Fuel Problems

[WhatAmIDoing]

Exhaust is your problem. If you have a cat efficiency code (I bet you do), unplug the downstream O2 sensor and the car will think the cat is fine, but you must disconnect the battery for 15 minutes to reset the ECU. Either get that exhaust buttoned up tight, or order a new one. Without seeing it, it's hard for me to say what would be better.

I don't want to confuse things, but are you sure you can just unplug the downstream O2 sensor?

From what I understand, people install spacers for them when they delete cats to make them less sensitive. This would prevent the ECU from throwing the inefficiency code.

I agree though, the exhaust has to be tight. That would be the very first step to solve this.

Yeah, you can’t just unplug the O2. The Long term fuel trim will get all crazy, and do exactly what OPs problem describes. There’s been some theory running around for a long time that the rear O2 just checks the health of the cat, but it seems pretty clear that both of them influence the LTFT. A spacer could work, but fixing the exhaust and getting a cat in there would be the only real fix.
Or modify the programming.

One thing that is concerning, is the throttle lag. It kind of points to an air leak somewhere, when there’s not enough correction in fuel trims to account for it at some specific load point. OP; have you read error codes, and if so, what were they?

- S

Yes you can, been driving with the downstream O2 sensor unplugged for 30k miles. Runs great, no limp mode, get 2 codes relating to bank1sensor2 voltage and heater circuit malfunction. Gas mileage is normal. Goes into closed loop. LTFT doesn't do anything weird. If I plug the sensor in, car runs exactly the same until it throws a cat efficiency code, then it runs like crap. Disconnect the sensor, disconnect the battery to reset codes and fuel trim, she's happy again. Maybe on the turbo ECUs they are tuned differently regarding what input the downstream O2 provides, as having it unplugged is causing no problems other than a CEL for me?

[wizechatmgr]

Neat little article that might want to be read by those doing a catalytic converter delete while keeping the car on-road -- specifically the fines involved... https://www.epa.gov/enforcement/clean-a ... esolutions

If one wishes to continue to roll the dice my advice is as follows (not that I condone it)
The ECU should be flashed or it is going to expect a change from upstream to downstream readings as if it had the presence of a converter.

[Sommerfeldt]

Yes you can, been driving with the downstream O2 sensor unplugged for 30k miles. Runs great, no limp mode, get 2 codes relating to bank1sensor2 voltage and heater circuit malfunction. Gas mileage is normal. Goes into closed loop. LTFT doesn't do anything weird. If I plug the sensor in, car runs exactly the same until it throws a cat efficiency code, then it runs like crap. Disconnect the sensor, disconnect the battery to reset codes and fuel trim, she's happy again. Maybe on the turbo ECUs they are tuned differently regarding what input the downstream O2 provides, as having it unplugged is causing no problems other than a CEL for me?

This doesn’t make much sense, tbh. The ECU doesn’t run a check to see if the cat is fine, it runs a diagnostic on the sensor itself. So unplugging it w/o disabling it should definitely lead to an error code, and also a CEL since it’s emissions stuff.

Also, if it does nothing but check the cat, plugging it back in should do nothing to the FT, as you say it does - it should simply report an issue with the cat and set a code. “Runs like crap” is a little vague, but I’d think it wouldn’t try to make the FT richer, since that could damage a cat, and going leaner is a risk to the engine. So what’s going on?

All in all, it’s clear that it’s far better to get a cat in there, and the sensor working, if you have one installed.

- S

[WhatAmIDoing]

Yes you can, been driving with the downstream O2 sensor unplugged for 30k miles. Runs great, no limp mode, get 2 codes relating to bank1sensor2 voltage and heater circuit malfunction. Gas mileage is normal. Goes into closed loop. LTFT doesn't do anything weird. If I plug the sensor in, car runs exactly the same until it throws a cat efficiency code, then it runs like crap. Disconnect the sensor, disconnect the battery to reset codes and fuel trim, she's happy again. Maybe on the turbo ECUs they are tuned differently regarding what input the downstream O2 provides, as having it unplugged is causing no problems other than a CEL for me?

This doesn’t make much sense, tbh. The ECU doesn’t run a check to see if the cat is fine, it runs a diagnostic on the sensor itself. So unplugging it w/o disabling it should definitely lead to an error code, and also a CEL since it’s emissions stuff.

Also, if it does nothing but check the cat, plugging it back in should do nothing to the FT, as you say it does - it should simply report an issue with the cat and set a code. “Runs like crap” is a little vague, but I’d think it wouldn’t try to make the FT richer, since that could damage a cat, and going leaner is a risk to the engine. So what’s going on?

All in all, it’s clear that it’s far better to get a cat in there, and the sensor working, if you have one installed.

- S

With the sensor plugged in, as soon as the ECU sets the DTC for low cat efficiency, car goes into limp mode and runs rich. With the sensor unplugged and ECU reset, sets code for unplugged O2 sensor and carries on like there's no problem with the cat. PO installed a one-piece SS turbo back exhaust with a high flow cat, so I'm not messing with that. I do agree that I'm not sure why it runs rich with a cat efficiency code, but I'm not the only one who has documented that behavior.

[Sommerfeldt]

Well... if it goes into some generic form if limp mode, then I guess running rich is logical. But still weird when the problem is the cat.
It’d be interesting to see if it’s possible to dig up the relationships between the rear O2, limp mode and fuel trims in the programming.

- S

[abscate]

ECM-4801 disables the rear heated OXS, I don’t see any notes about limp mode or disabling closed loop operation.