I just had a shop put on a set of GM 12558060 heads with matching GM 12496821 intake. Before, the car ran just fine. When I got it back, I expected I would have to make some tuning adjustments. Currently, the car won't keep a steady idle, and fluctuates between 600-1000, sometimes completely dying. After running a couple data logs, it seems the car runs extremely rich, as the BLM drops to 110-112, and the O2 is constantly 0.670-0.990. IAC also jumps to 90-160 as RPM hits 1500-2000. I've been subtracting fueling to try to lean the mixture out, and this helps minimally. My question is, is this the sort of behavior one would expect from new heads and intake? Everything else is a stock LO3, except for shorty headers and 3" cat-back. My concern is leaning the mixture too much and doing some damage.

Thanks in advance.

 

Id like to hear how this worked out for you. Im looking at going this route and trying to limit my problems after the install with the tune. Ill be using the EBL so that will help but just wondering if there was anything that could of help. Thanks!

 

Sounds like a leaking gasket.

Always a difficult problem to diagnose.

 

Are those 350 heads? On a 305, might have lowered the compression ratio.
Hook up a vacuum gauge to it, see what its pulling at idle, is the needle steady?