I have a 99 1500 with a 5.9L I started about 6 months ago intermittently. The iac would open all the way and would start sucking lots of air and loping down to like 3-400 rpm. The map sensor voltage goes clear up passed 4V. If i let it idle like that long enougj it will die. But i can start it right back up and it runs fine for a little bit. Not the TPS not the IAC. Has new O2 sensors. Fuel pressure is at 56psi. Vacuum is at 16.5psi. I just lust a new engine in from JASPER engines so its not internal bc it did this before new engine. It's getting severely worse with poor power and 13mpg highway .
Pretty sure its electical

PLEASE HELP

 

I would check all grounds first then the check the connectors on the PCM. Make sure they are firmly seated and no corrosion or debris inside the connector itself. Let me know what you find.


EDIT: Out of curiosity, what brand O2 sensors are you using. I have had bad luck with the autozone cheapies so I wouldn't use them.

 

56psi is high.. it sounds like your flooding it out.. it would be a good thing to get a a/f reading from it.. on a ram rail, @49psi, those 21#r's push around 23#'s (most injectors are rated at 43.5psi, unless they are FMS, and they are rated at 39psi).. @ 56psi, you're pushing into 25# territory.. your PCM can only curve so much.. you're drowning it..

what's up with your fuel pump? it sounds like that lil' dc motor is gettin' more juice than it is 'sposed to...

 

I've checked all my grounds and I have the Bosch O2 sensors from carquest. I don't know what would cause an over voltage to the fuel pump. I checked the pressure a week ago and it was at 54#s. I don't have any modifications to it. I did just recently put a 180 thermostat in it but I'm changing back to a 195 tomorrow but it was doing this before I swapped that out. Even with the old engine.

 

Don't the 1998 and up have a higher fuel pressure

 

No, older engines had fuel pressures around 39 psi, but from 96 up its definitely 49psi