1995 SL Idle Problem

I got a 95 SL from a friend. It apparently has the common Idle surging problems I have read about in this and other forums. Surging up to 2k, dropping down to almost stall etc... Have replaced all usual suspects, EGR, TPS, IAS, Temp sending unit, Blah Blah Blah. Last night I pulled the Brake booster vacuum line and plugged it with my thumb. The idle quality seemed to smooth out and drop below 1000 rpm. Couldn't tell because I couldn't see the tach. How can I test the Booster for a vacuum leak? Also with the car in neutral and accelerator floored, RPM peaks out and surges between 4k and 4500. Does this car have a rev limiter on it? 1.9L 5 speed. 213k miles. Any help is appreciated

You replaced the the idle air sensor right? the IAS takes a long time to learn, so you might have to drive it for a while to corect the problem, if that is the problem. Another problem might be a intake gasket, spray brake cleaner on the back side of the engine on the top side of the intake with the engine running to see if the idle changes, if the inake is leaking you will notice a big change in the idle