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You removed the oxygen sensor and replaced with twice new Bosch oxygen sensors.
Excellent engine compression numbers!
With your electric multimeter the replacement oxygen sensors appear lazy. Yet show a limited range delivering a voltage output at 0.5 volts DC and above with occasional dips to 0.4 VDC volts?
A working oxygen sensor:
- Encounters a rich, oxygen poor (relatively) fuel to air ratio and so the fuel ECU will lean the fuel to air ratio in what is called fuel trim control. Like an exhaust leak at the header pipe output to catalytic converter input leak that is in front of the oxygen sensor (more oxygen in the exhaust)
- Encounters a lean, oxygen rich environment, and the fuel ECU enrichens the fuel to air ratio. Like a high up exhaust leak at the cylinder head to exhaust manifold interface.
With exhaust works cold, a spray bottle with some dish detergent and soak the exhaust unions upstream of the catalytic converter. Use a remote start or use an assitant as you hurridly soak the unions for bubble.
The critical exhaust union at the header pipe output to catalytic converter input. Just upstream is a bracket secured to the bellhousing. There is an intermediate carrier angle plate between this bellhousing bracket and the header pipe. This exhaust support failed on my 1991 240 sedan and that exhaust union leaked both ways (sucking ambient air between exhaust pulses) and lowing MPG considerably. I had to replace the forward pointing studs at the catalytic converter input with beefier thermal proof hard (nuts, washer, bolts)
Yet the Bosch ECU uses multiple sensor and telemetry sensor inputs. There is a hierarchy of show the ECU uses and responds to sensor input.
Have you verified you mileage using a GPS device? Old Volvo odometers lose accuracy?
And you've replaced the FPR. A new part can be faulty. You've tested the fuel pressure and applied vacuum to the FPR vacuum line. High vacuum is throttle closed and less fuel injected. Low vacuum near ambient is gas pedal floored and throttle wide open and the FPR partially closes to raise fuel injector pressure.
Your 1992 Volvo 940 does not have a cold start injector, yet it may be useful to check for faulty fuel injector spray pattern? Maybe an injector is delivering too much fuel that shows a relatively rich exhaust.
I've not pulled an injector since the early 2100s. There appears description for testing injectors on the car so long as you have a receptacle large enough to contains the spray and fuel that passes into a container.
You get good voltages at the battery terminals with engine off, engine on not started, and engine start. A weak battery, poor charging less so, and corroded ground connections can cause poor fuel economy. I'll guess you've tested the electrics for these items.
And no brake drag. Lift your 940 and you can easily turn the wheels. Or after a sort of long drive, no one wheels is hotter then the others as brake drag can cause. You brake fluid is clear, not black? Black brake fluid causes calipers to become slow to engage, release, ultimately binding open or closed.
You've also verified proper air intake manifold vacuum be in inches of mercury or another fluid pressure scale at the air intake manifold?
And I'll guess you've read the Volvo 700 / 900 Series FAQ here or at the large UK site?
https://www.brickboard.com/FAQ/700-900/
https://www.volvoclub.org.uk/faq/FAQSummary1.html
If you have searched this forum for posts and threads that address poor fuel economy? Search at the UK site. Doubtful yet turbobricks.com in the Maintenance and Non-Performance forums may offer you insight. Helps to have a Turbobricks log in account.
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Woo.
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