O2 Sensor possible cause of rough idle.
 

I have an 87 stock 911 with 62Kmiles. The idle is constant in RPM but seems to cycle from smooth to slightly rough about once per second. Neither mode is very annoying, but the cycling between the mode is annoying.

Also, durring constant throttle application at constant speed, a simular cycling in noticable with a slight surge and lull in acceleration.

I am an engineer an somewhat familar with the Motronic engine management system. I have the Bently manual and am using it as a guide.

I placed a voltmeter on the signal from the O2 sensor and it swings back and forth between .1 and .9 volts. The swing in the voltage corresponds to the idle oscillation. It is more diffecult to match the slight surging durring acceleration to the signal I see on the volt meter. Durring higher RPM operation the voltage signal from the 02 sensor swings back and forth at a faster rate.

My question:

1. Is this a symptom of an O2 sensor or is the O2 sensor just indicating another problem?

2. Is the rate at which the voltage swings too slow? If it were to swing faster I wouldn't notice the changes in performance?

3. Any experience or advice on tracking down this issue?

4. An O2 sensor is recomended at 60K miles. What does a partially operating O2 sensor act like?

Unplug your O2 sensor and see if it still happens. My '84 does the same thing. I unplugged the O2 sensor and it stopped for a few hundred miles, but now it is back, not as bad as with the O2 sensor plugged in....

I'm interested in seeing other responses to this as the question has been raised a few times now.

I think you are talking about cross counts, the number of times the 02 sensor voltage crosses the .45V threashold as it swings from rich to lean and back again in a second.

http://www.wps.com/LPG/o2sensor.html

Cheers,

Joe

Anyone know if those voltages are universal? If so, why the wide variety (and $ range) of sensors, even among same # of wire sensors?

Has anyone ever hooked a potentiometer between their potentiometer and the computer to tune their emissions? Seems like it might be especially handy on inspection day with a cat bypass and chip installed.

I think the O2 sensor goes lean(low voltage) to rich(high) about 30ish times a second and shouldn't be able to be read by a DVOM. These things are probably the hardest things to relly test on the car.

I think the best way is to get a plotting osciliscope, or a scanner with memory recall and a good update rate(most cheap ones can't keep up). There are a lot of things to test with these things. You have to run it full rich and lean to see how far the voltage will swing, the number of cross counts (a lazy, unresponsive sensor may pass the previous test), and see if the slope of the plot is a nice smooth line.
OBD2 (94 and later) computers run a self test once and also keeps track of the voltages its putting out to compare it to "logic" tables to see if the info makes sense.

Also most cars go into "open loop" at wide open throttle-ignoring the oxgen sensor and using the throttle positioner sensor, the MAP and the engine ccoolant temp sensor for plotting fuel curves. A bad WOT switch could change the emmisions badly.

about once per second

How could you notice 30 cross counts in one second unless your idle counts were much lower? Seems like 1 or 2 counts per second at idle are more likely and about 10 counts above idle are what I get with my scan tool connected to my GM vin C.

I'm not sure if this is standard, but my scan tool gives cross count totals in 2.5 second batches.

Joe

30 times back and forth a second is too much for any 02 sensor, even a new one. At idle on a Motronic 911, with a new sensor, voltage will swing back and forth of stoichiometric from 0.1 to 0.8 volts about once every one or two seconds. Rev up the motor, and it may be twice that. It should be easily readable by any respectable DVM such as a Fluke.

If your engine rpms are oscillating up and down over a wide range, you may have intake air leaks or perhaps clogged injectors that the O2 sensor is trying to compensate for. The intake manifold gasket are notorious for shrinking, drying out, and letting in huge amounts of unmetered air. Spray some engine starting fluid (ether) at those points, and watch your idle shoot up. Idle on your 87 should be pretty rock steady and if there are any idle variations due to O2 swings, it should not be more than +/- 25 rpm.

Your acceleration should be smooth and I am surprised you notice some kind of cycling. Most likely you have other problems.

The RPM does not change rather the smoothness of the engine operation changes. It runs slightly rough when the output of the 02 sensor is lean (.1 volt ) and runs smoothly when the output of the 02 sensor is rich (.8 volt ).

I kept the fluke connected to the output of the O2 sensor and disconnected it from the harness. It output went to .8 volts ( rich ) and the engine idled smoothly. I drove the car with the meter connected and it ran great. So much better that I am asshamed that it was so poor before. It was much more powerful at low RPM. It was much more divable: I had tended to leave it in higher RPM ranges unknowingly.

I brought it back home after a 20 minute drive and reconnected the sensor. The voltage went to about .45 for about 10 seconds while the engine ran smoothly. I then began to cycle between .1 and .8 every 1-2 seconds and the annoying behaviour continued. I drove it again and the poor performance especially at low RPM was noticable. I stopped at a parking lot disconnected the sensor and returned.

What does this tell me?

Can it be the 02 sensor?
How do they fail or do the just get sluggish?

It seems to be working well but maybe there is a delay between the actual exhaust gas content and the measured content due to sensor contamination.

I like the thought of a vacume leak. My 77 beetle was very prone to vacume leaks causing poor performance of the fuel injection system.

I can use a portible scope from work and possibly do some more detailed investigation but I am not sure what else to look for.

I fear if I buy an O2 sensor, it will not solve the problem.

When I unplugged the o2 sensor, the car ran strong and smooth.

I finally changed the 02 sensor to a universal one: Bosch 13913, which was mentioned several places on this sight. Had to cut the connector and grommet off and crimp and heat shrink the new sensor on. $35.00

The car runs smooth and strong ( almost as strong as when the sensor was disconnected.

No idle hunting, no low RPM divability concerns.

The old sensor apears to have become slugish taking to long to respond and causing a minor instablity in the closed loop system.

Certainly nice to have the car back running smooth.