ls1 way too rich
That with the fact that you have open headers is going to make for a stinky stinky car.
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1. O2 sensors- tuner will turn off the rear O2
2. Water temp sensor - I tapped off the computer to feed my Autometer gage...big no no. It was reading -40 degrees on a 90 degree day. You must use a seperate sensor in the passenger head.
3. MAF - needs to be plugged in and properly plummed.
4. Vacuum leaks - make sure all the extra ports are plugged.
5. Factory tune was really rich on the dyno.
The good news is my stock 2003 LS1 made 350 rwhp once tuned.
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Also try holding your hand over the MAF with fingers slightly spread. My idle picked up with a little restriction.
The real problem with mine was it was not seeing the TPS because I had converted from DBW to DBC. Once we put the tune in from a 2002 TA it ran very well.
Do you have larger than stock injectors? If so, there is your problem - don't run it until you get a base tune in it or you risk washing it out. At the very least, you will foul the plugs.
The coolant temp sensor has to be hooked up - don't unhook it as directed above - that will cause it to run way rich too... IAT sensor needs to be hooked up as well...
Get it tuned.
A tuner will need to know exactly what is in the car as far as injectors, MAF, heads/cam, fuel system (including regulator) and probably a few other details...
Your other option would be to trailer it to a tuner... or get tuning software and tune it yourself...
There is a difference between "close enough to run" and spot on to the build IMO. Then again that depends on your tuning techniques.

