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Toyota oxygen sensor question

Recently my wife?s 1993 Toyota Corolla 4WD wagon (4AFE 1600 engine,

170,000 km) failed an emission test due to high CO levels. A quick test showed no output voltage at the oxygen sensor. Solution seemed straightforward- I simply installed a new Bosch OEM sensor. Unfortunately, CO levels did not go down and the behavior of the engine control system seems strange. The oxygen sensor output waveform (blue) and the feedback test signal VF1 (red) recorded in test mode (pins E1 and TE1 on the diagnostic connector shorted) are at
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In words, the sensor output fluctuates between 0.3 and 0.6 V with more than 10 cross-counts in 10 seconds. The output doesn?t swing to the expected 0.2V on the low side and 0.8V on the high side. The output responds quickly when the throttle is snapped shut, slewing to a high of over 0.8 V in about 0.1 s and then rapidly falling back to under 0.2 V as the feedback system regains control. However, I find no change in sensor output when I try to force a rich condition by pinching off the fuel return hose, and introducing a small air leak in the intake manifold by disconnecting a vacuum hose also has little effect. This certainly doesn?t seem correct. I wonder if anyone could comment on whether this is normal behavior for a Toyota engine control system, or have an idea on what might be wrong. The control computer shows no error codes in test mode. Spark plugs, wires and distributor cap are new. Compression tests at over 180 psi in all cylinders (specification is 195 psi for a new engine). The car runs fine and fuel consumption doesn?t seem excessive.

Thanks in advance,

Garry

Reply to
Garry Tarr
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You haven't posted any before/after emissions test results or the type of test administered. It would be nice to know as much info as possible. Emissions problems at idle can be nailed down pretty easy on this system, while problems that only occur under load could require considerable diagnosis.

Were you holding the engine RPM at 2500 during your VF output test? The narrow ranging of the O2 sensor would lead me to guess that you were doing your test at idle speed, or there is some sensor-related feedback issue.

I assume your 4WD is an automatic trans?

Things beside a dead O2 sensor that are big contributors to high CO are too high fuel pressure (bad regulator) or volume (stuck open injector), an out of calibration MAP sensor, or poor ground at the MAP sensor.

I would have gone with a Nippon Denso sensor instead of the Bosch unit

- regardless of "OEM" branding. Sure could save a lot of headaches.

Toyota MDT in MO

Reply to
Comboverfish

First test: (bad O2 sensor)

Idle HC 58ppm CO 0.05%

2500RPM no load HC 87ppm CO 1.58%

Second test (new O2 sensor)

Idle HC 193ppm CO 0.66%

2500RPM no load HC 138ppm CO 3.68%

I should note that the second test was conducted after prolonged idling, due to a long line-up at the test station. I should probaby have come back another day, but I was overconfident that the new O2 sensor would solve the problem.

The results shown were in fact at 2500RPM fast idle.

No, it's manual.

Today I had a chance to discuss the problem with a Toyota expert. He also noted that Bosch O2 sensors don't work well in Toyotas. He stated the sensor output should indeed swing from 0.2 to 0.8V.

Thanks for the tips on other possible causes. I'm suspicious of the fuel pressure regulator, since pinching off the return line has no effect on O2 sensor output. I will clean all the ground connections and test the map sensor.

Reply to
ngt

This isn't your problem.

I would be concerned that the Bosch unit was designed with the wrong circuit impedance or otherwise had a capacitance issue due to designing a "one sensor fits most" type of part with a Toyota pigtail harnass slapped on it. This is not to say that the O2 sensor is the root cause. I would put the old one back on if I knew that it worked OK, though.

With multiple symptoms like you describe, I would look at the basics. Timing belt and pulley alignment, fuel pressure and volume, spark quality, and plug condition (use ND or NGK). Then I would look at MAP, CTS, and TPS signals if the basics checked out OK.

Toyota MDT in MO

Reply to
Comboverfish

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