First thing we'd recommend is to ensure that in your vehicle is in fact emitting high levels of O2 during 2500 rpm. That is what a dilution problem represents; high levels of oxygen being emitted from the tailpipe and entering the smog machine during the smog inspection. A smog check station will be able to test your vehcle's O2 levels using either the smog machine (BAR-97 System) in diagnosis mode or by using a stand-alone 5 gas analyzer.
If in fact there is a high O2 problem, it could be caused by a lean air/fuel mixture. Normally however, we see dilution problems when there is a large leak in the exhaust pipe. The exhaust leak will let air enter the exhaust system and cause the smog machine to not allow a smog inspection or cause the vehicle to fail the inspection.
You should also check your vehicle's air injection system (if it's equiped with AIR), specifically the air injection diverter valve. If the air injection pump is pumping air into the exhaust stream at all times (instead of when the engine is cold and/or when the engine is accelerating) high O2 emission will result. Air Injection is used to aid in the burning of HC.
Follow the link below for more information on Air injection Systems.
Defective AIR Injection System May Cause Dilution Problem During Smog Test
