High Pressure then First thing to check is:Check the gauge...
Many gauges are not very accurate reading bottom or top off scale and most shop fuel gauges reads 0 to 100+ PSI.
I change the gauge on one test set to read 0 to 20 or 30 PSI for use w/ Carbs and Low Pressure EFI used by most 300 and 700 series TBI. (Many carbs use only 2 - 5 PSI Max. or fuel will flood the bowl.)
Then check return line damage. ANY pinching or crushing in that line can cause high pressure.
Pull return line and should easy blow air.
return can be block w/ crap or or crush where you can't see.
If tank or engine was pulled for any reason... vent and return can be connected wrong. Vent line is very redistricted at the tank end to prevent liquid fuel getting at EVAP "can."
If fuel pump was replaced, can be wrong pump... Pumps for Port Injection = made for high pressure and often higher volume and often "peg the reg" for TBI engines and can't dump fuel fast enough cause high pressure problems.
Note that ECM can have problems too cause engine running like crap or even setting 44 45 and other codes.
regulator can have problem and get block w/ crap in the feed line etc. You have 300 series TBI covered hundreds of doc's by GM and many others telling how it works and how to fix...
If TBI needs taken apart... first get a "rebuild kit" that have most parts/seals/etc.
------------------Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)The Ogre's Fiero Cave
[This message has been edited by theogre (edited 09-29-2021).]