crack

All, I need some input and specifically regarding the intake manifold. I have a 2003 Trailblazer with the 4.2. It's 2 wheel drive. I've had a bit of an engine idle problem and I did recently buy the car used (less than 8 months ago). It isn't anything major, but sitting at a light the idle...

I've been getting a P0440 code that won't go away. I have been chasing the usual suspects... I bought a new gas cap, I checked the evap vent valve in the back near the gas tank... it checked out fine (It activates and stops the flow of air/gas). I also checked the Purge Valve on the driver side...

I have a 05 TB 4.2L. I replaced original exhaust manifold less than a year ago and 2nd one is already cracked. Anyone know if they make these manifolds out of steel instead of cast. If not do u think maybe running a 180 or 190 thermostat will help keep the cast from cracking. Thanks for any...