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Hi.
I am new to posting here but have seen and used FAQs and posts for my ’96 940 8 valve 230FD and before that an ’89 740 230F.
My latest challenge has to do with bad gas mileage, most noticeably with city driving. With about 20% highway, I am doing at best 13 Ltrs/100 km (18 MPG-US or 21 MPG-CAN). I have never paid too much attention to fuel consumption in the past as I knew that it was generally high, but as I don’t use this car a lot, it was never much of a concern, until gas prices started hitting $1.30+ a liter ($4.90 a US gallon).
Published fuel consumption is 12.4 to 8.3 Ltrs/100km (19 to 28 MPG-US or 22 to 34 MPG-Can).
With 266K km (166K miles) on the odometer and a new 02 sensor with only 43K km (27K miles) that I put in 5 years ago when the factory original failed, I went looking for tune-up issues, fuel problems and engine air leaks.
Checked the 02 sensor for any obvious misses; found sensor heater good @ 2.5 ohms, sensor lead with .514 V powering up, going to closed loop in about 30-40 seconds and cross over cycling that appeared normal. As part of a somewhat overdue tune-up: replaced plugs, wire set, filters and cleaned/adjusted fuel body + TPS.
As found in the FAQs, tested for fuel pressure, residual pressure, FP regulator cut-in, ECT sensor (and EGR for good measure while I was in there) – all worked within spec.
Replaced a few of the small vacuum hoses that were not leaking - but, looked a little loose with hardening at the ends.
Pulled the injectors and decided to have these cleaned professionally. They were not leaking and, while the flow rates turned out to be uneven by 4-10% and that #1 cylinder injector spray pattern was too broad – all this was much better than I expected for originals that had never been serviced. They came back clean with new seal kits and on spec for flow rate, flow level and spray pattern.
With no noticeable improvement fuel consumption, I went back after the O2 sensor after noticing the cycling was only going to a high of .785 V down to less than .2 V - at times down to zero or very slightly negative, like -.025 V.
From FAQs, ran the tests to see if the sensor was picking up rich and lean mixtures.
At 2500 rpm, with a hot but disconnected sensor: sensor voltage went from a steady .7 V to .895 V when removing FP regulator vacuum hose and plugging it.
Again at 2500 rpm with a disconnected sensor, sensor voltage went from a steady .7 V to zero (hovering between -.025 V and +.025 V) when creating a vacuum leak by pulling the Idle Air Control valve hose from the air flow meter hose just ahead of the throttle body.
Without a scope to see the voltage pattern, I can only think that the 02 sensor is a little ‘tired’ and showing an average leaner condition than is actually happening and the ECU is responding with a slightly richer mixture, leading to higher fuel consumption.
My next step is to replace the 02 sensor. Having read that there is really only one way to confirm a flaky 02 sensor is to have another (good?) one to compare with.
Any thoughts or ideas would be much appreciated.
Thanks, André
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