Truck has around 150K miles and runs like new!...after it's warmed up.
After it sits all night, it will still fire right up, but shakes and shudders and won't hardly idle as long as it's cold. Throws code P0171 and P0174 - running lean.
I'm doing all the work myself, and just replacing parts as I can afford them. I've done search after search and get a variety of suggestions.

First I cleaned out the MAF, TPS, and intake. Got some gunk out of the intake but that didn't fix the problem. Still won't idle cold.

Next, I replaced the thermostat. Couple people suggested that. Made sense... Won't run until at a certain temperature, ok. Replaced that, no help.

Next, at the suggestion of a very knowledgeable mechanic friend, I cleaned out all the electrical connections with electrical cleaner, and replaced the Fuel Pressure Regulator and PCV valve. I just finished doing this, and it seems to idle better, but it is also around 50 degrees outside. I just know that tomorrow morning when it's 25 degrees, it's going to stumble and fall on it's face again.

I bought all new plugs too, but I haven't installed them yet. I can still take them back if I don't need them.

This seems to be a very common issue, and everyone has different variations of the problem and different solutions.

If anyone has this exact issue - rough only when COLD, but does start every time - and runs perfect when warmed up after a few miles... I would appreciate any advice from your experience correcting the issue. Thanks in advance!

 

This is the wife's truck, and I'm going broke trying to fix it and I can't afford to put money into MY truck! I can't feel good about working on mine until I know she can start the truck and reliably get to work in the morning.

 

There is a TSB that may apply. What is the 8th and 10th digit of your vin

 

8th VIN is "T" 10th VIN is "1"

Problem not fixed. I ended up going as far yesterday as replacing all the plugs AND wires, along with everything else I mentioned. Won't idle cold unless you hold it around 2000RPM.

 

Allow me to elaborate further...

It's 39 degrees this morning and my 1992 Dodge/Cummins with 300K miles will fire right up and run like new. Sure is frustrating when I can't find the problem with this Chevy to fix, especially after spending $200.00 and 8 hours of my time installing possible solutions.

Wife just left for work, and I went out to start it to see if it was fixed, and it get it warmed up. It fires right up, but after about 10 seconds or so of idling, it starts shuddering, shaking and wanting to die. If I hold the pedal down and idle it around 2000 RPM, it runs fine. When I leave it alone to idle at the normal 500-600 rpm, that's when it shudders and rev's itself back up to stay alive. It misfires too, and gives a misfire code.

By the time you drive 10-15 miles and get it warmed up, you can let it sit and idle all day with no problem.

 

Sounds like it has bad intake gaskets, low fuel pressure or bad coolant temp sensor. If the fuel pressure is good and the coolant temp sensor reading is correct replace the intake gaskets. Very common for intake gaskets on the z engine but I have done them on the T. The TSB only applies to the flex fuel engine (8th digit would be z).

 

A couple guys have told me to check/replace the IAC (Idle Air Controller). I'll add your suggestions to my list as well.

 

I'd like to chime in also. I'm having the exact same problem. As soon as the weather got cold, the rough idle came AGAIN! It did the same thing last winter. Spring and Fall, you cant even hear or notice the truck is on, but as soon as it gets cold I'm revin the engine at lights to keep from stalling. I have replaced plugs and wires 1 week ago, new PCV valve and will change upstream O2 sensors tomorrow. I want to change fuel pressure valve as well. I will clean throttle again. Understand that there could be pinholes in PCV hose as well. I have a spectre air filter in the OEM box. Besides all that where should I look that I have overlooked before I wave the flag and take it to the dealer to get robbed on a simple fix that I will have overlooked.....HELP!

 

OBTW, had the dealer change the fuel filter because it was too cold to get under it yesterday. For MDTAHOE, I have a Flex Z, how do I go about having it looked at per the TSB?

 

I'm going to try the intake gaskets next. It makes sense that when cold, they won't seal thus leaking air and causing rough idle. After warming up, they seal up and it runs fine. I've been told this by a few people now so that's my next attempt.
Replacing the IAC (Idle Air Control) did not help at all.