Engine Performance problem
1998 Chevy Tahoe V8 Four Wheel Drive Automatic 14,000 miles
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my 98 Tahoe 2-door had a new engine installed but the mechanic put the EGR valve in upside down.It was driven for 12,000 miles before I discovered the problem.Now the engine misses when I decelerate from 75 going down hill and hesitates when i feather the gas. It clears up again at about 60.the check engine lite flashes .If i accellerate hard the lite stops flashing. What.s wrong? Could the cats have gone bad? Help please?

First thing you need to do is have the codes read, Autozone does it free, Post the results with the code number(s). Even if the service engine light is not on, there should be freeze frame data stored.
If it won't make it to Autozone, buy a cheap scanner, they can be had for 30-40 dollars, a good investment, since they cover all cars sold in the USA from 96 on...
If the light is blinking, it means possible damage to the converter...service it right away!

I had the codes read and they are:PO171,174 bank 1 and bank 2 running lean, and PO300 multiple cylinder missfires. what does this mean?

http://www.2carpros.com/car_repair_video/test_fuel_injection_pressure.htm
Go here and checkout our video.
You can rent the test gauge at Autozone.
Not enough gas, or too much air...You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If that doesnt do it, you may need a new idle air control valve, it's bolted to the throttlebody