I just finished installing LT headers on my car and have noticed that when I start up my car, the idle is normal (around 1,000 rpm). However, when driving, if I shift into neutral while driving at any speed (I have an M6) the rpms slowly crawl up to 2,000 and idle there until I brake and slow down and then I am almost stopped, they crawl back down to normal, around 1,000 rpm. The car drives fine other than this and rpms seem perfectly normal while in gear. I have never heard of anything like this and was wondering if anyone had any adive? I put in the LT headers and ORP, took out AIR and EGR, currently have no rear O2 sensors or sims, I put in new plugs, had to take out and reinstall TCS during headers install, took off intake manifold to put in new gaskets... I think thats about it. That is all of the stuff that I did over the winter and I just started driving it again and notices this. Oh yeah, its an LT1 incase that matters for this. Thanks in advance for the help.

 

I finally figured it out. I had a vacuum hose that was broken. $1.05 to replace and the car runs great now. In my case it was the one on the passenger side of the TB just below the PCV hose. Hope this helps someone sometime.