need a little help from the experts

[DaddySS]

I ahve most of my tune really working well now and have it running really well. Idle, accel and decel, slow roll, etc. all are really excellent.

My problem is this - after about 40 minutes on the road it begins to deteriorate and runs choppy.

Has anyone else encontered this and what did you find?

[dfe1]

Are you sure it's time related? It sounds more like a temprerature issue-- perhaps after 40+/- minutes of driving, inlet air or coolant temperature rises enough to cause changes in timing or fuel. I don't know of any timers that would cause a change after the first few minutes of drive time, but there are a lot of temperature-referenced tables. Have you determined whether fuel or spark is getting whacked?

[DaddySS]

You're right, it's not timr but iat and ect are remaining constant. I get the distinct impression that it is running richer, like the 02 sensors are over heating or something but my 02 bung for the wideband needs to be repaired so I can't measure it. It's just strange as I don't see it in fuel trims, or 02 frequency but it definitely gets a distinct roughness, starts to feel "flat".

[dfe1]

What about spark timing? Any significant change in it?

[mr.prick]

IATs over 96*F tend to lean me out.

[DaddySS]

Interestingly, yesterday it drove perfect all the way, main difference:
Bad Day - IAT 63 at start of run, 64 toward the end
Good Day - IAT at 59 at start, 43 toward the end