Reason(s) for V8 not being smooth at idle

[yugrus]

My 2014 Tahoe (previous gen 5.3L) is silky smooth at idle. That is, when the car comes to the stop, and this lasts for about 5 seconds. Then the RPM begin to fluctuate quite a bit, between 490 and 530, and it is quite noticeable. It doesn't pass water bottle test at all, and the vibration could be felt through the body.
The MAF sensor and the throttle body are squeaky clean. LT fuel trims are the same and very close to zero under steady driving conditions, so I conclude the injectors are fine too. AFM is tuned out, and two-stage PCV catch can is used, no oil in the intake to report about. MMO in gas on regular basis.

I read the hint that ECU might lean out the idle to the edge of stability to save fuel. Very plausible, and if confirmed, would it help if I adjust injector's slope in the way that the computer would think that injectors are smaller then they are? So it would extend the injection cycle a bit, making the mixture less lean? I'm sure it is easy to do with the tuner, like Intune that I have.
Whold this work? Or the ECU would pull injection timing because of the O2 sensors?
What else could cause the rough(er) idle that you don't expect from a large engine?

Thank you everyone!

 

[KingCake]

A V8 should be very smooth at idle.

Sounds like you have a scan tool.

At idle, while running rough, you need to post the following data:
LT and ST trim B1, B2
O2 waveforms
Idle speed (Desired) or idle air desired.
MAF grams per second
EGR commanded and error
Vehicle speed
TCC lockup status/commanded

And mode $6 if your scanner can do it.


That should give us direction. Anything I could say right now would be a guess. Could be a lot of things. Base idle speed, idle air, maf, vac leak, misfire, etc, etc, etc.

Collect the data and post it before firing the parts cannon at it.

 

[eljefino]

You might have a bum motor mount.

 

[AZjeff]

Originally Posted By: KingCake

............ before firing the parts cannon at it.



Love it!

 

[nwjones18]

2014 not under warranty?

 

[Bgallagher]

Originally Posted By: nwjones18
2014 not under warranty?


+1 Bring it to the dealer. At the very least you can have a paper trail if it turns into a legit issue.

 

[meep]

how many miles? I think GM addressed the problem long before your year, but in the early 2000's they had a series of camshafts that were not hardened as well as they could have been. It didn't get a lot of press because it took tons of miles to cause any changes, and the impacts were minor, so nobody cared-- but the symptom that would make gearheads look at it was uneven idle. As I said it's not well known because by the time a V8 has 130k on it, an imperfect idle isn't really a big deal.

 

[SZR48207]

If everything is physically OK then I'd put my money on the ecu programming.

On my Tacoma, when I idle in park the vibrations from the engine create a really uncomfortable wave that resonates throughout the cab. There have been times in the past when I wasn't feeling well and the vibrations had me near vomiting. However, idling in drive with my foot in the brake puts me about 100rpm higher but no waves.

 

[pcoxe]

My 2009 Colorado was running a bit rough. Dealer claims it's normal and showed me other vehicles running a bit rough at idle. I installed a Superchips tune which raised the idle speed a little and it's much better now. Like several mentioned earlier, they are probably programming it to run too slow and too lean at idle.

 

[yugrus]

Thank you for all your replies.
It has 73K km on odometer, or 45K miles.
It doesn't look like the motor mount, as the engine doesn't rock too much while giving gas and stepping on the brakes. Besides, for about 5 seconds after coming to the stop it is extremely smooth.
The dealer indeed found nothing wrong on two occasions, and yes, I've been in my buddies' older Sierras and new Tahoes, Yukons and Sierras. Same faint shake at idle, even though the new gen 5.3L is quite a different beast, besides DI... Nature of the daemon? I guess, most people just don't pay attention or never notice. It gets me though, particularly after I put more then 100lb of MLV and other stuff under the inner skin, and it is dead silent inside.

I've played with idle speed using Intune, in 50 rpm increments, getting all the way up to 800. Indeed, it naturally feels better there, but that's not where it should be.

I've recorded my LT fuel trims on my way to work so far. At fully- warmed-up idle I have -1 for Bank1 and +2 for bank 2. Cruising at 40mph they are the same at +4. I will get other readings tonight.

