I have been trying to debug a problem of falling fuel economy on my
760 V6, 1988, LHjet 2.2. The fuel economy for this car is now 5km/l (11.7mpg) but used to be around 6.5 km/litre (15mpg) city driving. This despite increasing the tyre pressure from 28 psi to 35 psi. There is no other symptom, the car still drives very well.The oxygen sensor signal when the car is fully warmed up and idling is from .2V to .8V with a lots of .2V noise spikes that seem to come from the computer (the disconnected oxygen sensor signal is clean.) The signal makes a complete cycle (i.e .2V to .8V and back to .2V) in about 4 seconds which I think is too slow. The transition time is about .4 second. It also responds in the correct direction to a lean condition (introducing a vacuum leak) and rich condition (removing vacuum from fuel pressure sensor.) albeit after to couple of seconds delay (why?). This oxygen sensor is new and produces similar signal to the old one which I thought was faulty.
The injector pulse width (which I forgot to measure) with this oxygen sensor signal drifting up and down, is absolutely constant indicating that the computer is probably still in open loop mode and ignoring the oxygen sensor. In that case, what is causing the change in the oxygen sensor signal? Is this new sensor faulty (and the old one also faulty?) I'm more inclined to think that there is still something that stops the computer from entering close loop mode instead of both sensor faulty. On the other hand, the lack of transitions on the oxygen sensor is the only thing remains that could prevent close loop. Is there another sensor signal that I haven't thought of? It is probably not a temperture sensor since in this car there is only one temperature sensor and the temperature gauge is exactly in the middle. Also the rpm has dropped down to the correct value of 750 rpm from a cold value of 1000 rpm indicating the the computer knows that the car is already warmed up.
My questions are:
1) What are the conditions that must be met before the computer enter close loop mode? 2) How can I determine if the computer is in close loop mode? 3) Does anybody know how the injector pulse width shoud behave with respect to the oxygen sensor signal? The range in milliseconds? 4) How many transitions should the oxygen sensor make in a second with the car warmed up and idling. 5) If I looked at the Air mass signal, oxygen sensor and injector pulse width what should I see? Pin number on the AMM? 6) And finally, does anybody have a program listing of the LHjet 2.2 EPROM or similar?As you can see I haven't go a clue what the problem is yet and really want to make sense of these signals. Any suggestion welcome.