l have a 99 silverado l just replaced the fuel pump, fuel filter , and pressure regulater l am having a problem with the fuel pressure when the truck reaches 4000 rpms and its under a load l took itto the dealer and they tell me it is the brand fuel pump l used and l should buy oem for around $800 l only paid $300 at advance auto l also have an o2 sensor bad it really breaks up at 4000 rpms and wont shift unless l let off

That is correct. l have had experiance of 2-4 tries before getting a good fuel pump. this is not uncommon. The 800.00 ur dealer suggested also should include lifetime parts & labor warranty for ur truck. Sometimes worth the extra expense. The o2 sensor problem could be sending info that could also cause problems at higher rpms. indicating to rich or to lean fuel & causing the pcm to try to adjust ur fuel mixture to compensate. You really should adress this before going any further on ur fuel pressure concern. Some times the dealer may seem high priced but there can be good value in what they offer in the warranty that is offered

You should change the 02 sensor first and make sure that you have no vacuum hose leaks in the engine compartment (eg; vacuum leak at the MAP sensor).Make sure the fuel pump is properly grounded.Have someone else check the fuel pressure after the 02 sensor install and vacuum leak check.

If the truck seems to be running rich and you did tune up and all that other good stuff, veify which fuel injection system is in the truck. Their is a replacement unit out to superseed the old one. Good luck.

If the O2 sensor is reading a lean condition, you probably already understand the problem as this is one of the conditions that will set if fuel pressure goes low under load. l have seen this, so it is not unusual especially in a pump that is not OEM. There is a reason the OEMs are more pricey. If that is your only problem and condition, l think you have to do a do-over.