got a check engine light while running some errands last night. my obd2 scanner says it's "170 - too lean," and "171 - short term fuel trim."

obviously they're caused by the same thing... but what! i recently lowered my idle fuel pressures (although they're still higher than stock) and also have my vafc set to -10% at narrow throttle, just as a test. to complicate matters i just ran through a tank of regane.

what does everyone think the likely cause is? i have heard that regane affects the readings from the O2 sensor. i would assume the O2 sensor is triggering the code.

i have never heard of a CEL caused by the MAP signal from the vafc being too low. i also don't think it would give me that code, it would be 3 or 5, right?

the car drives fine. i haven't cleared the code yet.

 

damn!

i cleared it. it came back about 150 miles later. i have since zeroed out the VAFC on narrow throttle settings, and this tank doesn't have any regane in it. at least, just the tiny remainder from the gallon or two that was left at fillup.

i'm thinking it must be the relief hole.... but given that the fuel presures are still HIGHER than stock, why would it be reading "too lean???" the O2 sensor output appears normal except that if i hold it at 2000+ rpm with no load on the engine it just goes dead lean. zero voltage. driving around it oscillates normally.

 

i love replying to my own posts!

it turns out the jr ems/pos leans the car out a LOT during off
boost conditions (based on TPS). when i put the fuel pressures
back closer to stock when off boost it continued to lean it out,
generating the too lean code.

ironically, now that i have bigger injectors the close-to-stock fuel
pressures plus the leaning of the JR unit are putting me exactly
where i should be - the long term fuel trim is almost zeroed out
every time i check.

lol.

 

I got the same damn freaking problem with my car...99 EPB Si

My setup is a JR super charger @ 6 psi. a Walboro 255 lph pump. A VAFC which is set at like 5% at idle....now to fix this problem i did try upping the fuel presure using the JR FMU and Upping the fuel pressure ( still i get the same Damn code)....i do have a B&M fuel pressure regulator on the Way with a Fuel pressure Gauge...wondering how to fix this crap without going to bigger injectors like you said.