Run diagnostics on it and if no codes then have a O2 sensor bund installed and most shops can install temporarily a wideband O2 sensor and do a fuel mixture test at idle and up to 5K. If you've installed injectors lately, they could be rich for the ECU. Or, another guess is a leaking injector or leaking seal around an injector. Of course, these are all guesses as I ran my 300ZX rich with the wrong ECU tune and it had same behavior.