4.2 BFM system too lean only on bank 1

[josepe306]

Hello from Bulgaria.
As this is my first thread on the forum, please be aware my english is not something to write home about
In essence, i`m experiencing the following problem: a couple of days ago the chack engine light came on and as i took a look at it using vcds i pulled out the system too lean code but only on bank 1. I did delete the code and thus reset the fuel trims and went for a drive. Surprisingly, observing channel 33 i noticed that the car was running lean but only on bank 1 - adaptation values started reached 13,7% which is a real problem.
What was done on the car befor the MIL went on - both chain tensioners and chains changed with OEM, channel 93 show 0 and + 1 kw
New OEM MAF, new coils from BREMI, new spark plugs from NGK, vacuum leaks tested with smoke machine - no leaks, new PCV valve from VAICO, timing belt kit and water pump were also done, new air and fuel filter, new "main" fuel pump - fuel pressure is ok.
After i pulled out the code i checked the system again using smoke machine with no result. i got another new MAF from Bosch.
Car runs fine, fuel consumption is ok.
Important to mention is that i got the SAI system deleted from the ECU. Afterwards, working on the suspension, i removed physically the secondary air pump and sealed the hose that goes into the intake manifold. After a few cold starts i noticed the engine was running rough when started cold so it flashed upon me that no extra air goes into the intake manifold as the air pump was not there but the combi valves (also called egr valves) were still working thanks to the seconday air inlet valve. One can guess what i did - i sealed the vacuum line that goes into the valve without disconnecting it and whoa - engine runs fine when cold now.
To the problem - the lean condition on bank 1 appear only on partial throttle - shall i push the gas pedal to the floor and drive a couple of miles, the long trim fuel trim adaptation goes to 2% which, in my opinion, is perfect.
As to the vacuum leaks - i not only tested the car with a smoke machine but also pored gallons of break cleaner in the engine bay looking for a vacuum leak - as the most of you know, there are non - return valves in the vacuum lines that operate and open after certain rpms. I can positively say vacuum leaks are to be ruled out.
What i`m planning to do is to check the fuel injector for leaks.
I forgot to mention - the car runs on LPG and has a BRC LPG system installed. Bank 1 runs lean on load on both gasoline and LPG, which makes me think it not the gasoline injectors as well.
Another thing to be checked is the evap canister. Fuel tank has no vacuum when open , the cap looks ok, is not new but OEM. AS the car runs i can hear the evap valve ticking which makes me thing it works fine.
Any suggestions on what i`m missing and what shall i check? Thank you in advance and hope you understand my English.