Any smog experts out there?

cudamark said

This is what is puzzling to me. It's only the 25MPH Nox reading that is off. The 15MPH Nox reading is fine. The only difference running-wise is the RPM change of a little under 1000RPM, unless there is some component I'm not aware of that takes MPH into consideration and/or the computer does something weird at that speed. I was hoping there was something obvious that I've overlooked and that someone has run into that situation before who knows what it is.

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There will be more load (= higher cyl pressure) @ 25mph, which must be triggering the cause, I'm going to say carbon and a cyl. or 2 running lean. I'm also going to guess this pre-OBDII Magnum may need an intake & plenum gasket set, may be sucking oil & un-controlled air into a couple of intake ports, the system will dump enough fuel in to prevent a lean code.
Check fuel pressure.
I would drop the cat off, do an induction cleaning, change the intake set, add Seafoam/BG/etc. to the fuel & run that thing hard up a couple of long hills.
Also remove & verify the EGR passages are CLEAR, carbon build-up can cause insufficient flow even tho' the passages aren't blocked.
Then toss new plugs & O2 sensor in, put the cat back on run it again & then have it retested.