have a system lean code popping up. long term fuel trim at idle is 14. no missfire counts. adjusted fuel trim from full rich to lean still no missfire counts. when on test drive and found after accelerating on the freeway soon as i take my foot off the gas pedal long term fuel trim jumps to 29 and multiple cylinder missfires start to occur. take car off freeway and at stop light can feel missfire and fuel trim still at 29. coast it into the shop and at idle LTFT down to 10-14. sprayed brake clean all over intake manifold but wasnt able to register any change in fuel trim. fuel pressure is within spec. cleanned MAF and Replaced with known good. LTFT still high

 

Sounds like you've got an injector fouled or you've got a gigantic vacuum leak. Read my post on the P0420 code I wrote up. It will explain a lot for you.

 

Had problem like this with a toyota celica ended up being a maf sensor