Hi, I new here and hope I can maybe help at least some people who have that dreaded idle surging issue.

Bottom Line Up Front (BLUF) - Some people's issue may be caused by a vacuum leak between the Intake manifold and the Power Brake Booster (possibly a leaking brake booster).

Yesterday, while working on my wife's 93 Civic brake system, I pulled the master cylinder to replace it due to weak pedal before and still after bleeding the brakes.
At that time, I removed the brake lines and master cylinder but DID NOT disconnect the vacuum line from the intake manifold to the brake booster.

I started the car up to move it out of the middle of the driveway and immediately encountered the high idle and idle surging issue.
I been researching the internet and numerous possible causes and started troubleshooting.

Since my problem started right after I pulled the master cylinder from the brake booster, I took the vacuum line off the intake port and put a small piece of hose with
a bolt on that port and started the engine (see attached photo below).

Gone was my high idle and idle surging issue with the Civic back to a normal and steady idle.

I hope this helps...


Capped Intake Vacuum Port to Brake Booster - Possible Vacuum Leak in Brake Booster Causing Idle Surge