When I have my O2 sensor enabled on my PFC it makes my cars surge while moving. When I look at the readings on the commander it bounces from .7 to .3. I usally just leave it disabled but I want to know if this is just from a bad sensor or grounding or something else? Also when I come to a stop my idle will drop to about 600 rpms. It goes back to normal as soon as I come to a complete stop.

 

most people have the feedback disabled on the pfc

 

My guess would be low engine compression. Did it surge with the stock ECU installed?

 

The engine only has like 5k on it and I get 400mmhg at a 720 idle so I think the engine is good. I guess I just have to get a new 02 sensor and see. I just want to make sure everything is running right before I put on the midpipe and get it tuned.

 

How long have you had your PFC? Last time I did any major changes, my car did the same thing. I think it had to do with ignition and fuel. I also suspected that my computer was still learning (freshly installed PFC), even though I went through the idle learning. But it also did it on my stock ECU after some major changes to the car.

 

Autozone sells a compression gauge for $25. Check to see if the results are within spec. My guess is your motor is at or below 85psi.

 

Unless you need to pass an emissions test, I would leave it disabled. It will be much smoother. To echo mad_7tist, most PFC people really do leave this disabled. Most of the time when people use the datalogit software to "recalc" the PFC fuel maps it screws up the o2 feedback settings anyway, rendering it O2 feedback useless.

On the RPM dropping problem, you may want to try raising your fuel cut setting another 100 rpms or so for each idle setting. If it's set too low, when rpms drop it will take too long to recover and will do what you describe.

 

I think that's a little premature don't you? If he turns off the O2 feedback, and it runs OK, then it isn't compression... and besides, there would be lots of other signs of low compression like low vaccum and hard starting that he isn't seeing.

You can get a surge in these cars by just having a bad ground... but it's likely that either your sensor is bad, or it's competing with some other sensor/setting.

 

I agree with Ptrhahn. No obvious reason to assume a mechanical problem. The sensor is supposed to jump between .2 and .8 millivolts.

That means it's working, but it could still be lazy, skewed, or fouled.

I had a surging problem at cruise that went away with a new o2 sensor. If the problem goes away when you disable the sensor, try changing the sensor. Cheap test.

 

O2 sensors can get "lazy" as they get older, and they react slower. This can cause you to feel the ECU cycling at closed loop. Also, there should be a ground strap going to the exhaust at the main cat to cat-back flange - these are frequently torn/missing. A good ground helps the O2 sensor's signal quite a bit - it's such low voltage, it needs all the help it can get.

Dale

 

Im pretty sure its not low compression. Its probably just a faulty O2. The ground is still on at the cat-back but I dont know how good of shape it is. The idle thing is kinda weird I dont want to adjust anything. I think it should work without me adjusting anything. I think it may have just been something that happened when I first let the PFC learn the idle. Im gonna look for a oil leak this weekend port my wastegate and do a full tune up, then ill let it relearn the idle and get it tuned.

 

When i had my single turbo installed on my car, the O2 sensor caused the exact same reaction from my car, i dont have a clue why!