lean means not enough fuel for the air coming in, you need to add fuel to make it run right. adding fuel logicly will reduce mpg… albeit adding enough fuel to keep the trims in check to not toss a code will be marginal at best… so its not like your going to go from 31mph to 13 mpg… but never the less fueling it properly is a nessesity not an option for mpg expectiations.
Pulling the vac line off the regulator is just allowing the regulator to NOT open up and relieve fuel pressure at low engine load (high vac conditions) as in idle or partial throttle… therefore all its doing is making (for example) the fuel pressure at the injectors go from 20psi under low load/idle to say 43psi (3bar which most regulators are)… staticly across the entire rev/load range severly riching up the afr when the ecu/motor doesnt really want/need it. Unless the ecu is expecting a static 43psi or whatever durring all engine conditions this should not be done. (that regulator is just a simple spring and diaphram. the vac signal sucks against the spring pressure to unseat the diaphram to piss fuel past it and into the return line back to the tank. aka thats how fuel pressure is regulated)
The bottom line is you need to find out the “why” its tripping the lean condition. then work backwards to find the cause of that “why”. For all you know you could have one lazy injector thats leaning out one cylinder enough to throw off the ARF signal out the exhaust manifold to throw the cel. To check that pull all 4 plugs and read the electrode’s color to figure that example out.
exactly. the signal for the ecu is coming from that o2. the motor could be running 100% perfect AFR wise but if the sensor is getting/sending a shitty signal its falsely kicking a code. which is why another known good wide band should be used to verify it, why pulling a data stream from the ecu is needed, etc.