my mustang starts up at very high rpm's but it will not return to normal it just stays high. i have changed the tps and the idle bypass sensor , egr valve. also on the tps sensor should it have a constant ground?
i used to have that problem on my 89, it would idle at 3000 rpm, ya thats high. i found a vacuum hose that came loose and reconnected it and it was idling at about 2000 rpm, then i cleaned out the iac and messed with the idle screw till it got back to normal.
See the "Surging Idle Checklist for help with all your idle/stall problems. Everything you need to find and fix idle problems is in there.
The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.