I drive a 1993 Honda Prelude Si 4WS with about 175,000 miles on it. when the engine is cold, the idle will sit at around 2k rpm and then quickly drop down to about 750 rpm, rise back to 2k rpm, then drop. it does this repeatedly until the engine warms up. it does have an aftermarket high flow intake however i doubt this is related to the problem. other than this, the car runs great!
Thanks in advance for your help!
This is typical of a vacuum leak. If you have a good vacuum line guide, look for a vacuum line that runs to a vacuum temp sensor that cuts the vacuum once warmed up. The vacuum temp sensors screw into the intake manifold near the thermostat and typically have two vacuum lines running to it, one from a vacuum source, and another to a vacuum servo or device that is used when the engine is cold.
that would explain why my air vents wont change from floor to defrost, etc… how would i find where the leak is?
Look over ALL the vacuum lines for signs of age, cracks, deterioration, hardened, etc. Get bulk hose and replace all of it. After 17 years, they are probably all compromised. You should only need one size for the small lines (5/32" IIRC), then another larger line for the brake booster (like 1/2"), but don’t take my word for it. Take a couple of samples to the parts store and size some up. Always buy a bit smaller than a bit larger to make sure it will seal correctly. Replace the vacuum line one line at a time to prevent running one to the wrong place. Running a vacuum line wrong can cause the engine to run poorly, and be a devil to figure out without a good vacuum line guide.