High idle

[Jonty]

Hi all

Ever since getting my D back from having a water pump installed it’s been idling at 1000rpm both cold and hot. Occasionally it will idle at spec when warm but most of the time even warm it is sitting at 1000. I’ve done all the usual checks. My throttle linkages are all set properly and the idle micro switch is being engaged. I’ve tested the thermistor and it’s fine. I will be doing a smoke text later this week but no evidence so far of a vac leak. If my decel springs were worn then shouldn’t I see idle at spec when cold (as the idle motor should be able to compensate) then it speed up when warm? I’ve read all past threads and mine seems different in that it sits at 1000 both hot and cold. I’ve also tested the idle motor circuit and it’s all fine. Thanks

[David T]

I would suspect the quadrant link needs to be adjusted to get the throttle plates closed all the way. When the water pump was done the throttle spool was touched and that can change things enough to have to adjust the quadrant link. There can also be a little slop (play) in the ends of the quadrant link.

[Jonty]

I would suspect the quadrant link needs to be adjusted to get the throttle plates closed all the way. When the water pump was done the throttle spool was touched and that can change things enough to have to adjust the quadrant link. There can also be a little slop (play) in the ends of the quadrant link.

Thanks Dave - IÂ’ve back out the curb idle screw all the way and fairly sure the plates are closed

[82DMC12]

Before you touch the idle stop screw, try this:

I struggled with a high idle issue for a while after doing an intake manifold / valley dive. I had disconnected the throttle cable from the spool and probably didn't get the spool under enough tension when I put it back together. Note that the spool stop is NOT suppose to rest on the stop post. It should be 2 or 3 mm off the post and under tension by the throttle cable. Remove the quadrant link and get this adjusted FIRST. Then, remove the quadrant link and loosen the two 8mm nuts on either end. That way you can spin the link to lengthen or shorten it. Much finer adjustment than loosening just one nut and spinning the end piece. Spin the link so that it gets SHORTER, which will pull the throttle butterfly closed. Shorten it as much as you can until all slop is gone, then snug up the 8mm nuts. That should get it right as long as the butterfly and stop screws have not been messed with. See the photo below of how to keep the thottle spool off the stop.


PXL_20220903_163522470.jpg

[David T]

The only way you should adjust the throttle stop adjustment is with the throttle body out of the car so you can see the plates and make sure the stop screw is stopping the arm BEFORE the plates are completely closed. I turn the set screw until the plates *just* start to open. You do not want the plates stopping the throttle arm. You put too much stress on the throttle shaft, that is what that stop screw is for. Then you can adjust the set screw for the micro switch. The quadrant link has one end threaded left hand and the other right hand so, like a turnbuckle, you can get a precise adjustment by turning the link and then locking it with the lock nuts. If the ends of the quadrant link are worn and you have a lot of play, you shorten it so it pulls the throttle closed. While you have the throttle body out you can examine the decel springs and make sure they are OK. Make sure there are no air leaks too.

[Bitsyncmaster]

You don't let the throttle plates touch the body bore because it can make it stick closed. Both the plates and bore are soft metals and they will stick if closed with any force.

[Jonty]

The only way you should adjust the throttle stop adjustment is with the throttle body out of the car so you can see the plates and make sure the stop screw is stopping the arm BEFORE the plates are completely closed. I turn the set screw until the plates *just* start to open. You do not want the plates stopping the throttle arm. You put too much stress on the throttle shaft, that is what that stop screw is for. Then you can adjust the set screw for the micro switch. The quadrant link has one end threaded left hand and the other right hand so, like a turnbuckle, you can get a precise adjustment by turning the link and then locking it with the lock nuts. If the ends of the quadrant link are worn and you have a lot of play, you shorten it so it pulls the throttle closed. While you have the throttle body out you can examine the decel springs and make sure they are OK. Make sure there are no air leaks too.

Thanks David this makes sense. However the issue I’m trying to solve is with the throttle arm taken off the spool and the stop screw backed all the way out (so I know the butterflys are are closed as they can be) I’m having an annoying high idle just below 1000. Occassionally it drops to spec for a couple of minutes then goes back up. I’ve tested for vac leaks and my mixture is correct. I’m at a loss now and wonder if the decels are bad (but I would have expected it to idle on spec when cold if that were the case). Also the idle micro switch is being engaged.

[Bitsyncmaster]

Thanks David this makes sense. However the issue I’m trying to solve is with the throttle arm taken off the spool and the stop screw backed all the way out (so I know the butterflys are are closed as they can be) I’m having an annoying high idle just below 1000. Occassionally it drops to spec for a couple of minutes then goes back up. I’ve tested for vac leaks and my mixture is correct. I’m at a loss now and wonder if the decels are bad (but I would have expected it to idle on spec when cold if that were the case). Also the idle micro switch is being engaged.

My car would idle to spec cold but when warmed up idle at 1000 RPM with the idle stop screw all the way out. That problem was bad deceleration springs. So unless your springs are really bad that may not be your problem. I do know of one owner where the springs were really bad. You could see they did not close just looking at them without the engine running.

[David T]

Somehow, someway, more air is getting past the throttle plates. It could be leaks, it could be the decel valves not closed all the way, maybe the throttle plates have been deformed or bent, or the idle motor isn't working correctly. More air lets the motor run faster. Leaner for sure but also faster. Also check that the vacuum advance is working correctly. If it is not hooked up right or isn't working it can affect the idle speed. Check it electrically and with the vacuum. Make sure all of the hoses are hooked up to the correct ports and none of the hoses are cracked or split.

[Jonty]

Somehow, someway, more air is getting past the throttle plates. It could be leaks, it could be the decel valves not closed all the way, maybe the throttle plates have been deformed or bent, or the idle motor isn't working correctly. More air lets the motor run faster. Leaner for sure but also faster. Also check that the vacuum advance is working correctly. If it is not hooked up right or isn't working it can affect the idle speed. Check it electrically and with the vacuum. Make sure all of the hoses are hooked up to the correct ports and none of the hoses are cracked or split.

Thanks - agree I have to find this air bypass. Advance is all connected and working and all vac lines are correct and smoke tested. I’m convinced there is something funny going on with the throttle plates or the idle motor (the idle motor is working to an extent because if you disconnect the ecu the idle shoots up to 2000).