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Re: O2 Sensor Output - Good or Bad? (archive)

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Posted by Rick Stillwell on April 03, 2001 at 23:54:26:

In Reply to: O2 Sensor Output - Good or Bad? posted by Wayne McKay on April 03, 2001 at 13:05:44:


There is a really good manual on the Bosch ME-Motronic available as a download from SAE.ORG. It describes the operation and theory of the Motronic ECM, as used in the 750.

It says the two-state oxygen sensor, like the one used on the 750, measures the excess-air factor, or lambda, and once heated to at least 600 degrees C, produces a voltage based on the lambda. A lambda greater than 1 is lean, and a lambda less than 1 is rich.

The voltage JUMPS vertically from 0.200V to 0.800V at a lambda of 1. At lambda 1.1 the voltage looks like about .070V. At lambda 0.9 the voltage looks like about 0.925V.

Low voltage reading (below 0.200 volts) = Lean Mixture (excess oxygen).
High voltage reading (above 0.800 volts) = rich mixture (excess fuel).

I believe that if the voltage is jumping back and forth above 0.800V and below 0.200V, the oxygen sensors are working fine.

I kept getting a very low reading on both of mine at idle (.080V - .250V), and so I replaced both sensors. The new ones did the same thing. I heated the old and new ones with a mapp gas torch and they quickly jumped up over 1.0 volts, so I knew they worked.

I put the new ones back in and discovered that they were simply telling me that the mixture was very lean at idle. If I restricted the air flow in front of the mass air flow sensor with my fingers while the engine was running, the oxygen sensor voltage would immediately jump from about 0.02 to 0.90V.

I must have one of the following problems:

1. Air leak between mass air flow sensors and engine.
2. Low fuel pressure (unlikely on both sides).
3. Fuel injector clogged.
4. Bad reading from mass air flow sensors (they check ok, though).
5. Throttle open too wide at idle.
6. Air leak into exhaust.

This lean reading problem occured after I finished re-gasketing the valve covers. I used Permatex Hylomar (great gooey sealant) on the intake manifolds and replaced all gaskets and the mass air flow sensor boot.
New o-rings on the fuel injectors. I cleaned the injectors in MEK, but never could really test them, except to disconnect them one at a time with the engine running and listen for changes in RPM. RPM change = working fuel injector in my simple test. All injectors buzz ok while running. The resistance was ok on all 12 injectors.

My Mass air flow sensor wires properly glow red hot for a second after shutting off the engine after running at least 300 rpm.

I replaced the crankcase ventilation stuff (volume control valve and non-return valve) on both sides.

I'm still troubleshooting my lean mixture problem, but it looks like your oxygen sensors are ok.

The oxygen sensor voltages are supposed to jump back and forth as the engine quickly enriches and leans the mixture based on the feedback from them.
I don't know that the voltage should spend an equal time high and low, only that it responds extremely quickly and should jump up and down.

If it stays high or low, you'll get a check engine light.

If low, I think it will try to richen the mixture to a point, then assume it's getting a bad input and go into limp mode.

What I'm learning is that what appears to be an oxygen sensor failure may only be the symptom of another problem.

I will find the source of my problem if it kills me. I need to speed around in this thing again!

Good Luck!

Rick Stillwell
1990 750iL

Thanks Rick Stillwell for your timely March 28 post on getting inside the DME connector to check O2 sensor output. I was just at that juncture in my diagnostics and was thinking there must be a way to do it rather than dropping the exhaust. Just saying it was possible gave me the incentive to rip at it.

Bentley suggests the output should cycle between 0.8V and 0.2V. Mine started properly at 0.45V until they reached operating temperature but cycled between 0.88V and 0.08V! The top end isn't too bad but way low on the bottom end. The engine is currently running rich and therefore rough at idle.
1) Could these readings be the cause (i.e. tricking the DME to richen the mixture),
2) the effect (i.e. leaking injector(?), etc., causing mixture to affect the sensors),
3) or neither?
4) Are the sensors still OK?
5) Does a rich mixture produce a higher or lower output?
Can someone please enlighten me...I am assuming ideal sensor output would have equal duration at 0.8V and at 0.2V giving an "RMS" output (feedback signal) of ~0.5V. Then as the O2 content changes, the relative duration changes producing a higher or lower RMS voltage to the DME which adjusts outputs (injectors, etc.) to bring the system back to the 0.5V setpoint.

'88 750iL 192k kms (119k mi)
P.S. Good to have you back Max!!

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