Bad O2 sensor? Fault Light not on.

[zorin]

Hello,

I noticed an increase in gas consumption in the 300E '86, so I decided to check the O2 sensor and the diagnostic ports. After reading few posts and comparing the test results, I thick I have a bad O2 sensor. However The fault light is not on. It does light for few seconds when the ignition key is on, which proably means the light is OK.

Test results:

Key on , Engine off :
duty cycle 70%,
EHA +19 mA

Engine idling at operating Temp:
EHA gradually increased to +13mA.
O2 output fluctuates around 1 mA
Duty Cycle 92.3%
O2 heater plug : 14 V

The voltage at the ECU plug of O2 sensor is about 470 mV. Is this normal ?

I applied 1.5 V to ECU plug of O2 sensor (connected to pin 8 on ECU), EHA current gradually increased to -12 mA.


Either O2 is OK and my mixture is way too lean (but I think this would cause a rought idle which I do not have) or O2 sensor is bad, always showing about 0 mV, and the EHA is trying to correct it by sending +13 mA.

Changing the mixture with the 3mm allen key 1 or 2 quarters does not change the output at the O2 sensor (it remains close to zero Volt). Should I turn it more to see a chage?

I have left the O2 sensor disconnected and using the system in open loop state. The car nuns great but I have no way of knowing whether my mixture is close to optimum.

Unfortunately, I did the same tests on the 260E '88 and I have the exact syptoms.

Can I be sure that it s the O2 sensor before I spend $240 CDN on a new one?


Thank you.