Tom F&L GoR
Enthusiast
Since O2 sensors aren't usually a tune-up item, I tend to assume they're fine until they light a check engine light... wrong.
I have a '94 Gen 1 with 52,000 miles, therefore ODB 1. Engine has 3" exhaust, 1.7 rockers, and smooth tubes. I sort of got used to the snorty rough sound and feel after cold starts (blaming the factory fixed open loop fuel strategy not working well with the engine mods), but lately the engine had definite "shifts" that made it feel like someone was retarding the timing and then advancing the timing with a flick of a switch. This started happening only when cold, but then it would happen after a shutdown and restart that allowed the O2 sensors (but not necessarily the coolant temp) to cool.
I borrowed an OTC 4000 from work to watch the sensor inputs to the ECU; thinking it was something big like a throttle position or manifold pressure sensor. What I saw was that both O2 sensors would show lean, one side would occasionally "freeze" at low voltages (they should be 0 to 1 volts, midway being stoichiometric.) But because the signal abnormalities were only a second or two, it apparently wouldn't set a code. In fact, there were never any trouble codes stored. Running lean, the engine feels down on power and has the sound it gets on high RPM decels.
All of this wouldn't happen until about two minutes after the start, and then go away when it was fully warmed up.
I replaced both O2 sensors, and both the old O2 sensor heating elements read the required resistance, about 4 ohms. But the engine runs much "smoother" and all the rough start running is gone. I can only assume that even though the O2 sensors were being warmed electrically, they weren't maintaining enough temperature to operate correctly. What is still a mystery is that they were as bad as they were, but never set a trouble code.
Anyway, at about $90 list, $73 friendly price, (need two) it might be something higher mile cars want to put on the list. And if anyone has other experiences, please share.
I have a '94 Gen 1 with 52,000 miles, therefore ODB 1. Engine has 3" exhaust, 1.7 rockers, and smooth tubes. I sort of got used to the snorty rough sound and feel after cold starts (blaming the factory fixed open loop fuel strategy not working well with the engine mods), but lately the engine had definite "shifts" that made it feel like someone was retarding the timing and then advancing the timing with a flick of a switch. This started happening only when cold, but then it would happen after a shutdown and restart that allowed the O2 sensors (but not necessarily the coolant temp) to cool.
I borrowed an OTC 4000 from work to watch the sensor inputs to the ECU; thinking it was something big like a throttle position or manifold pressure sensor. What I saw was that both O2 sensors would show lean, one side would occasionally "freeze" at low voltages (they should be 0 to 1 volts, midway being stoichiometric.) But because the signal abnormalities were only a second or two, it apparently wouldn't set a code. In fact, there were never any trouble codes stored. Running lean, the engine feels down on power and has the sound it gets on high RPM decels.
All of this wouldn't happen until about two minutes after the start, and then go away when it was fully warmed up.
I replaced both O2 sensors, and both the old O2 sensor heating elements read the required resistance, about 4 ohms. But the engine runs much "smoother" and all the rough start running is gone. I can only assume that even though the O2 sensors were being warmed electrically, they weren't maintaining enough temperature to operate correctly. What is still a mystery is that they were as bad as they were, but never set a trouble code.
Anyway, at about $90 list, $73 friendly price, (need two) it might be something higher mile cars want to put on the list. And if anyone has other experiences, please share.
