Rough cold idle/flashing MIL light

My 02 Z71 has a rough idle, mostly when its cold. After starting, the engine will idle normal for 10 seconds or so then almost die. It repeats this process over and over. After driving (15 min) it will idle close to or completely normal. Sometimes this rough idle is accompanied with the service engine soon light flashing, but as soon as you start driving it goes completely out, and doesn't come back on. My gas mileage is pretty poor 12 to 14 MPG (I am not in 4wd) regardless of highway or city driving. The fuel pump was recently changed as it was only putting out 35 psi and I just took a look at my plugs the other day which were changed 2 months ago and they show heavy soot on them already. Any suggestions out there?

Like black soot?
Your symtoms may be a fuel pressure reg flooding it out.
If external fuel pressure fuel reg pull the vacume hose off and look for gas in the vacume line.
What engine?
Here is some guide line on a fuel pressure check that may show something.
Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

MT

I had a 99 with the 5.3 doing this same thing, it was the coolant tempature sensor, the one that sends the signal to the ecm.

Late getting back to this one.
Dont have the tools for the fuel pressure, but did just have the pump replaced. Any chance the 02 sensors are causing the ecm to run rich???

Late getting back to this one.
Dont have the tools for the fuel pressure, but did just have the pump replaced. Any chance the 02 sensors are causing the ecm to run rich???

Did they replace the fuel pressure regulator when the pump was changed? If it is leaking, it can cause it to run rich.

With a guage attached to the service port, it you turn on the ignition (but do not start the engine), the pressure should be 55-60. Then if you turn off the ignition, it should hold pressure. If it doesn't hold pressure, the regulator, an injector, or the check ball in the pump is bad.

EDIT: I should learn to read all the posts before jumping in. MT-2500 already mentioned to check the fuel pressure. Since you have a rich condition, you really need to check it along with it holding pressure after the ignition is turned off.

//2000CAYukon

Late getting back to this one.
Dont have the tools for the fuel pressure, but did just have the pump replaced. Any chance the 02 sensors are causing the ecm to run rich???

Like stated by 2000CAYukon and me you need to run the full fuel pressure test for pressure up to specs and leak down test.
That engine is very sensetive on fuel pressure.
Other things can caues it to run rich but tempt sensors maf sensor air intake sensor or eng miss.
But 02 sensors usually do not cause run rich.
If you do not have a fuel pressure gauge . Rent, borrow, or buy one or have a repair shop check it for you.
Good Luck MT

code reader said lean mix on bank one. Now i can't find a diagram of the fuel system, so correct me if i am wrong. Fuel gets pumped from the tank and goes to the manifold. The fuel pressure regulator doesn't meter incomming fuel (doesn't sit on the incomming line), but maintains the propper pressure by releasing fuel back thru the return line to the tank.
If thats the case, would a fuel pressure regulator that is dumping too much gas back into the tank cause the lean state encountered?

As MT said check the fuel pressure, self help here!