Ok i have a 89 rs with 305 has a high idle. Did the usual check vacuum leaks no leaks checked the idle air and it is good the ses doesn't come on when running but does during bulb check. Only thing that i can think of would be because i took off. But still no ses light and i don't know if ran like this before i got it because i had to put another motor in it old one had spun rod bearing.
Also the rpm gauge reads way off like 1200 is like 2500 inside the car. What would cause this?

 

I had the same issue on my 305 tbi. I unplugged tps while engine running and noticed very little difference. Pulled a few from the junk yard and it smoothed out my idle pretty well. Mine had a flat spot when checking resistance with a meter. Plenty of info on TGO on this subject. Just keep searching and reading. Good luck!

 

Remove the tbi unit & check the gasket, sometimes they'll get sucked in.
Check the coolant temp sensor.
Check the grounds, esp. the ecm grounds, near the t-stat or back of the heads.