I had the dreaded po300 code and wanted to share my experiance solving it as there is confusion about the code on the internet.. First thing I did was scan my Van with a snap on scanner it pulled the code and it also showed me where the misfires where coming from. I had random misses on all of bank 1 Mainly 3&5 but under acceleration the misfires would decrease in 3-5 and increase on 1-7. I knew my ignition was all good having tuned it up less then a year ago with mostly OEM parts with the exception of Denso iridium plugs so I was sure the problem was the injection system. Did the leak down test. Hooked up my fuel pressure gauge turned the key on watched the pressure build then it immediately dropped down to zero. Got on the internet and learned the best option is replacing the entire injection system with the upgraded AC part number
217-3029



I got it on e-bay for 310.00 here
ACDelco 217 3029 New Multi Port Injector | eBay
So I proceed to remove the old one remove the upper intake and all fight it out install the new injection system test it engine runs perfect. If you get a PO300 first thing you need to do is the leak down test. If it fails replace the crummy injection system. Dont know why GM used that crap on there engines. Worst fuel injection system I've ever had the misfortune of working on down right nasy. GM had a great injection system-the Tune port setup the used on Corvette and Camaro. This system is absolute garbage. I'm sure it was a money thing. Could have built a better injection system for a few hundred more per truck sold.. Darn shame.
This is an 2001 GMC Savana 3500 with the 5.7

 

yep. I'm glad that system is long gone.