Rough Idle

Paradigm49 · March 5, 2010

Sort of at wits end here.

bought a 2007 gmc sierra this past august, has LMG engine in it. When i originally purchased the vehicle, i couldn't feel the engine run (which i was very impressed with) however a few days after that started to notice a slight twitch in the engine, almost like an occasional miss.

took it back to the dealer, no misfires were detected, but you could physically see the engine sit and twitch. So they ended up changing the Mass Airflow Sensor, MAP sensor and then the ECM.

didn't really do much of anything, but i was getting fairly decent gas milage, didn't have any other issues, so i just ignored it.

A few weeks ago however, this "twitch" is now feeling like a full on misfire. I have replaced plugs and wires (first set of plugs i replaced again because i incorrectly gapped them) but...still to no avail, same problem is there. Noticed that most all of the original plugs were soaked in gas, and the rear most plug on the passenger side (i'm not sure the numbering sequence in the engine) had oil on the threads of the spark plug which i thought was odd. I've ran a can of Seafoam through the oil a few miles before an oil change, ran an entire can of Amsoil PowerFoam through the intake, and still no resolution. On a whim i also went cylinder by cylinder disconnecting the coils to see if unplugging one of them wouldn't make a difference in the way the engine performed, but each time i unplugged one, obviously the cylinder wasn't going go fire. Took it to another dealer the other day, and they scanned it, came up with a P0102 code (mass airflow sensor) so that was replaced...again.

Didn't do anything.

I'm still covered under the 100,000 powertrain warranty, but do these engines typically run not so smooth? I had manager of the service department show me 4 brand new 2010 sierras with less than 20 miles on them and they seemed to idle a bit rough but the oil on the spark plug threads concerned me, and i was under the assumption that the spark plugs shouldn't be drenched in gasoline when you pull them out. I've contacted GM regarding the issue even though i really didn't want to, but its now beginning to show in the gas milage, and the fact that the computer isn't throwing any codes isn't helping matters much either.

Do i really have a leg to stand on, or is this just something i will continue to have to deal with?

Any help would be immensely appreciated!

grose67 · March 5, 2010

I'm not overly familiar with the generation 2 and 3 engines, however what you are explaining does not seem normal at all.

Paradigm49 · March 5, 2010

Actually I believe this would be a gen 4------it's a 2007 sierra with a 5.3 LMG engine.

and your right, it doesn't seem normal at all, and it's annoying the hell out of me. The only performance mods i've done on it are a magnaflow cat back system and an AEM Brute force air intake. Stock and aftermarket intake---same problem, actually it's more pronounced with the aftermarket.

The unfortunate problem is that i have a good friend of mine that has been a mechanice for 30+ years and has a good idea of whats wrong with it, but he won't delve into it because 1-he works for a dodge dealer, and 2-it would void warranties---something of which i would prefer not to do. what pisses me off is that because its NOT throwing a code, the technicans won't investigate it further. My sneaking suspicion is that i have low compression on at least 1 cylinder, and that i may have a intake manifold leak (though i can't pinpoint where it would be coming from) I've noticed that when i go to apply the brakes the RPM's will go down as normal, and then for a split second it will jump up about 150-250rpm, then go back down again...and hold pretty steady. When this orgininally started, i had driven the truck down a decently bumpy road...not offroading or anything of the sort but it was a backroad next to a lake to go fishing. Considering i bought the z71 package with the truck, I never assumed or expected the engine to run like this. The other thing that i have noticed is that there seems to be a bit of play in the MAP sensor---it's all the way in the intake manifold like it should be, but the retainer clip seems to give it way too much slack otherwise.

The only other items i have noticed that caught my attention is that on a few of the wires that go to the injectors, the tape that was around them seems to have been unwound and on the 2nd cylinder on the passenger side, there seems to be discoloration of the wire but i don't see any bare wire itself. The last time i cleaned the intake manifold I did notice that there was still a considerable amount of exhaust that was still visible in the intake manifold, which also brought the question of the valves possibly being bad. But again...no error codes...won't get fixed by a dealer.

Is there a simple way to diagnose a leaking intake manifold without annoying the living hell out of a dealer?

I'm not overly familiar with the generation 2 and 3 engines, however what you are explaining does not seem normal at all.

jro909 · March 5, 2010

my gen 3 5.3 does it also, if the trucks not throwing codes theres nothing wrong...somebody way back when told me the ethanol in the newer fuel triggers that..

