1999 Suburban 5.7 vortec has me stumped
 

My suburban started stumbling up to about 2000 rpm then seems to catch up with itself and runs fine. Burban starts right up no problems ever!. When I drive on the interstate it sometimes pops a p0171 and p0174 code but that is very random. last code it popped the other day was a p0101, p0303 and a p0440 all at the same time. I have replaced my MAF, ambient air temp sensor, EGR, Fuel pump and filter, Idle control sensor, throttle control sensor, pcv, O2 sensors, checked for vacuum leaks (none), checked exhaust backpressure (below 1), seafoamed the upper intake through brake booster vacuum line and lots of injector cleaner in the gas tank. I had the spider injectors upgraded to the MFSI injectors last year (long before this issue). Still stumbles until about 2000 rpm. I put a vacuum gauge on it and I pull low normal spectrum. Timing was set by mechanic to within 2 degs of TDC. Fuel pressure at idle is 54psi. Pump said working pressure should be 62 but that never happens and the truck ran fine before at this pressure. Does anyone have any idea? I am beginning to think that my lower intake gaskets might be the issue but oil shows no signs of coolant contamination and my coolant level is consistant. I replaced my distributor in 2016. I have not changed my crank position sensor because I don't have a scan tool to relearn with. Any help or ideas would be greatly appreciated!!




thanks all for any help you can offer!

The 303,i think the same,lower intake gasket,that one caused the 101?one thought,is the new MAF,a quality one?

autozone MAF. I have thought about that but hate to replace just to find out it's not the problem...any suggestions?

Duralast from Auto zone,bad news,I would think it's underreporting,so #3 might be a small % less then others and the MAF is making it worse,and once I saw a restrictive air filter cause a problem,you could disconnect MAF for a test,see if studder disappears,make it rely on the tps

I have not tried that, but will try it when I get home weather permitting. I will let you know what happens.

thanks,