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In Reply to: Re: Fuel Consumption posted by vytas on June 26, 2001 at 09:32:55:
Voltage on one of the O2 sensors varies between 0.2 and 0.8 V,
- This is as it should be
while the other (slowly) between 0.4 and 0.6V. The car just failed emmisions.
- This is not as it should be and is at least one source of your problems. You should first check if the heater circuit is working for this one (corroded connector, etc.) before going to the extent of replacing the sensor.
I also cheked ECT inputs on both DME's (pin 78). While warming their reading deviate (0.96 and 0.91 V respectively at fully warmed engine).
- The Bentley give the resistances for the sensors but I don't have voltage specs for the output...would need to know the loop current which I doubt is readily available. You will need to either compare with others here to get a feel for typical output or more simply check the resistances at various temperatures:
-10 +/- 1ºC (14 +/- 2ºF) --> 7,000 to 11,600 ohm
+20 +/- 1ºC (68 +/- 2ºF) --> 2,100 to 2,900 ohm
+80 +/- 1ºC (176 +/- 2ºF) --> 270 to 400 ohm
I believe I recall the thermostat setting is about 85ºC which would put the sensor resistance roughly in the 200 to 350 ohm range at operating temperature. Assuming the A/C is not on, the low speed fan cuts in at 91ºC and the high speed at 99ºC so you can get an idea of the operating temperature your engine when you do the test.
If you have the Bentley (pg. 130-35) you will know this but just in case, the ECT sensor for the DME is the one furthest away form the angled one on the end of the crossover pipe underneath the coolant reservoir tank (don't know of an easy way in without removing this tank).
a) pull the connector, turn on the ignition and measure voltage between the center pin and each of the outside two. There should be 5V.
b) turn off the ignition, go to the sensor and check resistance vs. temperature between the center pin and each of the outside two. They should match the above table.
Is this normal and what could be causing rich condition? MAF was replaced last year, o2 sensor codes sometimes appear.
- From what you have been able to provide (good info by the way - accessing the DME connector is great stuff for diagnostics), it would appear your problem stems from one O2 sensor. I don't expect the ECT sensor will end up being your problem from the voltage numbers you gave. For example, a reading of 5V would indicate an open circuit and 0V a short so your 0.9V represents a relatively low resistance corresponding to what you would expect from the chart.
- As for what is causing the rich condition, a low voltage output from the O2 sensor tells the DME that you have a lambda ratio >1.0 (lean) and the DME will increase the injector duration time to compensate. I wrote a little of the theory earlier in this string - see the link for more info.
Hope this helps,
'88 750iL
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