Recently rebuilt motor, and the idle hunting came back with the new motor...
Have spent a fairly long time trying to adjust and check the choke actuator to fix it, and in the process, pure laziness found out some info on the way the system works, and an easier way to detect and rectify the problem. I think this should be added to the Infobase, as it will save you a lot of time diagnosing the issue.
1. Idle hunting, anywhere from 600 - 2500 rev bouncing once the car is warmed up
2. Cold idle does not drop to normal 650rpm idle once engine warms up
- Warm up the car, take it for a few laps, until the engine has reached running temp.
- Once car is warm, take it home, and turn off the engine
- Pop the hood, and use a phillips screw driver to remove the tin plate from the Idle control/choke assembly This can be found by looking over the back of the plenum (red bit on intake) and you might have to pop the air temp sensor plug out of its clip to see it (turbo 1)
- Take both screws out, and lift the plate off, put them somewhere safe, not on the car or engine.
- check that the O ring is ok (the one under the plate), but don't pull it out, unless it comes away (don't want to have to buy a new one)
- Start the engine.
(The choke and idle assembly draws air from above the throttle plate in the side of the tube under the tin plate you just removed. Removing the plate does not interfere with the idle control and choke, as the air above the throttle plate is at normal atmospheric pressure (at idle))
- make sure you have no grit on your hands etc, and don't drop anything down the hole, as it will get straight into the engine.
- first trick, place your finger/thumb over the hole at the base of the tube, this will seal off the choke, and should drop the revs and stop the hunting.
- If this stops the hunting, and the car is definitely at running temp, then it means:
a) The choke has been mis-adjusted
b) The choke temp control is wearing out and not moving as far as it should
or c) The choke control has failed and is not closing the choke off at all when the engine warms up.
What you can do:
- using a wide screw driver/piece of steel (make sure if is spotless, no chipped paint etc) while the engine is still running, turn the plug in the bottom of the turbe clockwise to screw it down towards the part that closes off the choke.
- keep turning it down until the revs stabilise and then try the finger trick again.
- if the revs don't change when you block/unblock the choke plate after winding it down, then it has closed the choke, the car should be fine to tune the idle as normal for the rest of the settings (step ups etc)
The last test:
- The process of closing the choke with the engine running and warm will reset the choke off position, however, if the choke control bulb, a temp sensitive bulb, is stuffed, the choke now won't open (as the plate is wound right to the bottom).
- To test this, stop the car, put the plate back on, and wait till it cools down
- Start the car after an hour or so (or longer if it takes that to get cold) and see what the idle is like, for me it worked fine, but if the temp bulb has failed, then you'll get a low possibly rough and you will need to look into another control bulb, possible from a PGM-FI early integra/rover or something...
How to Test & Adjust a C-T Autochoke - Can Cause Hunting
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