Questions on ECU, oxygen sensor signal, injector pulse width for 700 series.

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.
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I haven't read all your post, just skimmed it (I will reread later). Have you tried resetting the ECU? Have you then take the car for a run and let it remap?
AJS
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snipped-for-privacy@roads.vic.gov.au says...

This is all based on the ignition plugs, wires, cap and rotor all being in good condition. Engine settings otherwise in stock or as originally set condition. Any Bosch Fuel injection system, starting with the 1970 system, reads the coolant temperature first. In the LH cars this sets open loop and increments the fuel map values as the coolant temp increases until it reaches the threshold switching value to go to open loop operation. At this point priority goes to the MAF. The air mass measurement is converted to a dwell angle or pulse width signal to the injectors with consideration given to whether on not the throttle is closed. The validity of this calculation is compared to the expected O2 value versus that reported by the O2 sensor. Whatever discrepancy is seen at the checkpoint time is used as a fudge factor to adjust the injector pulse width, and so on and so on.
There is a test point located in what appears to be a two pin AMP plug, IIRC, along the right front fender. There should be SB/W wire that when grounded closes the idle air control valve. A DMM connected between the G/W wire and ground should read a fluctuating voltage between ~2v and ~12v. This sweep is obtained by turning the adjustment screw on the MAF--you may ahve to drill two 1/8" holes in the aluminum plug inorder to remove it with needle nose pliers. The voltage sweep when reading the analog scale across the boot, of the meter should cycle evenly about once per second. A second meter connected across the O2 sensor and ground will sweep slightly slower but not as slow as .25 cps.
Lazy O2 sensors are mainly caused by tip contamination. Silicone spray used on intake hoses or seals can be deadly to O2 sensors. Antifreeze from leaky head gaskets or raw fuel from streaming injectors can also be a problem.
I'd check the thermostat first, to make sure the engine isn't running too cool; coolant reservoir for CO, to make sure the engine hasn't at some point over heated and damaged the head gaskets; coolant temp sensor for proper value, if the coolant temp signal is low the engine runs richer. Pull the vacuum hose from the fuel pressure regulator to make sure the diapragm isn't leaking. If that didn't turn up anything I'd make sure the injector grounds on the manifold were clean and tight, retest, then bypass the fuel pump relay to run the pumps with the key off, then pull the fuel rails and using a plug with pigtails and two jumper leads (switching the ground side), fire each injector in turn to check for wet tips, drips and streaming.
Other than a bad control unit, I think that's about it.
Bob
--
The goal of driving is to miss the maximum possible number of objects.

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Does this means I can influence the sweep rate by altering the AMM output? How easy is it to restore the orginal setting? simply remember which way the screw pointed? If the sweep rate comes up to 1 cps does this mean the AMM is just about correct?

I have already replaced the thermostat sometimes ago because the temperature gauge wasn't where it should be (in the middle pointing straight up.) Don't know how to find out that the engine is definitely warmed up though.

Don't quite know how to do this. Shouldn't there be "hard to start", "oil in coolant" and such symptoms?

Already done this, found no leak.

I'll do this ASAP. Perhaps this is the reason why the oxygen sensor signal looks funny.

Thanks Bob for a very detailed response.
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better to measure the exhaust CO reading ,it'll indicate the outcome of all you mentioned.
Duong Nguyen gJG

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