LS3 Crate engine won't idle
 

I just got my Hot Cam LS3 crate engine going in my 71 Chevelle. I ran it down the road to break in the rings but I have to use the gas and brake to keep it running at lower speeds. Sent off the PCM last week to TSP to have it tuned and the idle speed bumped to 800RPM. I can get it to idle now in park and drive but you really have to try a bunch of times to get it to idle. Then if you press the gas it instantly dies out. A sure way to kill the motor is rev it and drop the gas BAM instantly drops dead like you turned the key off. Runs good at anything over 1500 and has good power. Checked everywhere for vacuum leaks and can't find any. Drove it about 25 miles and no codes being thrown. Any ideas?

sounds like the throttle blade is closing to much when you come of the gas and not allowing enough base air through. Its an easy fix. Just give TSP a call and have them adjust the airflow final minimum.

Thanks that sounds like a possibility. Sucks to have to send the PCM in again it took 3 weeks to get it back last time. However, now it is throwing a code P0102 Mass or Volume Air Flow Circuit Low Input.

I tried attaching the evap canister. I plumbed a vent from tank to a three way evap canister which sucks fresh air from downstream of the MAF and now sucks it into the port behind the throttle body. It is much better now doesn't stall when you drop the gas. I had left it unattached until I could find out how to make it work properly. But it may work this way who knows. I think this is bypassing enough air to make it idle better.

Well that was only a temporary deal its back to stalling again every time
you drop the gas pedal or even if you release it slowly. You have to slowly
release the gas pedal over the course of about 5 minutes to get it to idle then
it idles good. Really hard to drive it dies out all the time. It will sometimes
backfire through the intake too when you first give it gas off idle.

Now its throwing these codes P0102, P0171 & P0174

It certainly loading up on fuel low speeds certainly not lean like its saying it
is. Changed 02 sensors and still same problem. Fuel pressure is steady
at nearly 60PSI. I tried mocking up the intake simpler and that didn't help
either. I looked everywhere for leaks and can't find any vacuum leaks.

I took the valve covers off and closed all the intake valves, plugged off the PCV port and sealed the end of the air intake and pulled a 25" vacuum on a vacuum port and it held vacuum for 15 minutes without dropping at all. So, I am sure I don't have a vacuum leak

I know this is random but did you have the vss sensor hooked up to the P C M? i had a similar issue but mine would not read until the speedometer was hooked up and calibrated...

Having a similar problem with my LS1... Will not even attempt to idle. Have to keep it running with the foot feed. Misses like crazy under 3000rpm, intake backfires all the time, WOT it runs good, but will go from 6500rpm to totally off if you push the clutch immediately. Smells like gas bad. O2 sensor extensions fot cut, assuming thats the problem... Hope you figured this out.