Erratic RPM at Idle

[Gabriel Iacoviello]

Hey all, so I have a 2006 GMC Sierra 1500. I bought it used with 135,000 miles on it and I have put maybe 4000 miles on it since. Ever since I bought it sometimes at idle the RPMs will slowly bounce between something like 600 and 900 RPMs, also while this happening the battery gauge also fluctuates up and down and the lights (headlights, radio, odometer etc.) flicker. Furthermore I have a random misfire code that comes and goes every couple of weeks. Ive been having trouble figuring out whats wrong with it, or if all these problems are connected or separate issue I have to deal with. I would appreciate it if anyone has any good ideas of where I could start looking for solutions? 

 

Thanks,

Gabe

[Jsdirt]

Usually this will happen with a throttle body that's never been cleaned. Be advised that after cleaning, you'll need a bi-directional scan tool to relearn the idle, otherwise it'll idle at 2 grand for what seems like forever.

 

Another possibility is a severe air leak through intake gaskets, or a broken hose. Live data on a scan tool should tell you what you're dealing with. High positive fuel trims at idle that get better with RPM are a dead ringer for a vacuum / air leak. High load percentage is usually an indication of a dirty throttle body.

 

Yet another problem could be dirty or clogged injectors, or a weak fuel pump. Fuel trims will go positive in this case as well, but get worse with more load.

[garagerog]

OP, since you mentioned the flickering lights you may want to start with your electrical system, i.e. battery condition, negative battery cable, grounds and even alternator before you go down the rabbit hole of chasing what ifs.

[Jsdirt]

Lights are flickering because it's idling too low.

[Gabriel Iacoviello]

Thanks for all the input, I’ll see if I can get my hands on a good scan tool.

 

thanks,

Gabe

[garagerog]

2 hours ago, Jsdirt said:

Lights are flickering because it's idling too low.

Not sure that a 600-900 rpm idle is to low.

[Jsdirt]

That's what the gauge is saying - what it's actually doing is probably right at the charge threshold from the OP's description. He said, "something like ..." which says to me it's probably lower intermittently, which accounts for a flicker.  Of course it never hurts to check the electrical system, especially on modern vehicles that have some years on them, but from his description, it's more than likely idling too low.

Edited April 10, 2021 by Jsdirt

[Gabriel Iacoviello]

So JSdirt, it actually doesn’t ever drop below 500 RPM’s

[Jsdirt]

500 is pretty low. On my older cars that would definitely cause a flicker with any lighting. I haven't experienced RPMs that low on the newer stuff, but I'd imagine the same would happen on these. Electronically controlled, but the same physics always apply.

[Gabriel Iacoviello]

So quick update/question on the truck. I have been cleaning the ground wires off and checking the electrical system to the best of my ability and still no results. However I did notice that the symptoms get worse ie. more noticeable flashing and more drastic jumps/drops in idle when I put the a/c on and even more so the higher setting I have the a/c on furthermore if the a/c is not engaged sometimes the idle stays steady and the lights are normal. This makes me think its defiantly an electrical problem but what do y'all think?

 

Thanks again

[Jsdirt]

The A/C compressor puts a decent load on the engine - if you're already idling at 500 RPM, compressor engagement will undoubtedly drag that down lower. You need to find the source of your low idle.  Pull the intake tube off and check around the throttle body butterfly for excess carbon. If in doubt, clean it anyway. I use a toothbrush and brake cleaner.

 

If you still have the issue after a cleaning, is the check engine light on? If so, what are the codes? Is your air filter clean, and the intake tube unobstructed? MAF clean? Is there a K&N filter on there, and if so, is it over oiled with residue in the intake tube and MAF?

[CamGTP]

For what it's worth the factory idle settings are 525 in park and 550 in gear when warm.

Edited April 15, 2021 by CamGTP

[Jsdirt]

Never saw mine that low. Interesting. Can't say I'm shocked GM gauges aren't accurate ...

 

The joys of internet diagnostics! Are we chasing a TB issue or electrical issue? That's the million dollar question ...

Edited April 15, 2021 by Jsdirt

[Gabriel Iacoviello]

So I cleaned the throttle body (very dirty) and checked out the MAF and air filter yesterday, everything checked out. Didn't fix the problem unfortunately though still happens, mostly at night and mostly when the headlights and blower motor are running. So I'm thinking alternator?

[Jsdirt]

Can you get ahold of a multimeter? Set it on AC volts and put the positive lead right on the B+  alternator output post, and ground it to the engine or battery. You shouldn't have more than 100mv AC. 50MV or lower is better. You need a QUALITY multimeter for this test.

 

Better yet, an oscilloscope. You might be better served to pay a tech to perform this test with a scope, just to rule this out. Some high end scan tools like Snap On brand have one built in. In my old cars I'd say this is dead ringer symptoms for it, but on modern vehicles it usually causes more problems than just a low idle, so I'm not 100% sold yet ...

 

You really need to get a scan tool on this thing and let us know of any pending, history, or hard codes, plus a few other parameters. Don't wing a $150 parts store alternator at it - you could be wasting your money. TEST FIRST.

Edited April 18, 2021 by Jsdirt