Paradigm49 · March 5, 2010

Ok, so basically i'm just paranoid and the fact that they are putting more and more ethanol in the gas now is whats causing it?...thats sort of annoying....well not sort of but really annoying, though lately it's gotten worse, maybe it's time to try a different gas. No, no codes are being thrown, just the engine idling and feeling like one of the coils or plugs went bad because it's shaking that much, but---in reality nothing is unplugged and the plugs that are in there now have maybe 10,000 miles on them...thanks for the heads up though!

my gen 3 5.3 does it also, if the trucks not throwing codes theres nothing wrong...somebody way back when told me the ethanol in the newer fuel triggers that..

Nytemare · March 5, 2010

Give your TB and plenum a cleaning with Seafoam. Could be due to crap in the plenum or you have a small vacuum leak if its getting worse.

Check the torque on the plenum bolts and make sure that its cinched down properly,(WITH a torque wrench, not what you think is tight)...you could be getting a leak there. Also ensure your o-ring where the vacuum line connects to the brake booster is sealing properly and is nice and pliable, not rigid. If it is, get some Dow-corning 55 and apply a light coating to soften it up.

Run some of the Seafoam through the fuel tank also, and maybe a little in the oil about a tank of fuel before the next oil change.

Paradigm49 · March 5, 2010

Shouldn't be an issue on all of those, i'm actually heading out to work on it now...does anyone happen to know of the top of their head what the torque specs are for the bolts on the plenum bolts?

Give your TB and plenum a cleaning with Seafoam. Could be due to crap in the plenum or you have a small vacuum leak if its getting worse.
Check the torque on the plenum bolts and make sure that its cinched down properly,(WITH a torque wrench, not what you think is tight)...you could be getting a leak there. Also ensure your o-ring where the vacuum line connects to the brake booster is sealing properly and is nice and pliable, not rigid. If it is, get some Dow-corning 55 and apply a light coating to soften it up.

Run some of the Seafoam through the fuel tank also, and maybe a little in the oil about a tank of fuel before the next oil change.

jdbankman · March 15, 2010

Had similar rough idle on my old truck and cleaned the TB and idled great after that.

Nytemare · March 15, 2010

Oh yeah...sorry guys, Erin PM'ed me saying it was the Brake Booster grommet that was not seated properly. He re-seated it and the rough idle went away.

03YD163 · March 26, 2010

Oh yeah...sorry guys, Erin PM'ed me saying it was the Brake Booster grommet that was not seated properly. He re-seated it and the rough idle went away.

Could you go into a little bit more detail, e.g. where this is at? I'm having the issue on my 03 Yukon Denali, have had it since I owned it (6 months or so) Went through the same steps you did, plugs, wires, seafoam, thoroughly cleaned the throttle body, even replaced the intake manifold gaskets, still doing the same thing and is seeming to get worst.

I just figured that it was the mileage (168K), but if it's something else i would like to fix it!!!!!

Thanks, Adrian

Nytemare · March 27, 2010

Oh yeah...sorry guys, Erin PM'ed me saying it was the Brake Booster grommet that was not seated properly. He re-seated it and the rough idle went away.

Could you go into a little bit more detail, e.g. where this is at? I'm having the issue on my 03 Yukon Denali, have had it since I owned it (6 months or so) Went through the same steps you did, plugs, wires, seafoam, thoroughly cleaned the throttle body, even replaced the intake manifold gaskets, still doing the same thing and is seeming to get worst.

I just figured that it was the mileage (168K), but if it's something else i would like to fix it!!!!!

Thanks, Adrian

Hey, yeah no prob

Heres a pic showing everything, your checking 4,5,6 and 10 for cracks and fitment.

kmirts · September 12, 2010

i have an 08 sierra with rough idle not sure how the brake booster would affetct this? could u post some pics in detail as to what this is, the pic above doesnt really look like mine.

jro909 · September 13, 2010

i have an 08 sierra with rough idle not sure how the brake booster would affetct this? could u post some pics in detail as to what this is, the pic above doesnt really look like mine.

the booster uses vacuum from the motor, if its not seated right it will cause a vacum leak...what do you guys consider a rough idle? like bouncing rpms or what?