down. It's very random. But does it with or without AC on and more in gear than idleing in park. I'm not getting and SES thrown so I have a feeling the dealer won't know what to do. It runs great besides that, just aggravating when I got rid of a 1999 S10 that purred like a kitten at idle. I would figure this has to be somewhat common with other trucks?

you got a cold air intake or any after market mods?

jro909 · July 16, 2011

vortec v8s idle rough....its from the computer leaning out the mixture...mine does it and probably 90% of other peoples do, its nothing to worry about unless your rpms are bouncing

jdubub · July 19, 2011

my truck runs good and strong, but in park it does idle rough. The truck will move, but the RPM's will not. It's just another "characteristic" of these 5.3 Vortecs

mkelly · July 19, 2011

Add mine to the list, it idles rough once its warmed up.

I have noticed that If I get on it after a stop light it will idle smooth at the next stop light..... It will also idle smooth after I get off the interstate running at 75 or so.

I've seen this out of all of the newer 5.3s, but did not see the problem in the 5.3s from 99-05 range. I read somewhere that the newer programming dropped the idle very low to save fuel and thats what causes the problem.

semper3318 · July 18, 2011

Yeah my RPM's fluctuate also, you can watch the tach dance from 500 RPM then shoot up to 700 RPM and back and forth. Bad thing is my SES light isn't coming on so to the dealer nothing is wrong. It seems this has to be an issue with others. Besides that the truck runs flawlessly.

vortec v8s idle rough....its from the computer leaning out the mixture...mine does it and probably 90% of other peoples do, its nothing to worry about unless your rpms are bouncing