When I ran my OBD codes for the first time, I got a 33 and 34 (relating to EGR function). My first action was to pull off the EGR valve and see what it looked like inside. It was almost completely (~ 95%) full of carbon coking, and the pintle would not move. I soaked it in carb cleaner and gradually scraped all the junk out, and finally got the pintle to move.
So I cleared the codes and ran the test again. I got the same codes. So I pulled the EGR back off and took a closer look at the EGR spacer. Yep, the holes in the spacer that match up to the EGR were also 95% full of coking. So I pulled off the throttle body & spacer; the gaskets literally disintegrated. I went to Advance for some gaskets, and I picked up a can of CRC throttle body cleaner. (Says it won't harm to teflon coating.) Turns out all of the chambers inside the spacer (all around the intake opening) were full of coking too. This was terrible! I cleaned it all out using the T.B. cleaner, and while I was at it I cleaned the T.B. and then dried everything with compressed air.
I reassembled everything, and took it out for a test drive (besides, I had to heat up the engine before I could run another KOER test). It seemed to run a lot better, and had more throttle response. (I know, this was probably 50-100% psychological.) So I got back, ran the tests again and all I got was a pair off 11's.
So for those of you with high-mileage cars which might not have ever had the EGR & spacer cleaned, spend a couple of hours and give them a good cleaning. You'll be happy with the results. While you're at it, clean out the IAC valve, and (although this is controversial) you may consider removing the mass air sensor and cleaning it with electrical contact cleaner. I cleaned my mass air about a year ago (almost certainly the first time it had been cleaned) and I definitely noticed a seat-of-the-pants improvement in power.
Now, if I just knew someplace to take the upper intake to have it cleaned out... (it was all crappy looking inside too, but I don't think I have the means to clean it properly myself.)
John
93 GT