I'm not exactly sure how to figure out whats going on in my data logs.

At idle my BLMs tend to wander down to 108. I recall that the ecm ignores the O2 for idle and uses a different BLM cell. Either way, it starts smelling like burnt Fruitloops (tm) if it sits long enough idling (5 mins or so).

Going 35 it goes anywhere from 108 to 118.

On the highway, it tends to work its way to around 118-124 going 55 mph. While its doing that, the O2 seems to be indicating slightly lean. It was my understanding that if the BLM is below 128, it should show as running rich. My air/fuel gauge pretty much goes out completely or wanders around the first 2 dots if i'm going long enough.

Going 65 it stays right around 128. I haven't datalogged much going 65 to say much more

I know that comparing speed the BLM isn't exactly the way to do it, but the vac readings change a lot and it would make more sense to look at the log than to have me post it all.

Is this a problem with my worn engine or am I not reading the results correctly? I can e-mail ome log files if that helps.

Its stock '92 305 TBI

 

During idle the ECM stays in closed loop and uses the O2 sensor. Otherwise the BLM value would never change. On the highway the ECM will go into a lean cruise mode. During this time the O2 sensor value will drop into the double digits and the INT will lock at 128.

During cruise when not in lean cruise mode the O2 sensor should oscillate high and low (200 mV - 800 mV, approximately).

Canister purge will cause the BLMs to drop. This should only happen during non-idle times. A leaky EGR valve will do the same thing. If the EGR or the canister purge valve is a little leaky that will cause the idle BLMs to move around, more so at idle (hi vacuum, low air flow).

How does the car run otherwise?

RBob.

 

I thought it might be going into lean cruise but I wasn't sure. I was pretty sure that 3rd gens missed the boat on lean cruise, but I'm glad to hear its present.

I'm trying to think up excuses on why it wouldn't be my headgasket that is blown. Right now it runs fine except for the smoke and the overheating. I try to keep it around 200 F with a 180 F t-stat and my right hand hovering over the A/C switch.

it smokes pretty bad at start-up and then pretty bad once it's decently warmed up. I can't see or smell it on the highway

 

My 8746 from a caprice goes into lean cruise. Really weird to see he A/F mix gauge disappear lean only to jump up to about middle 14.7:1 after about 1 minute then disappear again. Punch the pedal a little and it goes back to full rich. Can't complain about the over 20 mpg fuel mileage though.