Rough Idle

[retiredsparky]

Have you checked fuel pressure? Fuel pump in the tank, fuel pressure regulator on top of engine can be culprits. There is an idle air controller (IAC) that can fail. The PCM reads the crank sensor to determine engine rpm, then adjusts the idle rpm to match the programmed idle number. If I was guessing, I would say the IAC is the culprit. Or take it to an independent shop and maybe spend less than throwing parts at it.

 

[Bigbertha78]

From everything I've read if the crank sensor was bad it would not crank at all. Fuel pressure regulator is good, but as far as fuel pressure, something I need to check. I'll also see if there is a way to test the IAC, sucks on these SUV's because there is just a plug going into the throttle body so if it's bad the whole throttle body will have to be replaced.

 

[Mechanic421]

Need to read scanner data when it is running poorly. Almost sounds like mass air.

 

[Dneel81]

when you decide to fix the header bolt, pull the whole thing off the engine and weld a nut onto the broken part of the stud. quickly hit the nut with impact wrench (reverse) and it'll come out. Much easier than the easy out crap, angled bits, extractors, etc.

 

[Bigbertha78]

So we moved from AZ to Florida, and just today I notified the truck had been doing fine since last post. It's been raining a lot and very humid and today I pull into garage truck is fine, come inside for an hour or so then go to leave again and as soon as I crank Tahoe, RPMs are fluctuating from 800 down to 400 back in forth in Park! Tap the gas and it responds fine then back to the fluctuating RPMs put truck into reverse and the issues goes away and never had another problem on the ride. Any takers?

 

[Dneel81]

vacuum leak.
fuses all intact?

Also if you're attacking a problem, be sure to diagnose the problem correctly and not just throw parts at a problem. Very rarely is the MAF to blame. Usually it's dirty. Usually the power supply to it is damaged somehow. Usually the MaF is fine itself...sure sell alot at the local parts store though.

 

[chevyv8]

check 02 readings while running rough. rich/lean condition can cause this. also try to get a scanner than can read live data (specifically misfire and fuel trims)

 

[Bigbertha78]

I do have a code reader that reads live data but while looking at the fuel trims, 02 readings, etc.. what should I be looking for in particular? Specific Limits? What's good and what's bad? I'm new to the scanner and learning as I go.

 

[Larryjb]

If you can get the misfire history, that could be easy to tell. On mine, most of my cylinders were in the 10-20 misfires, but cylinders 3 and 4 were into the hundreds. However, my misfire is in park AND drive in idle, and I have old plugs and wires. For me it makes sense to do plugs and wire first. If you can find which cylinders are misfiring then move a coil to a different cylinder, if the misfire moves with the coil you've got the culprit.

As for fuel trims, I can't remember the numbers so I can't help you there. Upstream O2 numbers should swing something like once every couple of seconds as the computer adjusts the air fuel mixture. Downstream should be fairly constant, as long as the cats are using the remaining O2 to clean up hydrocarbons.

Larry

 

[jbyler]

Haven't found the issue yet, currently about a thousand miles away from home with my truck. Went into the dealer last weekend and talked with one of the service department mechanics, his thinking was to try changing the spark plugs and wires, if that didn't help then come back and he could hook up the fuel system cleaner for around $160!! Don't have any tools with me but when I get back home it will be the first thing I change. Let me know jbyler if you get a good fix and like wise if I get one. I still have not gotten any codes yet.

So, the issue continued on until one day, the engine basically went into limp mode. My friend was close by and we limped to his house, hooked up the reader and it said ignition failure code. We took the coil off and it had shorted and melted... We ran down to the auto parts place, got a new coil, replaced it but the engine still misfired. Now we are getting the injector circuit code, so I'm ordering all 8 from Amazon (unbelievable (wonderful) price for OEM parts) and also changing the fuel filter at the same time. Crossing my fingers that this works... Stay tuned... Meanwhile, I'm going to replace the rear HVAC module (great youtubes on how to do everything!) because that went out awhile ago (no heat)...super simple fix...ordering the part on Amazon/same day delivery! I still have to figure out why my door lock controller on the driver door doesn't work when the door is open. When it's closed, it works... And, lastly, the 6CD changer is eating the CDs and the single changer is doing the same, so have to figure that out, too. I tow my vintage trailer with my Denali and I want to get it running again poste haste...Summer's wasting away...