I thought I was again approaching the end of my "To Do" list when something just jumped up and bit me. I'd had noticed my MPG has been lower than normal for the last tank or two so I decided to temporarily install an Air/Fuel ratio meter as a trouble shooting step. This allows me to monitor what the L-Jectronic computer should be receiving from it's Narrow Band O2 exhaust sensor.
Once this older style O2 sensor is up to operating temp it provides a toggling feed back of either (aprox) a .1 volt signal when it sees a lean exhaust or a .9 volt signal when a rich exhaust mixture is sensed. If everything in the L-Jet system is working properly and it is happy with all it's other inputs - the computer reacts to adjust the fuel injector pulse width by chasing these two O2 sensor voltages. The computer initially fires off the injectors and then waits for the O2 sensor to tell it "What's Happened" in the cylinders. If a .1 volt (lean) signal is received, the computer immediately enriches (lengthens) the next series of injector pulse widths. This generates a slightly richer exhaust moment. Then after receiving the resulting .9 volt (rich) signal from the O2 sensor, the computer now shortens the next series of injector pulse widths to once again lean out the mixture. In theory the average of the toggling lean/rich injector pulse width adjustments are supposed to produce the IDEAL fuel MPG vs emission setting over all. The newer "Wide Band" O2 sensors can produce a vast variety of stair step feedback voltages which gives their computers a lot more information to work with. With today's faster computers, the older designed "Narrow Band" O2 sensor is considered a bit crude when it comes to controlling the engine emissions during a heavy acceleration or hard decelerating event. Although it takes a little longer (compared to the Wide Band Systems) for the older computers to catch up on it's calculations, it will eventually return to producing it's toggling injector command sequence once a steady speed is reached.
My MPG problem has turn out to be the result of an unhappy computer commanding a constant rich mixture for some reason and ignoring it's normal toggling lean/rich injector mode of operation. The O2 sensor is generating a constant .9 volt signal as seen on the Air/Fuel mixture meter.
I'm now assuming Miss Purdy is operating in the OPEN LOOP MODE and the problem is not a failed O2 sensor because I just replaced it with no change in the symptoms. The heater supply voltage source is good and the new one tested good using a butane torch before installation.
I think it may be now down to a couple of wiring possibilities: 1) The O2 sense signal is not reaching the computer 2) The Temp II Sensor has failed and/or not connected to the computer - the computer may think the engine is always cold ? or 3) One of the Fuel Injectors and/or Cold Start Injector is leaking 4) Fuel Pump pressure too high 5) Miss Purdy is a closet hypochondriac .........It's actually still running pretty good - any additional suggestions would be appreciated.
My wife always feels the best way to fix a 928 is unscrew the radiator cap and drive a new car under it.
Michael