Cat bypass - O2 sensor question

[jackb911]

I did a search and didn't find a definitive answer, so here goes...

1987 3.2 Carrera, bone stock engine except for a GHL cat bypass pipe. I've been told that I should unplug the O2 sensor. I've read that this will make the engine run too rich (around 11:1 as opposed to an ideal 14.7:1).

Should I unplug it since the car is de-catted, or leave in place?

Thanks in advance.

[Bob Kontak]

I am pretty sure it is disabled under heavy acceleration. Why piss away several mpg when you are just bumming around on the street?

[rattlsnak]

Who told you to unplug it? I wouldnt...

[KTL]

Under full throttle the full throttle microswitch tells the ECU to default to a specific mapping in the chip. Meaning, the O2 isn't used to monitor the AFR for the stoichiometric 14.7. When you disconnect the O2, the ECU runs in open loop mode. Kind of like a limp mode to allow for decent running w/out the monitoring effect provided by the O2. The only reason you would want to run w/out the O2 on a Motronic car is if you have a chip programmed to do without it.

[jackb911]

Thanks, guys...great information. It will remain hooked up. The car runs perfectly as is so I assumed it was best to leave well enough alone.

This clarifies why most of the the Cat bypass pipes and pre-mufflers that are available have O2 sensor bungs.

[nkotselas]

I've got a question about something similar,
I have a euro 3.2 in a euro 78 sc. Neither is supposed to have an o2 sensor but there is one on my car. I don't believe it was wired into the dme after the engine conversion. I have now bought a euro premuffler. Should I disconnect the o2 sensor and replace it with something just to close the hole? Or should I just install the sensor because not being wired to the dme, it will have no effect? I don't think the premuffler has a hole now so i hope don't have to drill one to accommodate the sensor

Thanks

[lucittm]

Nick, where is the O2 sensor physically connected right now? You said it is "on" your car, can you be more specific. If it was not wired-in, the connector would be dangling in your engine compartment, is it? When you say "disconnect the O2 sensor" what are you disconnecting it from and is this a physical disconnect or an electrical disconnect? You said you may "close the hole", what hole are you referring to? If the premuffler has no hole, why would you contemplate drilling one, welding in a bung, installing the O2 sensor and routing the wiring, if there is nothing to connect it to?

More questions than answers here, sorry. But I had a 1977 S model, and I have been where you are now.

Mark

[nkotselas]

Mark,
The car is a euro sc. It was imported in the early 80s to CA. To pass smog a catalytic converter was installed with an 02 sensor. I just this year installed a euro 3.2 but kept my cat and muffler on the car. I don't believe the 02 sensor was wired to the dme. Is it then ineffective? I have just bought a euro premuffler without a hole for the sensor. Do I need to make one or will the car run how it was originally built without the sensor?
Thanks for helping out

[lucittm]

I understand now that you have a mix of exhaust pieces in addition to an engine swap. I was not aware the euro cars did not have the O2 sensor because this is not an emissions piece, more of a fuel efficiency piece. I would check with someone that knows the euro specs to be sure. The O2 sensor is your friend, it makes for better mixtures across the RPM band. Without it, the system operates in "open loop" and is not as efficient.

Next issue. If you are talking about removing the cat and installing a premuffler in the good state of CA, I would caution against that. There have been many posts in this forum regarding CA rules on cats. No one will advise you to remove a cat because when your car gets sniffed by the side-of-the-road emissions folks or your car is inspected at random, they do not want to be responsible for the impounding or fine. If you are racing at Del Mar, that's different.

In any case, I would not drill a hole in a perfectly good euro premuffler. I would install the O2 sensor (weld the bung in place if necessary) upstream of the premuffler where both pipes come together.

Mark

This is one of the reasons I left San Diego (El Cajon, actually) several years ago...

[nkotselas]

I don't remember exactly how the euro cars worked without the sensors but im almost positive that is the case. It was a good idea to move out, nothing is more frustrating than these smog laws.
I do know it is not legal but I got the impression that many people were doing this. I plan on swapping the cat back on for the test every other year but I have never been checked besides that so I doubt I would get caught. That's also why I prefer the premuffler, it looks like a cat. And people probably can't recognize a 30 year old exhaust system either

[Josh D]

Originally Posted by nkotselas

I don't remember exactly how the euro cars worked without the sensors but im almost positive that is the case.

The RoW cars '80-'83 work the same way as the US '78-'79 cars without O2 sensors. No O2 sensor, no frequency valve.

[nkotselas]

Thanks, so am I in the clear to disconnect it? Was it perhaps only put in place to pass inspection?

[rattlsnak]

What is it connected to? The Euro harness does not have the O2 plug.

[nkotselas]

It is not connected to anything, I figured out the problem in another thread here:
o2 sensor and CA smog questions