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Old 06-29-2006, 11:50 AM   #1
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Default O2 Sensor bad but no CEL?

This is on a 93 Miata.

I've just installed an intercooled greddy kit a couple of weeks ago. AFPR based with a bipes for timing control. Car seems to run pretty good. I have a part throttle in boost stumble I'm attempting to sort out with FMs O2 Clamp. Previous to the turbo install the car has always suffered from idle dip. It was really bad in the begining due to a failed clutch switch, and since has never been a serious problem. When coming to a stop the idle will generally go down to ~500rpm and then up to ~1000rpm and then settle right around ~900rpm. I just took this as character in the car. I've replaced plug wires and just recently plugs.

As part of installing the O2 clamp I was attempting to messure the voltage of the O2 sensor before and after the O2 Clamp so I could dial it in. Long story short the voltage observed at the O2 sensor with the car fully warmed up and idling was around 30-50mV. At one point the O2 sensor showed -275mV. So I disconnected the O2 sensor harness at the engine and did my readings from there and got similar numbers. My understanding is I should have been seeing 0V-1V.

No big deal, I have a spare 4wire O2 sensor. Then I got to thinking, why didn't my ECU throw a code for the O2 sensor (17 & ?). I wouldn't have any stored codes in the ECU because I disconnected the battery and ECU to install the O2 Clamp, but I did the diagnostics anyway. The CEL reacts normally, so I know it's working. But no codes (as to be expected).

I've never gotten great gas milage on this car, honestly I never figured out the mpg and don't really pay too much attention to how many Km per tank (I know, BAD car owner). Since installing the turbo gas milage has been harsh, but I expected that since I'm running rich because of fuel pressure and that I'm running 1.8l injectors (installed with the turbo), still have tuning to do.

So it's obvious that the O2 sensor is done, unless my testing procedure is suspect. How hard will this be to remove from the stock Greddy downpipe with the turbo/manifold still installled? I'm not worried about the faulty one being difficult to remove since it's only been in the DP for two weeks and I used anti-sieze when I installed it. Any tips on changing it out? It's either 7/8 or 22mm right?

The question that really plagues me now is why my ECU didn't throw a code for this.

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