[mwebb]

you claim to be operating with 15% diesel ?
i tried 4oz of Shell Rotella diesel fuel additive to 10 gallons of fuel in a 10G Geo metro
lost about 10% FE that tank and

the additive did NOT combust in the engine so when it got to the exhaust
the 02 sensor
saw lots of combustibles so it thought " Rich condition - subtract fuel " not good , not a good idea at all
documented with scan data and scope waveforms

as you know
the 02 sensor does not see 02 , it sees combustibles in the exhaust stream .

15% , a gallon and a half in 10 gallons --- would be catastrophic in my car
so
kindly elaborate on this .......
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then we get to No EGR ?
when you wish to improve FE , you have removed the EGR
that does not click either .

as it is well known that EGR improves FE by reducing relative engine size AND decreasing suction throttling loss .

so , to IMPROVE your FE and reduce NOX production
reconnect your EGR system ,
yesterday -
it is likely the ECM will respond to your tampering by retarding ignition timing
as it knows the EGR is NON functional , which will further reduce FE over just lacking the EGR .

so you are taking what is bad and your ECM is making it worse .