bogus wrote:
I find that rather hard to believe, really.

You are still using the stock manifolds. If anything, I am wondering if the EGR solenoid failed.

With the older Vette's, wiring and vacuum lines become real problems. I don't know exactly how much diagnosis you did, but, if the vacuum is being pulled, and the EGR is open, I don't see how it will set a code.

The codes are set when the valve - a vacuum servo (dashpot) - is opened and indicated as such at the time the computer says so.

It doesn't really care about what volume of gas is going in.

Short of the EGR temp sensor, it's rather ignorant of the entire process.

How did you deduce this to be the problem?