Fuel leaning out while cruising

[Nis-240sx-san]

Here's the low down. I have an 1991 KA-T coupe. Lately i've been noticing something that is becoming more increasingly annoying with it. Day driving (lights off, Rad fans on) is perfectly fine. My AFR's are where they are supposed to be, yadda yadda. BUT here lies my problem. At night (light on, rad fans OFF this time), while holding the throttle steady (say on the thruway) my AFR's gradually decrease, like 14.7-15.2 then lower to 15.6-16.8 and eventually 16.8- --- full lean on my gauge. Then if i step on it and boost for a few seconds, it'll stay at proper AFR's for a while, and then repeat.

Now a few things i've noticed, if i turn my headlights off, the car will run fine with no leaning out issue. BUT the more electrical accessories i have turned on (ie. lights, rad fans, inside fan, fog lights) the leaning out issue happen more extremely.

I'm pretty much stumped here, but the issue has got to be either electrical, or maybe my ecu is just going brain dead.

My fuel pump is hard wired, i'm running an adjustable fpr which is functioning properly, a nissan quest alternator...i'm just trying to list things that i think may have something to do with this issue.

any input is much appreciated!

[kouki munster]

Is the car exhibiting any driveability issues, low on power, bucking, ect..... while the afr's are reading lean. Also pull out a plug and see if it shows signs of being run lean.

[float_6969]

IF the car is running OK. I'll bet you have low voltage and you're getting an erroneous output from the wideband.