 

[Silverado12]

Originally Posted By: yugrus
Thank you for all your replies.
It has 73K km on odometer, or 45K miles.
It doesn't look like the motor mount, as the engine doesn't rock too much while giving gas and stepping on the brakes. Besides, for about 5 seconds after coming to the stop it is extremely smooth.
The dealer indeed found nothing wrong on two occasions, and yes, I've been in my buddies' older Sierras and new Tahoes, Yukons and Sierras. Same faint shake at idle, even though the new gen 5.3L is quite a different beast, besides DI... Nature of the daemon? I guess, most people just don't pay attention or never notice. It gets me though, particularly after I put more then 100lb of MLV and other stuff under the inner skin, and it is dead silent inside.

I've played with idle speed using Intune, in 50 rpm increments, getting all the way up to 800. Indeed, it naturally feels better there, but that's not where it should be.

I've recorded my LT fuel trims on my way to work so far. At fully- warmed-up idle I have -1 for Bank1 and +2 for bank 2. Cruising at 40mph they are the same at +4. I will get other readings tonight.


I would think that 750 is right where the idle should be.

 

[yugrus]

Originally Posted By: Silverado12


I would think that 750 is right where the idle should be.


Yeah, 520 is what ECU commands for idle speed in my case. I just wonder, may be they decided to go lower on idle for fuel economy? Still, at 750-800rpm idle I can feel the irregular shake, muted indeed by the nature of higher rotational speed.
There is no bogging or stumbling when the throttle begins to open. So this has to do with how the idle is handled.

 

[raytseng]

bad tank of gas?
I would try a new tank of gas without the MMO and perhaps redline si-1 at half treatment .
If you can't find or don't want to use si-1; I'd say try a tank of shell nitro; All gas maybe the same, but I think shell adds an UCL in their addpack that somehow seems to smooth idle. The si-1 has the same effect in my tests.

maybe it does nothing, but this is a relatively cheap experiment

 

[yugrus]

I've got the car two years ago, and tried few things in gas. MMO, Lucas UCL, SI-1, 2-stroke oil. It didn't care, and was doing same thing all this time.

 

[DriveHard]

Does it do it if you run E85?

 

[yugrus]

Don't have E85 in my area. But I fuel up at top-tier stations only.

 

[Oli20]

My truck will do the same thing, but only when the AC is running. If I turn off the AC it will immediately stop doing it.

 

[yugrus]

With A/C on I like it even better, that little shake is not as noticeable.
Oh, and some time ago I put the smoke machine on it, but that revealed nothing. No vacuum leaks.

 

[yugrus]

Here's what I managed to get out of my Maxidas scanner. Fully warmed-up engine at idle, in park.
Desired idle: 600 rpm
Engine speed: 593-616, varies fast, and that's the issue.
MAF: 5.27-5.33 g/s
Air flow calculated: 4.99-5.21 g/l
APP indicated angle: 0%
TP indicated angle: 3%
MAP: 32KPa
Intake manifold pressure: 69 KPa

Commanded A/F equivalence ratio: 1. This stumbled me. It was I red, and I don't think that this is right...

HO2S B1S1: between 70 and 800 mv, cycling nicely.
B2S2: 725 MV
B2S1: same as B10
B2S2: 700 mV

Fuel trim cell:6. Not sure what it is, but should be 0...30
Vehicle speed:0
TSC slip speed:between 5 and 15. Could be -200...4000

No EGR valve on that motor, as far as I know.

Will wait on what you guys think...

 

[brave sir robin]

Originally Posted By: yugrus


Will wait on what you guys think...


I think your truck is fine and you are looking for a solution to a non existent problem. I have been there and know these feels VERY well.

Modern engine idle speed is not a static thing. shoooooot.... even on my 68 Buick the idle speed will change if there is a change in electrical load (however small) on the alternator.

Be careful futzing around with things because the more you mess with it (looking at you electrical connectors) the more likely you are to break something and actually need to replace parts.

If there is an issue I doubt its a fuel, air or a spark related and is more likely an issue with the accessory drive or other rotating pieces like the fan or its clutch.