I replaced my intake manifold gasket at about 80,000 miles on my 2003 Avalanche in 2007.
Here were the symptoms:
1. Ran like crap when ambient temperature was less than 45 degrees F.
2. Once warmed up would run fine.
3. Even after vehicle sat for several hours it would run fine.
4. If it sat all night in cold weather it would run like crap if the temp was 45F or colder.
5. Check engine light with a code that read LEAN BANK 1 & BANK 2.
The intake manifold gasket was shrinking in cold weather, and then of course expanding when it got hot. When it shrank it would let air in. When it warmed up it would expand and fill the void. I did the ole "spray intake cleaner all around the intake and see what happens" trick, but that didn't prove anything, so don't bother with trying that.
I did this 3 years ago, so forgive me, as I dont remember everything, but hopefully what I do remember and what pictures I took will help someone.
You'll need one specialty tool - a fuel line disconnecting tool. They sell it at your local auto parts store - its probably less than $10.
Unscrew your gas cap to relieve pressure on the tank. Pop the tool over the line, push it up in the connector to expand the splines, and pull the two lines apart.
The next tricky part is disconnecting the injectors...just be patient, study how they are connected and disconnect all the injectors.
The only other thing I remember that was minutely tricky was removal some other device attached to the intake manifold. I just looked at the pictures, and I think it might have been that solid plastic line with the green cap on it. Just look at my pictures and remove accordingly.
I am not a mechanic, I am regular ole guy with a lot of patience who loves to figure stuff out on his own and refuse to pay anyone to do anything for me. My motto is "if you can read, you can do anything." From past experience, nothing gets done right unless I do it myself, and if it isn't done right, the only person you can blame is yourself. Even if you break something trying to learn, its still cheaper than paying someone else to do a job you can do.
Anyway, thats my rant. The pics are below. Enjoy and good luck!
The design of the intake gasket makes it nice and simple! No more having to line up the holes and using silicone and all that crap to hold it in place. It just clips on!
The finished product!