I'm starting to get the hang of the lean burn on the VX. It feels strikingly familiar to the Insight lean burn. First, some info from the OBDI training manual.
The LAF (Lean air fuel) sensor can be used to detect AF ratios in a range from approx 12:1 to 22:1. These were used on the 92-95 VX and 96-98 HX. If an AF ratio richer than 12:1 is required, the ECU will go into open loop.
A good way to monitor AFR is to attach the positive lead of a DMM to the Pump Cell Control O2 lead and the negative DMM lead to the O2 reference voltage lead. This shifts the O2 output such that 0V is stoich, a negative reading is rich and a positive reading is lean. To test the LAF, when introducing propane to the intake would create a rich condition resulting in a reading of -1.3. To create a lean condition, you could disconnect one of the fuel injectors which would result in a 0.4V reading.
In practice, I have found that lean burn does indeed result on a reading of 0.4 to 0.5. That is similar to the reading that you woulld expect when you have one injector disconnected. I could then logically conclude that lean burn could reduce fuel consumption by as much as 25%.
Of course it takes time for the cat to warm up enough to enable lean burn mode. When starting cold, you will notice a reading of -0.77V. This is also what you would read if the LAF was disconnected, and similarly, you get the same reading when the LAF is dead. After a few minutes, the reading will oscillate between -2.0V to 0V. After another minute or so you will notice the reading stabilize around 0V and then climb to 0.40 to 0.45V during a lean burn cruise. During idle, the reading is cut in half to about 0.22V.
Lean burn on the VX has more range than on the Insight. I can climb even the steepest hill on my commute in 2nd gear and maintain lean burn mode. Moderate hills can be climbed in 3rd and more gradual hills in 4th. On the highway, lean burn mode can be held at speeds up to 65-70 MPH, much higher than the Insight. Once your vacuum drops below a certain point, it seems like VTEC kicks in and your instantaneous FE drops immediately by 1/3. For example, While climbing slight grade in 5th at 55MPH, I can hold 45MPG indefinately. When encountering a steep grade, you would open the throttle to hold your speed. As the injector pulse width reaches about 6.3ms before dropping out of lean burn. The instantaneous FE drops immediately to 30 MPG. This is very similar to what you would feel during a NOx purge in the Insight.
There is a downside to lean burn in that when you don't have it, the FE is worse than what i got with the Del Sol. And after a long engine off coast, the LAF has to warm up all over again. This means that I need to rethink my route to take advantage of lean burn. I am still learning the tricks and techniques and Im not doing to bad considering the fact that I have a bad fuel relay and I have stoppedengine off coasting. As soon as my relay arrives, I will try to push the VX to the limit. Until then, I will use the time to learn how to hypermile all over again.
You're crazy! How long does it take to warm back up?
It depends on how long the engine is off. It might take 30-60 sec on the longer coasts. Often I am on, off, on and back off again over a 2 or 3 minute period. In that case, I would lose lean burn completely. It is possible that idling would result in better FE in that case because the LAF will stay warm. It must really suck in the winter. I might need to wrap my LAF in foil.
I think I might finally had the SuperMID calibrated. The problem I have been having is that my tank is very difficult to fill consistently so I don't know if I am consistently filling to the same level. But the last two fiulls, I jiggled the car after topping it off and the level didn't drop. In both cases I could see the fuel at the top of the filler tube. Hell, when I got home from the gas station, 0.116L later I checked the filler tube and I could still see the fuel. I'm pretty sure it is full.
Woooooooot! 708 miles, 9.715 gallons, 72.88 MPG!
The MID showed 67.0 MPG.
I have recalculated my fuel parameter to 14219. Ben, it is prolly safe to set yours to that number also.
What this means is that the numbers that I have been posting are about 8% low. It appears that the VX does indeed perform better than the Del Sol, which also makes sense.
So what I thought was a 73MPG PR to work turns out to be close to 79 MPG with the new calibration. I just got back from a highway run at 55MPH using DWL up the hills. Since I was in a 65 zone there was lots of traffic passing by pulling me along with them. The 34 mile RT (all in lean burn mode) was worth 76.1 MPG at 85F.