Code 13

it very well could be the mods. I take it you set the idle where you like it (might this be a bit higher than 672 RPM? I bet so). Was the car to full operating temps during the test?

Here is my take: If you notice no drivability issues, don't sweat it. You could dial the idle down to remove the code, but it might create other issues in the process.

my two cents. good luck.

that might be endemic to an AOD with no shift kit (if you dont have one). it seems like some AOD's just like to short shift. But I am not 1/1000000000th of Jerry when it comes to autos.

you have a pure AOD (not AODE), right? If so, there is no computer control (basically, the trans is not connected to the puter in any way; the TV cable is about the only connection)? I think so. Therefore, I dont see how the tranny would know if your IAC or idle is a little high (under 1000 RPM should not be a shift point, it does not seem to me).

The IAC and your idle being a smidge high would not be unlike if you were just ever so slightly feathering the pedal, which should not produce shudder, I dont think.

Like I said, I am not that up on autos, so I am really just typing what came to mind.

good luck with it.