Hello guys.

I'm having an annoying idle issue that's driving me nuts!

When first started in the morning, it runs ok, but after running for about 1-2 minutes, when its at idle, it starts to sputter and just sound like crap in general.

Once its warmed up this issue completely goes away and it runs fine.

Stuff I have replaced / done:

- Plugs, wires, cap, rotor
- Plenum and other intake gaskets
- New Catalytic Converter
- New serpentine, idler pulley and tensioner
- Ran seafoam through the gas tank and brake booster

Anyone have any idea what else it could be? Im thinking maybe the IAC? but I dont know because like I said it runs fine when warm...and if it was the IAC it would run like crap all the time.

I know the battery is weak, it starts the truck fine, but it tested weak at autozone, could that somehow be it?


Thanks guys.

 

Sounds more like a failing O2 sensor. Though, a bad battery does tend to cause some interesting issues.... Since you know the battery is bad, replace that first, since it needs it anyway. If the doesn't solve the issue, have a good long look at the pre-cat O2 sensor. They are recommended maintenance items at 75K miles anyway......

If you had to replace your cat because of a blown plenum...... probably killed the sensor too.

 

When they replaced my cat they had to replace the "Downstream O2 sensor" (Reading off my bill) because it was seized into the pipe and had to be broken off.

Is that the pre or post cat sensor?

 

That one would be the post-cat O2 sensor. In all reality, it has little to no effect on how the engine runs, just tells the PCM if the cat is doing its job or not. The pre-cat O2 sensor though.... it has a GREAT deal of affect.....

 

think of the exhaust as water flow...

pre-cat is upstream
post-cat is downstream.

it might? be running poor when it opens up the evap system to suck in the canister vapors.

or it might? be running poor as is begins to reduce the cold idle rpm.

IAC can fail and need replacement.
TPS can fail at certain temperatures.
both of these are fairly cheap, about $40-50 each. if you do replace them, replace them one at a time, and keep your old ones. its nice to have good spares to swap in for troubleshooting.

 

This sounds like exactly when its happening actually.....

So you think its my pre-cat O2?

 

^ i don't.
if you've not done so, i'd replace the IAC and clean the well/port on the back of the TB that it goes into. don't forget to include the o-ring on the new one.

if you don't replace it - at least clean the pintle and well on the old one. any gunk interferes with precise idle control.

here's why. it the IAC's job to control idle speed during cold and warm idle by moving the pintle in/out of the idle port. its a little stepper motor. it does this under instruction from the pcm to move in/out precise amounts. its supposed to have this perfect motion (and air) control to adjust idle down from cold (1000 rpm) to warm (600). when unexpected things happen is when the engine starts sputtering.

its my uneducated understanding - that the PCM only uses the O2 sensor readings when its all warmed up to operating temperature, and running in "closed loop mode".

you could test this theory by temporarily unplugging the electrical connector on the O2's. expect a CEL on this and don't overreact to it.