Are your LTFTs 20%, or is this a hypothetical question?
High LTFTs cause AFR to be whacked.
The easiest way to find out is to log WBO2 and see how BENs are in CL.
For me, BENs are between .99 to 1.02 with LTFTs averaging -2%,
being a little lean from idle to 1200 RPM.
LTFTs that high could be an intake or exhaust leak.