Some things to think about:
Nitrogen oxidizes at very high temperatures, so there may be "hot spots" in
the combustion chamber (carbon build-up?), or the old cat-con was
overheating. (The EGR valve is a good guess, since the purpose of EGR is to
control temperatures in the combustion chamber.)
Try a different fuel. Some fuels contain more nitrogen. There's already
plenty in the air, but more nitrogen in the fuel will mean more nitrogen in
the combustion chamber, and more NOx in the exhaust.
As for why you can't see the EGR valve, it may be incorporated into the
exhaust and intake manifolds in such a away as one or both has to be removed
to see it.
Chris K.
On Mon, 26 Sep 2005 19:02:18 -0700, Bob Spidell wrote
> Hope this isn't too far off-topic:
>
> My girlfriends '95 Jeep Cherokee--4.0L six--has excessive NOX emissions.
> A new cat got them down low enough to pass smog, but I figured it was
> due to a stuck EGR valve. Well, I looked, and the engine doesn't
> appear to have an EGR valve (it is FI).
>
> Anyway, anyone know if this engine has an EGR and, if so, why can't
> I see it? If not, what would cause an otherwise good-running engine
> to emit so much NOX (400ppm+)?
>
> TIA,
> bs
>
> ***************************************************************
> Bob Spidell San Jose, CA bspidell@comcast.net
> '67 Austin-Healey 3000 '56 Austin-Healey 100M
> ***************************************************************
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