I got a new exhaust manifold because my old one was cracked. The mechanic said it looked like it had been really hot and said it could have been caused by a stopped up cat. I think that is possible because I am only getting 20 mpg and all the ignition stuff is new and the vehicle is stock.
My muffler has some rust around it so I am thinking about getting new 2" exhaust all the way back and removing the cat when I have him put on the manifold. I think there is an O2 sensor behind the cat. Will removing the cat have a negative affect on my MPG or the way the engine runs? Do I need to put the second O2 in the pipe behind where the cat would go? or should I just unplug it and leave it out entirely?  Will the 2" exhaust without a cat have enough backpressure to properly operate the primary O2 sensor?
It is a 16V engine.
Thanks
Jarrod

Your ECM will NOT like not having a second O2 sensor!

The main O2 sensor should work OK, it might just take a little extra heater power. The rear O2 sensor will throw a Check Engine Light if the sensor and/or cat is removed. Its an OBD2 emissions test and won't affect the engine running. Why not fit a free-flow cat?

Why not fit a free-flow cat?

Because a straight pipe is almost free$

If a free flow cat were cheap I would consider it.

If it doesn't affect how the engine runs or my MPG, I am not really worried about the check engine light.

You aren't in part of TN that gets smogged, are you?

No.  We don't have emission tests. I'm way out in the country. 9 miles out of the nearest town (with a population of about 1200.)

Your ECM will NOT like not having a second O2 sensor!

What will it cause the ECM to do other than throw a code? Would it be better to just leave the second O2 sensor off?

Leaving the second sensor off would put the computer into a permanent and worse version of having the second sensor read a bad cat.

Not sure but on some other vehicles if the readings get bad enough the engine goes into a sort of "limp home mode" with drastically reduced performance.  The thought is that the cat is clogged so you just need to get home or to a shop to get it fixed.

second o2 sensor?  when did they start doing that? 

What will it cause the ECM to do other than throw a code? Would it be better to just leave the second O2 sensor off?

It'll just throw a code. The second sensor is only there to check that the cat is working OK. Only the first sensor is used for mixture correction and only in closed loop mode. If it detects the first sensor as faulty then it'll remain in  open loop mode which isn't anywhere near as drastic as limp home mode.

second o2 sensor?  when did they start doing that? 

1996 officially, sometimes a few years sooner.

Some vehicles have 2+ sensors.  Our XL7 has at least 3, maybe 4.

Quote from: kirknd4spd on March 22, 2007, 10:18:13 PM

second o2 sensor?  when did they start doing that? 

1996 officially, sometimes a few years sooner.

Some vehicles have 2+ sensors.  Our XL7 has at least 3, maybe 4.

damn, that seems kinda pointless.
like my rx7, for instance, only one o2 sensor, but 3 cats.... car manufacturers do some interesting things sometimes

Here's what the O2 sensors do.

The first one checks the exhaust for oxigen.  If there is too much oxygen the motor is running lean if there is not enof the motor is running rich.  So this one effects the motor directly.

The second is after the cat.  It tells the computer if the cat is working or not.  The second one wants to see a lot of oxygen.

There are plug in chips that will allow you to remove the second O2 because the chip tells the computer that the cat is working great.

I have a friend that was telling me that early chevy trucks have a real problem with the second O2 sencers.  So he put the 2nd O2 in a can/box and mounted it under the vehicle.  The can/box keeps the O2 clean and the O2 sees all the oxygen it needs.  I have never tried this though, he claim it works great!

I've only seen the chips on the internet.  Don't remember where?

Summit Racing has them.



kirk, the V6 trucks have two cats, one on each side directly aft of the exhaust manifolds.  There's a O2 sensor in front of both and one aft of each cat...I think.  I've seen similar under most vehicles.

In order to meet everyone's favorite emissions control standards, this is what you must do.

What year is your RX7?

Summit Racing has them.



kirk, the V6 trucks have two cats, one on each side directly aft of the exhaust manifolds.  There's a O2 sensor in front of both and one aft of each cat...I think.  I've seen similar under most vehicles.

In order to meet everyone's favorite emissions control standards, this is what you must do.

What year is your RX7?

yeah, 2 cats like that makes sense, but rotary engines only have one exhaust manifold. one rx7 is a 91, the other is a 85.