L31MaxExpress
I'm Awesome
MFI update spider is ~58 psi KOEO and drops at idle.A new fuel filter never hurts. Your pressure might be a bit low, I think these Vortecs call for 60-66 KOEO and tend to drop into the 50s while running. Generally a failed regulator causes the pressure to bleed off after shutdown in my experience, so at least your new injector units regulator seems OK. New filter should bring your pressure up a bit, so retest with the new filter on and see what you get.
Other than intake gasket, and assuming your new injectors aren't a factory defect part, I'd take a look at the distributor itself. If it's an original plastic-body unit they like to crack which can throw things off.
Also recommend taking a look at your idle air control valve. If that gets sticky or fails it'll starve the engine for air when the throttle plate is closed (like when coasting, or at full idle) and can cause rough running in those conditions.
EDIT- I still do not get how GM gets away selling that updated spider without reprogramming the PCM with the correct injector data. What is in the PCM for the stock spider is wrong for the updated spider. I put the newer injector data in mine years ago and it ran much better all around.
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