3FE - how do they run w/o o2 sensors?

[PabloCruise]

Hey All,

I do not know much abouth the 3FE.

I drove a 3FE Cruiser that had the 02 sensors disconnected (new exhaust being fabbed), and it ran like poo.

Not much power, a little trouble idling...

How do 3FE's behave when the o2 sensors are bad or offline?

TIA,

 

[robjam]

If I understand correctly, the O2 sensors control the fuel/air mix. When the sensors fails or is removed the ECU uses a set fuel/air mix and can not adjust for changing engine performance / environment. So, it should run, but in a 'safe' or 'limp' mode since it can not regulate the changing needs of the engine.
Hope that helped.
Anybody got a better explanation, please?

Rob

 

[Pskhaat]

I don't think it runs poorly, but from when my O2 sensors disconnected appears to me to run consistently rich probably to avoid a lean condition.

 

[PabloCruise]

Anyone else?

 

[Mace]

I think Rob nailed it.

the computer should have a "limp home" mode. so when you disconnect the O2 sensor it goes into a mode that will run crappy but still run..

 

[PabloCruise]

Should this include trouble idling while coasting?

 

[Mace]

I would say yes, the computer really has no idea what the mixture is so at idle I would imagine it would run like crap..

 

[import silvia]

It runs like crap without the O2 sensors. The idle is all over the place, and there is no real low end torque to speak of. The engine finally gets running decent in the middle rpms, but your dumping fuel the whole time.

 

[Pskhaat]

My experience is that it is hard to start w/o O2s but should idle just fine.

 

[euclid]

You might do some research into putting a dummy load where the O2 sensors were. If you could figgure out what the ECU is looking for you could probably trick it. Another option is that if it runs rich you could modify the Air Flow Meter a bit to account change the fuel air mixture.

 

[Cruisergreg]

the computer should have a "limp home" mode.

It's not really a limp mode, but an "open loop" mode, which means the computer is not using feedback from the O2 sensor to manage the fuel/air ratio (which is set a 14.7:1, the most effiecient stochiometric ratio for use with the cat). The motor runs in open loop until it reaches normal operating temp, and sometimes at idle, or when the O2 sensor is dosconnected