I've been having this same issue. My FD drove fine one day and then, the next day the idle started surging while I was on my way to work. I checked all vacuum lines, gaskets, and you name it but nothing was unplugged or out of the ordinary. After a few months I pulled the engine and replaced it with a spare one because the original engine lost compression on the front rotor. The idle surge did not go away. So it's not engine or vacuum related.

I was able to minimize the surging by playing with the TB idle screws for a while and by cleaning/unsticking the TB butterflies. I still get some surge but it's only about 100rpm of surging and only when the car is warming up.

The car will idle fine at start up, then it'll surge as the car warms up and until it reaches 72*C. At 72*C the surging starts to slowly go down. After driving for 10-15 minutes, the surging completely goes away and the car idles smoothly.

I'm thinking that it's more electrical or tuning related. Maybe even fuel delivery related.

 

It's been a couple months – did you ever figure this issue out? I'm having the same problem on my freshly rebuilt engine.

 

I'm interested to know as well, what ended up being the issue? My recently rebuilt single turbo FD exhibits a similar hunting idle on warmup.

 

After my last few days, I'm going to ask the same thing that helped me sort my car out - are you checking the right clutch switch? There are two - the white plastic box bolted to the pedal bracket, and the smaller one that threads through a mount between the pedal and the dash. The breaky spring switch is the one that threads through the mount, between pedal and dash, not the plastic box bolted to the pedal bracket.

 

Last weekend I was installing an electric water pump was tested my radiator fans because I wanted to wire them in to the EWP controller. I discovered that one of the fans on my radiator wasn't spinning. The fan was seized and whined when I tried turning it by hand. I did not notice this earlier because the car has been off the road since the fall and hasn't been started in months. I unplugged the bad fan and, after finishing the ewp install, turned the car on. The idle surging seems to have gone away. I'll do some more testing when the car is back on the road in the spring.

 

You may be on to something regarding the fans. I am also having the idle hunting/oscillating problem. I've isolated it to when the fans turn on. Before the fans turn on, idle is fine. Once the car reaches temp (99C), the oscillation begins. It stops when the fans turn off. Today I had the windows down when the problem occurred and noticed a fan was whining when it turned on. A seized/stalled fan could cause excessive voltage drop and increased load on the engine (via the alternator). Either of these could cause oscillation.

BTW, here are all the things that did not fix the issue for me.

• Vacuum leak. Replaced all intake gaskets, including one fiber ACV gasket that was actually leaking. Inspected vacuum hoses and connections.
• Grounds. Checked and cleaned all engine bay grounds.
• ISC. Disconnected, the problem still occurs, albeit at a higher rpm.
• Throttle binding. Found and removed a zip tie around a vacuum hose (stock pressure sensor) that was binding the ISC.

 

Mine was doing the same thing after removing / replacing the UIM. It was because I put the metal throttle body gasket in backwards. It's indented on one side, and I didn't notice the upper corners are different, so the indentation was facing the wrong way (DOH!). It allowed too much air to get past the ISC passage in the throttle body, so when the ISC was working the idle bounced around. Unplugging the ISC cable would stop the hunting but it wasn't an issue with the ISC solenoid, just the gasket backwards. Just something else to think about if you replaced the throttle body gasket and then the issue starts.

 

vacuum hoses... probably one in the vacuum chamber behind the alternator... in the input tube...

 

try adjusting your idle air control screw on the front of the throttle body to shut the butterfly valves more.

Typically when a car is doing that it has a couple of problems.

Base map is off, The powerFC is jumping between a few cells which have drastically different values and its basically shooting fuel in on one cell and dry on the other.

Another issues is you have 1,000 rpm set for idle, the air control screw which sets the butterfly valves is open too much. The powerFC tries to pull down the rpms but the engine has too much air. It cycles between this the entire time causing the idle to surge a bunch.

The screw underneath the elbow that opens and closes the air valve for the idle air control valve needs opening if your rpms cannot be caught coming to a light and the idle keeps dipping too low. The idle air control valve can always bring the idle higher but cannot lower the idle rpms.

So the way you want to set the idle for these cars, start the car and when its warming up try to get the idle set screw on the front of the throttle body as close as you can to being where you want the rpm's to be at. Then open up the idle air control screw to "stablilize" the idle at the rpm you want. disconnect the battery with car off and try again, once you get that dialed in the car should run very well since its basically dialed in before the computer starts to make a whole bunch of corrections.

A smooth base map also does wonders for the cars drive-ability and idle.