EGR Valve

http://tinyurl.com/5kg4a

Post cat is indeed heated.

Interestingly, Nissan appears to have been using that downstream sensor as
a fuel trim device, not as a cat efficiency meter, in its pre-OBD-II setup.

http://www.asashop.org/autoinc/oct2000/obd_ii.htm

Excerpt:
"Most manufacturers use the rear (secondary) oxygen sensor for fuel control
or to 'trim' or correct the output from the front (primary) oxygen sensor.
The rear sensor can be used to correct the front sensor for aging,
poisoning or deterioration resulting in offset or drift. Some manufacturers
set limits on the correction by the rear sensor and, when the limits are
exceeded, store a fault code and illuminate the MIL."

It appears the use of the downstream sensor for cat efficiency monitoring
only started when that function was mandated for OBD-II.

 

So what I get from this is that if the exhaust
gases after the cat are too rich in O2, the post-
cat sensor will trim back the fuel mixture as
well. This means that the cat is not damaged
from the pre-cat sensor being unplugged, right?

t

 

Could be.

If the pre-cat sensor sees too much oxygen, it assumes a lean mixture and
richens things up. If it sees no oxygen, it assumes a rich mixture and
leans out until it does see oxygen, at which point it flips back again. The
sensor therefore will flip-flop several times a second as it adjusts the
mixture rich/lean/rich/lean, etc. In the end it's supposed to reach a happy
medium that results in maximum fuel economy and lowest emissions overall.

It's a bit like alternating current electricity: It ranges from +110V to -
110V, and the average voltage is ZERO.

Right now I have no idea how Nissan would use the post-cat sensor signals
to adjust fuel delivery. All an O2 sensor can see is oxygen. If there is
none in the exhaust, the post-cat sensor has nothing to do.

 

Im out of town for a couple of days...will get the exact codes for you
once I get back. And as I said, appreciate all the advice..ur awesome!

 

I have a 1992 Civic EX auto. Does it have an EGR valve, and where
approximately is it located. I have a shop manual for the car, but I
can not seem to find any reference to an EGR valve.

Thanks,

Jim

--

 

If you have EGR, the valve is located just behind the valve cover, probably
on the passenger's (right) side of it.

The control box is on the right side of the firewall, next to the wiper
motor. If you just have a big open space on the firewall inboard of the
wiper motor, then you have no EGR.

 

You have the D16Z6 engine, and it did not have an EGR valve regardless
of what transmission it was bolted to.