i have a Q after all these test..have you ever checked your fuel mix,yet??

Ivan

This test indicates that DME ECU is fully opening the ICV when it should not, most likely a bad transistor in the ECU.
Another test should allow the engine to warm-up, turn it off, and then remove the valve and position the vane in the middle.
Then re-install the ICV and re-start the engine without connecting the ICV connector. The engine should idle very close to normal (700-800 RPM),
assuming the fuel mixture is OK. If the ICV connector is then installed later, the surging should re-occur, confirming a problem in the DME ECU.

Correct, this is the 3rd test I did. Left the vane part open and the idle was a little fast but stable, as the car warmed a little surging returned but it took a while so presumably this was a symptom of the mix needing to change as the car warmed.

I can't get an appointment for a few months, the garage I spoke to said it will be tricky to set whilst it's surging.

Given that manually positioning the vane in the ICV half way keeps it stable for a time will probably manually set it and get them to verify the mix is ok.

Not sure if this has been done to check for a bad transistor in the DME: Disconnect the ICV and unplug it’s harness connector. Move the vane to its center position. Turn the ignition ON but don’t start the engine.

When you now reconnect the ICV harness connector a bad DME will make the vane snap immediately to one of its extreme positions with a loud metallic click sound. Either fully open or fully shut. A good DME will make the vane vibrate and jerk slightly about once per second.

With your previous test results I’m almost certain you’ll find this DME test passing since you indirectly ruled out an ICV at one of its extreme positions.

I've done some more testing and if I part close the vane, connect power and start the car, let it run for a while, stop the engine and then inspect the position of the vane, it's opened wider but not fully opened. Will verify its operation with the suggestion above-thanks.

To me that sounds normal. The ICV is driven by two transistors in a push-pull configuration. If one dies (fused short or open) the other will inadvertently drive the vane all the way to an extend in a millisecond. I’ve never seen a partially damaged transistor that introduces an offset for the ICV or slow response.

The fact that your idle feedback loop somewhat works but then goes into oscillation is more likely due to an offset of an input (AFM signal at idle, throttle plate opening when at idle, amount of unmetered air (intake leaks, AFM bypass screw), amount of fuel. Or the propagation time is different and the two variables are the ICV speed and the AFM flapper door speed. If either is too slow it’ll cause oscillation. Did you check that your flapper door of the AFM moves freely?

The ICV test was referenced in post #5 on 11/10/21 and again in post #26 on 11/29/21 with a full explanation, and no real conclusion was reached
as to whether the DME ECU is not the source of idle surging. Here again is the procedure and it's hoped you now understand it and can make a conclusion.

It's that simple!

I believe that is the procedure I followed and updated on today. Apologies but this is pretty much my first time at trying to resolve an issue on a new (to me) car (or any car for that matter).

To summarise the results:

1) Valve force closed, disconnected, no start
2) Valve force closed, connected, starts and surges
3) Valve force opened, disconnected, starts and surges
4) Valve force opened, connected, starts and surges
5) Valve centrally positioned, disconnected, smooth idle
6) Valve centrally positioned, connected, surges

I'm going to try centrally positioning the vane, powering on and watching the vane behaviour to see if it snaps open or fluctuates (as suggested recently)

The #5 & #6 tests (warm engine) indicate your surging is related to the ECU ICV function.

Completed the DME test this morning, fully closed the vane, connected the power and the ICV snapped to a central position and began jerking slightly.

https://drive.google.com/file/d/1-iWikEUfScTj5vwLgeCknR28vfyK5DgH/view?usp=sharing

Out of curiosity, how does cold ICV behaviour differ between cold and warm engines? My car behaves the same way whether hot or cold; from my understanding the main difference is that the o2 sensor isn't used on a cold engine (therefore in my case this can be ruled out as a potential cause).

Hopefully, you've been installing the ICV in the correct way, i.e. the arrow on the valve pointing toward the intake and not the air filter.

Yes, arrow downward towards the intake.

From this test I can see that the vane positions itself centrally but this is too far open to hold a low idle; when I left the valve disconnected and manually positioned the vane slightly more closed (1-2mm), the car settled into a slow steady idle. Equally, if I start the car and inspect the vane position after a few minutes it has opened up even further (exacerbating the problem); does this imply the mix is too rich and its trying to open the valve further in an attempt to lean it out by adding more air?

That indicates you have no air leak! Your problem relates to the functioning of ICV when connected.
Continue to manually re-position the ICV so the idle stays at about 800 RPM with the ICV disconnected.
This should help conclude that the ICV's operation is your problem.

The ICV does not have a direct effect on the mixture. It just bypasses air around the throttle body butterfly, as if the throttle linkage were moved.

I've manually positioned the ICV whilst disconnected several times and can reliably get it to run stable (even with a little throttle it settles again); generally I have to close it more than half to get the idle down to 880RPM (at the vane's mid point the RPM is about 1,200 RPM. I'm assuming this might be because my idle air screw is not set correctly (albeit per previous posts, turning it doesn't seem to do a lot)

Is there a chance that someone messed with the throttle stop position and/or idle switch position? The mechanical stop defines where the throttle valve rests when the accelerator isn’t pressed. In a normal setup the throttle return spring pulls the valve against this mechanic stop and then shortly thereafter the mechanism actuates the idle micro switch.

If someone changed the micro switch position to where it holds the throttle valve open too much this could cause your issue where there is simply too much air coming through the throttle body.

Check the following: With the engine off gently start applying throttle by pushing the throttle lever where the cable connects to the throttle body. You should first hear the micro switch disengaging and very shortly thereafter the throttle valve starts opening. When releasing you should feel the throttle valve settling against the mechanical stop and then very shortly thereafter the micro switch gets actuated.

besides all the control checkings ..can you see if you still have this plug in your AFM?It goes on from the bottom of the AFM .If it is still there the fuel mix was not adjusted for a long time if ever..
http://forums.pelicanparts.com/uploa...1638717703.jpg
http://forums.pelicanparts.com/uploa...1638717827.jpg

Yes, it might not be. Try turning (clockwise) the throttle body air screw completely in, and then connecting the ICV connector.

Idle adjustment screw o ring
 

My 86 had a surging idle problem. Then I noticed a whistle on acceleration. Sounded cool but meant an vacuem leak. Another Pelican member had same issue which I found while searching. I removed the screw about 19 turns lubed the o ring with dielectric grease ,reinstalled ,set idle, and boom no more embarassing surging idles at stop. I know alot has been discussed here and I quickly browsed it but sometimes simple is easiest. Hopefully this is something not looked at but dixes the issue. Goodluck. Thanks Rob. Usually these cars are pretty reliable but age take its toll on gaskets ,o rings and electrical components.

I wound the screw right in and the idle dropped from around 1,200rpm to 950rpm - it was stable for about 5mins and then minor surging started (not as severe).