I understand next to nothing about "closed loop" and "open loop" conditions. From what little I've researched over the last few years, it sounds like when boosting the car is in open loop and when off the boost the car is in closed loop. I couldn't explain what that any of that REALLY means though.

My question comes after having a conversation with a friend of a friend and an ASE certified mechanic of 20+ years. He asked me what I'd done to the car. When I mentioned the test pipe he told me I'm wasting tons of gas because the ECU is stuck in a "closed loop". He also mentioned something about the o2 sensor. I was totally out of my element. He can talk about this stuff in his sleep. Anyways I threw a couple things out, "I have a CEL fix flashed into the ECU". He said nope, something totally different. I told him the car's been tuned with the test pipe, AFR's are all between 10.8 and 11.2, and the knock sums are well within the desired range.

Can somebody tell me what he's talking about and if it's accurate?

 

I'm no mechanic, certified or otherwise, but what you're saying doesn't make much sense to me. Closed loop simply refers to the logic of the control system ajusting its output depending on the environment. In other words, it makes changes based on what the sensors are showing. Your car operates in "closed loop" when cruising. The ECU is trying to maintain a perfect 14.7 stoichiometric ratio of air to fuel. So, it basically "listens" to the O2 sensor and if the ratio is too high (too much air) it adds fuel, if the ratio is too low (not enough air) it removes fuel. On the other hand, if you go WOT the ECU switches to open loop mode where it simply adds the fuel specified by the tune. It does not "close the loop" by making adjustments based on (for example) the O2 sensor.

Now, what I don't see is what this has to do with adding a test pipe. The O2 sensor in question is *before* the cat (or test pipe) so removing it wouldn't have much (if any) effect on the readings. The O2 sensor that is after the cat is simply there to determine if the cat is working or not. Finally, removing the cat means the exhaust is less restrictive and thus it takes less "effort" on the part of the engine to remove the exhaust gases. If anything, that should improve gas mileage (although be less environmentally friendly). So, I can't see why adding a test pipe would waste "tons of gas" or keep the ECU "stuck" in closed loop, but I'm a layman, maybe there's something I'm missing.

 

After a little research I was asking myself the same thing. In any event I've spoken to my friend after I learned a little more from a similair thread I posted on the NorCal boards. He said "Ohhhhh" when I explained where the 02 sensors are and what each is responsible for. I also mentioned the stoich AFR of 14.7-1 while in closed loop. Instead of going into a crazy mathematical equation, I just said that when off the boost the car is in closed loop and when on the boost it's in open loop. He told me "you should really take it for a tuneup". Once I'd explained that it had been dynotuned (probably 50+ dynopulls) he changed his tune. For some reason he was under the impression I'd made all sorts of hardware modifications to the car without a dynotune and flash.