Hello. I have a 1988 Chevy K2500, 5.7 Liter. The following problem came up suddenly. It Starts fine in the morning, drives fine, but if I shut it off for more than 15 min. and go to restart it, It runs terrible. Like it has no power and is running very lean. Check engine light comes on and the trouble code I get is a 44. Sometines it will clear up and run fine as I drive and sometimes it will continue to run bad. I replaced the O2 sensor and it did not help. The fuel pump is a after market item about 4 years old. Where do I start?