1991 Chevy with 350. When I bought it it already had headers and true dual
> exhaust, as well as a hypertech chip installed.
>
> Last year I had to replace the EGR and one of the cats to pass. After that
> I did a full tune up, plugs, wires, ignition the whole thing.
>
> I also recently had the oil changed.
>
> The tech at the shop thought the following numbers indicate the EGR and
> possible cat.
>
> High Speed
>
> Standard Current Result
> HC 126 149 FAIL
> CO 1.38 0.5 PASS
> NOx 1269 2568 FAIL
>
> Low Speed
> Standard Current Result
> HC 129 153 FAIL
> CO 1.08 0.41 PASS
> NOx 1369 2371 FAIL
>
> I am planning on removing the Hypertech chip, and will check the EGR valve
> to see if it is working properly. Any other thoughts?
>
> Troy
Troy,
The hypertech chip forces the engine to run a richer A/F mixture, hence more HC's in the exhaust. By screwing with the fuel map and ignition timing it also produces a hotter burn, which increases NOx levels. I've yet to see an EGR take a dump within a year of being replaced.
Remove the hypertech chip, dump the stock PROM in there and see if she'll pass. If NOx is fine and HC's are still up, the cat may need to be replaced.
Doc