Yesterday morning, wake up and head to work. Start the monte and she stutters a bit like she wants to stall, lightly rev the engine and the rpms jump on their own to about 1500 to 2000 rpms. After about a minute she calms down and idles fine. Didnt think anything of it at the time, until after work. Same thing. Then this morning, hoping it was just a freak oddity, I start it and again, same thing. This time I dont raise the rpms myself, let it go and it stalled. So i restart her again and its stuttering horribly and if i rev it the rpms jump high on their own, occasionally it will rev itself up. and then it will just switch to idling like normal, like nothing happened...Please someone help me figure this out before she goes into the drink.

 

Could be MAF or IAC valve, two possibly faulty items.

Mine had your symptoms a while back, swapped those two and problem went away. I couldn't pin point which one, but I am leaning towards bad MAF.

 

Probably a lot of carbon sticking in the TB and the IAC. Get a can of seafoam deep creep, take the TB off (get a new gasket) and clean that up really well.

Also get a can of MAF cleaner spray and give the MAF a spray. Try to clean the MAF every oil change.

Torx 20 Security Bit for the sensors.

Torque the TB back on at 89 in per lb.

 

Would this issue throw a p0420 code? Or do i have other issues too?

 

That code is actually a clogging catalytic. Which could also possibly cause this stalling (but let's face it, that TB could still be filthy)

I would get it to an exhaust shop and get it backpressure tested and get a new cat on there if it needs one.

 

The cat is 3 weeks old. Its magna flow direct fit

 

Decided not to take the Monte to work, but I started it before leaving and it started and idled perfect. I'm gonna clean IAC, MAF and throttle body this weekend and see what happens. Thanks gentleman, always a great help.

 

Then you need to either get that P0420 code deleted from your PCM or look-up how to create an anti-fouler.

That magnaflow doesn't work as strictly as the stock one, so the car is going to think something is wrong.

 

I thought that the 2004 and prior models only used the rear O2 sensor for emissions and not to control fuel trims. The 2005 model uses the rear to control more than just emissions, I don't have a cat & removed my error codes, tried running with no rear O2 and I was running a bit rich;
How old are your O2 sensors and have you ever ran leaded fuel in the car?

 

Ive driven myself crazy looking for the cause of this issue. Ive cleaned IAC, checked fuel pressure regulator, cleaned throttle body and MAF...during live data i was having crazy random misfires on every cylinder. Well its been 3 days and the issue is 100% gone. After cleaning everything, nothing fixed it, until I put fuel treatment in and refilled tank. took two days and issues are all gone. Its been 3 days now and No CEL, no hesitations. Nothing. Im guessing the issue was bad gas? All that I can think of now.