Car going rich on the dyno? Any ideas? At what coolant temp does the car pull timing?

[Cobra4B]

Took the car over to All Aspects for a fresh NASA verification dyno. I last dyno'd summer 2010. This winter I swapped the OEM intake for a Halltech filter and deleted the cats. I also converted to a single Spal fan.

The car's air/fuel was very erratic on the dyno. The first pull it was lean at 13.6:1... figured that might be the case with the intake/cat delete w/o any tuning.

Tuner is Ed Hutchings who is the best in the area and has tuned 4 C5s for me... we figure it'll be an easy fix to fatten it up to account for the mods. As Ed starts making changes the car isn't reacting to them properly. He's making small changes and the AFRs are going way rich in the 10s or way lean into the 14s.

First we suspect bad wideband. We have the wideband in the driver's side rear O2 bung (I have both rears gone). We put a 2nd in the tail pipe and it matched the one in the car so not the wide band.

Then Ed put it in speed density to take the MAF out of the picture. Car is still acting up. I decided to put fresh gas in it as the car had 3/8ths of a tank and it was gas I had put in at Road Atlanta in December. I put 10 gallons in and it was like magic. The AFRs became consistent and the car was reacting to the tune changes properly. Ed dialed it in in SD then turned the MAF back on and dialed that in.

We had the car making 368-371 rwhp with a 12.6:12.8 AFRs consistently. Then we let the car cool off some to make the 3 back to back to back pulls required for the new NASA dyno procedure.

The first two pulls the car was a 1/2 point richer running 12.0 to 12.3 AFR and made 365 each pull. On the 3rd pull the car went way rich into the high 10s and made 356 rwhp and 352 torque.

I'm stumped... we had been on the dyno all day fighting a basically stock 2001 Z06. I'm taking it back over there on Friday and Ed is going to wipe the car clean with his Tech 2 and upload a fresh tune and start from scratch.

My main questions are what causes the car to go into "piston protection mode" or whatever it's called? The coolant was a max of 221 at the end of the 3rd back to back to back pull. When we started the pulls the oil was 180. I'm not sure what it was the last few pulls before the back to back ones when it seemed to be acting fine.

We checked fuel pressure as well and that was fine. Ed was shocked the gas made any difference because "bad" gas usually makes the car go lean and only lean... not be erratic.

Any ideas? Any suggestions?

Cliffs:

1. Car has very erratic AFR on the dyno
2. Fresh gas fixed it
3. Car is dialed in making 370 rwhp with 12.6 to 12.8 AFR
4. Let the car sit for a few minutes to do the 3 back to back pulls for NASA's new procedure
5. Car goes richer first two pulls (12.0 to 12.3) then way rich on the 3rd(low 11s high 10s)

WTF? Is 220 engine coolant temp a number that triggers something?

[pluslt]

My first guess was the O2 sensors, but it sounds like you ruled them out. There is a machine (can't recall the name) that you can plug into the OBDII that will show how well they are acting and if they're slowing down from being clogged up. Good luck.