Rick Berman wrote:

<Snip:
Has anyone had a problem with the Cat. unit causing the HC to go high on =
idle. We have a 500+ PPM on HC at idle all the gases look great. We have =
done the usually thing and still can not get the HC down.
\Any help
Snip>

I don't think its the CAT.  What are the other readings at idle.  500 + ppm
HC would mean a lean mixture to me.

If you graph CO and HC by fuel mixture you get the following:

CO Emmissions

6
5.5
5       *
4.5       *
4           *
3.5           *
3               *
2.5               *
2                   *
1.5                    *
1                        *
.5                         *
                                   *
 0                                         *
%CO              Rich        14.7:1           Lean   (air fuel mixture)

HC

1200
1100
1000                                 *
900                                 *
800                                *
700                               *
600                              *
500                             *
400                            *
300                           *
200   *                      *
100             *           *
 0                         *
 HC ppm      Rich        14.7:1           Lean   (air fuel mixture)
                        Stoich

Connect the dots and you can see the relationship between CO and HC.  Your
situation of high HC could  means  "way to lean" , like false air some
where in the system or the AFM leaned out too much.  Also out of spec spark
advance can cause highHC.  Whats your timing set at?  What are the CO and
NOx figures.   I'll bet your CO figures are under 0.35 or a little to lean.
What are the 2500 rpm figures and what year & model?

The CAT generally fine tunes the gases At Stoich to reduce CO and HC
slightly but its really there to reduce NOx at stoich.

Let me know what is happening.

Have fun

Steve Blackham
Centerville, Utah
77 Bus converted to camper (westy top)
87 Weekender