Vacuum leak?
Does the MAP ever read 100 KPA but then go down to 88?
I'm sitting here thnking...I've never looked this up on an NA motor. 88 would still show just a bit of vacuum. Which technically you are pulling in air.. Pulling this off the hip though
You reading this on HPT's graph?
There is a glitch that won't allow it to read higher than 88kpa.
I'm not 100% sure that this is your issue, but check it out, and let us know what you find.