Isles rough but otherwise runs fine

[Richard Blakely]

My 2012 Tahoe, when stopped at a traffic light, varies in rpm from 500 to 600 rpm. Once I accelerate, all is well. I have replaced spark plugs and wires, mass airflow sensor. Still with same problem. I have 151,000 miles on 5.3l v8 using regular gas. On earlier models, there is an idle control valve, does this model have one? Other suggestions?

 

[swathdiver]

My 2012 Tahoe, when stopped at a traffic light, varies in rpm from 500 to 600 rpm. Once I accelerate, all is well. I have replaced spark plugs and wires, mass airflow sensor. Still with same problem. I have 151,000 miles on 5.3l v8 using regular gas. On earlier models, there is an idle control valve, does this model have one? Other suggestions?

Pick up that Throttle Body and Mass Airflow Sensor cleaner kit, your throttle plate opening may be a little wide because it's dirty. Tuners routinely raise our idle about 35-50 rpms, they're a little low to maximize emissions performance. Run a few tanks of ethanol or high test gas through it too.

https://www.walmart.com/ip/CRC-MAF-Throttle-Body-Single-Use-Twin-Pack-1-Kit/36755294

 

[Richard Blakely]

My battery died this morning. Been threatening to for past 2-3 days. I replaced it with new battery, and found no more idle roughness and idle rpm was about 100rpm higher. I can’t see the connection between rough idle and dying battery I know the alternator gauge has been reading low since I bought it. With new battery appears to be normal at 14v read from gauge. Am I crazy??!!

 

[Doubeleive]

My battery died this morning. Been threatening to for past 2-3 days. I replaced it with new battery, and found no more idle roughness and idle rpm was about 100rpm higher. I can’t see the connection between rough idle and dying battery I know the alternator gauge has been reading low since I bought it. With new battery appears to be normal at 14v read from gauge. Am I crazy??!!

no not crazy these trucks are very touchy about voltage makes them do all kinds of weird stuff, but I would as James suggested anyway as a preventative measure and toss a couple bottles of injector cleaner in the gas tank for the next 3-4 fill ups that should help

 

[OR VietVet]

Also, for future reference, you kept saying a rough idle but described it as varying between 500 to 600 rpm. There is a big difference. Rough idle is a type of idle that can shake the engine to varying degrees like you have a fouled cylinder of some sort or an arcing plug wire or.....etc. The vary in rpm from 500 to 600 rpm is just a difference in idle that goes up and down like a roller coaster, if you can picture that. The battery problem can cause that type of symptom but is rarely the cause of a rough idle. This would involve two totally different diagnostic paths. Clean the fuel system up and the throttle body, make sure to follow directions completely, and run some mid grade or even premium fuel for a few tank fulls because they are not only higher octane but they have more cleansing additives in them. I recommend that after that you run at least mid grade all the time of a major supplier like Chevron or Texaco or whatever is in your area.

 

[Richard Blakely]

Great suggestions, thanks. I may have over-dramatized the rough idling. I’m just comparing our ‘07 tahoe in the same light, which is very smooth and no change in rpm. Both are at 150k miles. The tach jumps up and down on my ‘12 by 100 rpm which causes a ‘jumping’ action. Cruising is fine, no problems.

 

[OR VietVet]

Ok, can you see the engine "shake" at idle or is the rpm change like a surge effect?

 

[91RS]

Does it go away if you shift into neutral?

 

[Richard Blakely]

I don’t believe it goes away when shifted to neutral. I’ll double check. I just went on a 1200 mile journey to Colorado. I used mid grade the whole way. Ran great and actually 19-21 mpg! But I still noticed the idle rpms jumping. Throttle body cleaning will be next

 

[shegarty]

Also check for a small vacuum leak. You can spray some of that cleaner you'll use on the throttle body in and around your intake manifold and listen for a slight increase in revs. A leak can cause a somewhat variable or lumpy idle and then disappear on acceleration as the volume of air leaking becomes much less than that being seen by the meter.