thomadyeoh,
After all done, my engine idle at 1.5k rpm, with air-con 1.2k rpm. I already lock the idle screw to max for your information.
DIY Option : take off the throttle body from the intake plenum, you could see a hole in the mouth of the TB, at the bottom part.... SEAL IT. That's where your throttle body is leaking air into the intake causing high RPM because your ISC unit detect low heat and kept the leak valve open. Can stuff it with JB-Weld compound (or any other compound that hardens up like cement and able to withstand high vacuum pressure, or face the bugger being sucked into the engine).
WHY : The ISC unit at the bottom of the TB, is a 'heat-by-water derived & electronic controlled choke', which it has 2 way of working.
1. Cold engine choke - ISC sensor detects water temp, if too cold > engage gear to open air leak valve and allow air to enter the hole at the mouth of the TB and out into the intake plenum from a hole behind the TB's butterfly.
2. A/Cond Compressor Drag vs Engine Rev Control - this is another job of the ISC. The temp sensor inside the a/c blower system in the car, being connected to the aircond compressor and the ISC together. Air cond compressor when it engages its clutch (ON), there's a significant amount of torque drag it will give to the engine causing the engine to be heavier and works slower... so, to counter this, the engine has to be revved up abit, and this happens when the temp sensor at the a/c blower inside the car detects more cooling need to be done to get the desired temperature. The temp sensor also sends a signal to the ISC unit to leak some air from the TB into the intake plenum, and that is through the small hole at the bottom of the TB's mouth where the air flows through the ISC's valve.
If the a/c temp sensor sense the cooling temp has reached the desired temperature, it will shut off the a/c compressor and sends a signal to the ISC to close the valve. It's the same concept as the Toyota's FICD. AC compressor clicks on, FCD opens valve to leak air into the plenum, the AC compressor off, it shuts the valve.
From my POV, if you experienced this problem after removing the water hose, then just plug it back. it may also indicate your ISC, while maybe cacat abit, but still working to give that smooth idle.
Other answers : It's okay to unplug the ISC socket and run around. It doesn't make any effect to the car if you're not using ISC anymore and have alternative way to control the engine's idle during cold start and turned on aircond. I've ran my 92p for many years without using ISC because using Mivec TB (stepper motor) and used Toyota FICD. Just cold start seemed to be a bit problem because I had to manually rev the engine to warm up, or it'll just die.
What I wish : (read with Jeremy Clarkson's voice)
Someone could invent a standalone ISC unit, with adjustable leak control valve, that no need to plug at the throttle body, so people who use aftermarket tempayan throttle body, fat plenum, hi-cams, that couldn't be plug with the stock ISC, could sleep happy, because they could still use an aircond in the car! Wonder why petrolheads with agressive cams that has low vacuum stroke due to the intake valve opens too long below 1200rpm, couldn't just keep their AC comfort while idling below 1200rpm instead of at least 1500rpm! Hammer please!
Hey, I've came to know Works Eng. did come out with something called Air Control Valve... is that it?
