[brucepick]

Eureka. HX Lean Burn apparently requires 2400+ RPM

Sometimes I miss the most obvious things... In this case, in my search to learn the requirements for lean burn in the HX, I missed rpms.

2400 rpm and higher, to be specific. Just over 2500 on my dash tach but 2400+ on the ScanGauge. Or, in fifth gear, over 65 mph (on dash speedo). I've seen LB in lower gears so road speed isn't the requirement, it's rpms.

Below those levels, lean burn is iffy at best. I'm sure I've seen LB below 2400 rpm but at least in tonight's conditions that was the requirement.

Tonight, in conditions apparently perfect for Lean Burn, my '97 HX would not stay in lean burn and would go in only when it's "mind of it's own" decided to. Having noticed that increased speed sometimes helps, I brought it up over 65, and voila! Lean Burn. Next, I carefully monitored rpms and road speed to deduce what I wrote above.

OK, the other things I've written in the past still hold (notes below). But it seems they have an additional requirement, namely, 2400 rpm or better. Of course, attempting to go into LB at say 2410 rpm is probably a losing proposition because if you accelerate up to 2410, as soon as you let up on the throttle for the necessary TPS 20 or less, you'll lose rpm and will be below 2400, so no LB. Aside from that caveat, I seem pretty confident in this.

The other requirements:
TPS (throttle position) of 20 or less (as shown on SG) to enter LB. TPS over 31-32 will end LB. No air blowing on the cat converter - my grill is thoroughly blocked aside from the four segments in passenger side lower grill opening. Wet weather seems to reduce possibility of lean burn, but maybe that's due to the fact that I rarely drive over 65 if it's raining!

One other caveat. YMMV. I suspect my cat converter may not be functioning optimally, as I occasionally get a CEL with a code that references cat function. So it's possible that with a perfectly functioning cat the requirements for lean burn might be more lenient.