Any GM mechanics....I have a question

[72Blazer]

I have been going round and round with my Chevy dealership over a problem with my 05 Silverado with the 5.3L/auto.
The problem is that when the motor is cold,under 180 drgrees,when I drive around at about 45mph and let off the gas there is a clunk in the drivetrain and the truck jerks/hesitates.Sounds like backlash in the drivetrain.Driving slow the engine seems to run rough.My around town MPG has gone down to 12mpg from 15mpg.When the temp gets to 180 degrees or above its fine,it runs great with no hesitation at all.My highway mpg is still at 20mpg and the truck runs great when its warmed up.The techs at the dealership say that there are no codes and they can't seem to find anything wrong.But there definately is.I had the injectors cleaned just in case.
Since it seems like a temp related issue with the electronic choke system I told them to look into that.
I don't know if there is a temp relay for the fuel system or an electronic choke of some sorts.
Could someone with experience with GM tell me what part/system I should have the dealer look at? I want to go to them with something that makes me look like I know what I'm talking about.I know in theory what it could be but I don't know what components make up the choke system.Hope I make sense.
I'll probably take it to another dealer but would rather not because of I have a dealership just down the road.I need to find a mech that can can troubleshoot problems and not just fix what the codes say is wrong.

Thanks
Rob

[STANG KILLA SS]

there was a throttle body recal.. my dad took his 2000 with the 5.3. he was having some troubles. seemed to fix it.

[72Blazer]

Its 2wd.The problem went away for about 2 weeks recently and than returned.Thats why I think its a choke problem.Also because of the bad mpg around town while highway mpg is fine.I'll ask about the recall,THANKS.

[racerx]

I don't know what I did but my other post disapeared??? Anyway, anything with a fuel injection system does not have a choke. The ECM controls the air/ fuel mixture by legthening the or shortening the time the injectors stay on (open) A choke actually closes off the flow of the air coming into a carburator to richen the A/F ratio.

[rockmike]

Anthying under 185* is going to make the ecm be in choke mode[dumping extra fuel]The dealerships are lazy you just got to hound them,basisly make it there problem cause right now it sounds like they are in the mentality that its youre problem.So go in there rant and rave and make it there prob and they will have a little extra motivation to get it fixed and have you off there ass.

[72Blazer]

Originally Posted by racerx

I don't know what I did but my other post disapeared??? Anyway, anything with a fuel injection system does not have a choke. The ECM controls the air/ fuel mixture by legthening the or shortening the time the injectors stay on (open) A choke actually closes off the flow of the air coming into a carburator to richen the A/F ratio.

I don't know what to call it other than a choke(from my carb days).My Toyota had a cold start injector and the Mazda I had had a thermal relay.Don't know what GM calls it.Basically whatever it is alters the air/fuel when the motor is cold.
I'm just trying to find out what it is called so I can go in and say something other that "than choke thingy is bad).