Idle was around 450-500rpm. Adjustments made, now idles at 850-950rpm.
When I first bought my AE82 Toyota Corolla FX16 GT-S with an AE101 20-valve 4A-GE silvertop in it, the idle was high and all over the place.
It turned out that the throttle cable used was not adjusted properly. It always had tension on it, so when the gas pedal was all the way up, there would still be some throttle applied via the cable, which was enough the keep the idle high. You could manually turn the throttle all the way to close in the engine bay, and it would stretch the cable slightly. The idle would then drop.
It was also random how high the idle would be, because the throttle cable often would never quite rest in the same idle spot. Revving the engine once would often cause the idle to change to a completely different speed.
So step one was to get the cable to allow the throttle to truly be at 0% when the car is at idle. I simply bent the bracket using long pliers so that there's a little bit of slack in the cable.
This made the idle steady. However, it was now very low, around 450-500rpm most of the time. Often, the car would die. And sometimes the idle would be so low while the car was cold that you had to give it gas to get the car to start, and give it slight gas constantly to keep the car from dying.
I tried all sorts of things:
- Checking for vacuum leaks (I replaced a couple cracked vacuum caps, no effect)
- Removing and cleaning the ISCV (no effect)
- Checking timing (was right at +10° when in diagnostic mode)
- Adjusting TPS (I've had ongoing TPS issues so I've been doing this a lot)
- Replacing spark plugs (no effect)
- Taking apart and cleaning the intake tubing, AFM, plenum, factory plastic velocity stacks, and butterflies (no effect)
But what finally worked for bringing my idle up was adjusting these four screws, circled in red below:
A detail shot. It's the screw in the square.
(Both photos from Chin Fa Cheng's excellent site)
When I had my plenum apart, I noticed that my screws in my manifold looked suspiciously far screwed-in, compared to the photo above. So I backed out each screw one and a half turns (counterclockwise), put everything back together, and the car fired right up and idled at 900rpm. Problem solved.