i have a 1989 gmc r3500 with a 454 tbi that i just checked the fuel pressure on and its showing about 11 to 12 psi when you turn the key on but right after it gets to 12 it will shoot down to 0 psi.im trying to fix a lean code 44.also it does it on both my fuel pumps so i dont think its the fuel pump.and when idling it stays around 10 psi i believe.the way i have to use my gauge makes it impossible to do a driving test with it.

That pressure is ok for TBI......

last time fuel filter changed?

Exhaust leak?

Vac leak?

Old/worn O2 sensor?

no i put a brand new fuel filter on because i had to install the tbi fuel adapter to it so i just replace it at the same time.no exchaust leaks as i know of and i replaced all vac line and o2 sensor is 1 month old.ive been trying to fix this lean code for a long time know and replaced alot of parts.i think im going to buy the tbi repair kit and replace all the tbi gaskets in case its leaking and it comes with a fuel pressure diaphragm so i can rule that out to.

I would spray around the engine, and the base of the throttle body with carb cleaner to see if there is an rpm change...

i replace all the seals on the tbi and the truck has never ran better.ty for all the info and help.