2008 Silverado 5.3 fuel pressure drop

Josh29 · July 11, 2017

Hello. New poster here. I have a 2008 Silverado extended cab 4x4 with a 5.3 j vin code.

I replaced a leaking fuel sending unit a few weeks back. Truck ran ok for a few days then quit. Would turn over but not start. I had no power coming out of the fuel control module above the spare tire. So I replaced it with a GM oem control module and had a dealership program it.

After that was programmed it felt real sluggish and slow. Started pinging bad under heavy loads. So I checked the fuel pressure. Should be between 55-62psi. With the key on engine off it jumps right to 62-65psi. Start the truck and it maintains that pressure for about 10-20seconds then drops straight down to 43psi. It's not a slow drop, it's immediate. So I assumed I had a faulty fuel regulator on the new sending unit, so I replaced the whole sending unit with another brand new one (ACDelco). Didn't change anything. So I replaced the inline pressure switch even though it read the same on my scanner that a mechanical gauge read on the fuel rail. No difference.

I took it back to the dealer that programmed the new FPCM and asked them to diagnose the pressure drop. Meaning I wanted them to see if the new FPCM was faulty or possibly the program job. They charged me $220 for diagnostics and said I had clogged cats and said it could be causing the computer to force the pressure down. So I went home and replaced the Y pipe and cats which were clogged. Fuel pressure still drops.

Anyone have any Idea how this can be happening? I can command the fuel pump on with my scanner and it will jump right back up to 62psi no problem. So why is the computer commanding the pressure down like this?

Josh29 · July 11, 2017

So I just took it out for a beat. Runs way better with the new cats. No more hesitation and pinging.

My long term fuel trim is still wacky. Bank 1 is minus 1% and bank 2 is positive 8%. I don't see a deviation in air fuel mixture like that from one side of the engine to the other as a sign it's running right. And I just noticed on the scanner that the desired pressure the computer is asking for is 43.5psi. And it's maintaining that beautifully. For some reason the computer is just asking for a lower psi than what the specs call for on that engine.

I can command the pressure up to 60psi and it does run stronger throughout the rpm range at full throttle than when I go full throttle at 43.5psi. I just can't think of any other reason besides bad or wrong programming in the FPCM.