"closed loop" and test pipe

[Steiner]

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 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?

[earlyapex1]

Closed loop is when the ECU is looking at the front 02 sensor for feedback and changing fuel trims to make sure it hits stoic, 14.7:1. This occurs during idle, cruise and light throttle. It has nothing to do with the rear 02 sensor. The rear 02 sensor is there just to make sure the cat is running correctly, if not, it throws a cel, nothing more.

Open loop is when the ECU is going off the Fuel map to hit the target fuel. It doesn't use the front 02 during this. This is during WOT, high throttle, higher load, and cold start.

[EFIxMR]

Closed loop means the ECU is using the 02 sensor during cruise to adjust the AFR to 14.7

Open loop means the ECU is reading the tables in the fuel map under heavy load (WOT for example)

The test pipe does not affect gas mileage because the main 02 sensor which is used for Closed loop is before the cat. And is still functioning 100% as it would from the factory, otherwise it would throw a check engine code.

The rear secondary 02 sensor behind the cat from the factory checks the efficiency before and after the cat. If the secondard 02 readings stray from normal, it tells the ECU that the cat is broken or worn and throws a check engine code.

The mechanical 02 fix tricks the secondary 02 sensor by limiting the exhaust flow to the sensor when a test pipe is installed. The ecu believes the cat is still there and there is no check engine light activated.

On your car... It is only running a 10.8 - 11.2 AFR under heavy load or open loop.

Under cruise it is still using closed loop to maintain a 14.7 AFR.

So in short your friend is incorrect despite his 20+ years of being ASE certified.

[MarkSAE]

The ECU only uses your rear o2 signal to determine how efficient your cat is. The more exhaust that goes by the o2 sensor, the faster the signal cycles. The rear o2 signal is supposed to cycle ~4 times slower than the front. If it goes out of range, the CEL gets thrown.

The mechanical fix works by moving the rear o2 sensor further away from the stream of the exhaust. This will slow the signal down and helps it get back in the range needed not to throw a cel.

[lqdchkn]

Ne1 know where the thresholds, that the ECU uses to switch between modes, lie?

[fusionchicken]

this is why i love car specific forums. thanks for the responses, you knowledge is greatly appreciated :)

[earlyapex1]

Ne1 know where the thresholds, that the ECU uses to switch between modes, lie?


There are some tables in the EcuFlash that probably control some of this, but I haven't messed with them yet, not sure why you would want to unless you have larger mafs, etc:

Open Loop Throttle Low Load
Open Loop Throttle High Load
Open Loop Load #1
Open Loop Load #2

[lqdchkn]

*EDITED*

N/M reading comprehension owns me right now. :lol:

[Steiner]

Thanks for the info guys. I knew I wasn't running some sort of ghetto tune but I didn't know exactly how to explain it.