Cold idle stumble

[eagle mark]

I've even seen a bad CTS not set a code!

I'd never rule out anything and like to hear other peoples problems and solutions. Problem with this being a CTS is I've never seen one do anything but work or go bad/fail and when they do it's -40C all the time. So even warmed up your still running MAX choke. This issue has not mentioned pig rich forever, warm or cold... and he wouldn't be getting 10 MPG, he'd have black soot exhaust, gas contaminated oil... way worse then this little issue.

I understand your theory of it being off and causing this, but have never seen it happen.

How many miles on this rig? TBI still original? You could pop out the plug hiding the throttle blade set screw, this is not an idle screw, it is a minimum air setting. As the TBI wears the throttle blades close. It would not hurt a thing to turn this screw in 1/4 turn. Letting more air past throttle blades helps mix fuel (injector above throttle blade) with air.

Have you checked fuel pressure? As mentioned very important to good running, will run much lower, but not good.

 

[chulisohombre]

If the engine is cold it will be running rich until it warms up to keep the engine running. It will stumble a little if you punch it til it warms up a lil. A bad temp sensor will keep the engine running rich thinking it cold all the time and give bad mileage all the time. Look at the jets and make sure it's spraying a good clean even spray when it injects the fuel. That'll rule that and the fuel pump out. Change out the fuel filter if it hasn't been done in a while. Mine had all kinds of crap in it when u did. I don't think p.o. Did much maintanence on
Mine except to keep it running. Ran much better after. Check all intake bolts and tbi bolts. Loose may be letting extra air in somewhere. Check vacuum lines for wear and tightness. When in doubt replace it. Check egr and PVC valve. Again any extra air will make the engine run funny. I'd try some Lucas fuel cleaner additive to the gas tank. Will help get the grime out of the system after fuel filter change. That's a few things besides what you got so far. Cap and rotor are easy to pop off and look at as well.

 

[eagle mark]

You can not judge fuel pressure by injector spray pattern.

You can see dirty/clogged/leaky injectors by spray pattern.

 

[chulisohombre]

You can not judge fuel pressure by injector spray pattern.

You can see dirty/clogged/leaky injectors by spray pattern.

Yeah thatll check that and Low fuel pressure will cause the injectors not to have a clear clean spray as well.

 

[black dawg]

can you describe this stumble a little more? Does it completely go away as it warms up?