High idle speed and no more surging after intake rebuild

[82DMC12]

OK I am going to try to forget about the idle hunt because maybe I have fixed so much stuff that my hunt will actually go away.

In the shop manual, it says the thermistor in the Y-pipe will provide for a faster idle if the coolant temp is below 59F. However that faster idle value is not given. The shop manual only describes that idle speed to generally be 775 +/- 50 RPM.

What should I expect as an idle speed in very cold conditions? I suppose this speed will not last long since it doesn't take too long to heat up the coolant.

[Bitsyncmaster]

OK I am going to try to forget about the idle hunt because maybe I have fixed so much stuff that my hunt will actually go away.

In the shop manual, it says the thermistor in the Y-pipe will provide for a faster idle if the coolant temp is below 59F. However that faster idle value is not given. The shop manual only describes that idle speed to generally be 775 +/- 50 RPM.

What should I expect as an idle speed in very cold conditions? I suppose this speed will not last long since it doesn't take too long to heat up the coolant.

If your engine hunts right away on a cold engine than probably the idle motor is sticking cold.

My car will hunt for a minuet or two right after the engine warms up enough to start the lambda running closed loop. What causes that is the lambda turning on before the O2 sensor is really warmed up enough. After that it never hunts. When I was running a wideband unit (heated O2) I never had hunt ever.

The stock idle ECU has one set idle speed (775 RPM). When cold the ECU just limits how much the idle motor can close hence causing a faster idle depending on a lot of other adjustments in the engine. When I was running the stock idle ECU I never saw any fast idle even down to 0 deg.F.

[82DMC12]

If your engine hunts right away on a cold engine than probably the idle motor is sticking cold.

My car will hunt for a minuet or two right after the engine warms up enough to start the lambda running closed loop. What causes that is the lambda turning on before the O2 sensor is really warmed up enough. After that it never hunts. When I was running a wideband unit (heated O2) I never had hunt ever.

The stock idle ECU has one set idle speed (775 RPM). When cold the ECU just limits how much the idle motor can close hence causing a faster idle depending on a lot of other adjustments in the engine. When I was running the stock idle ECU I never saw any fast idle even down to 0 deg.F.

Thanks Dave. Unfortunately we got a cold front and it's far too cold to be standing in the garage with the door open messing with idle. So, I'm just doing some basic checks for now. So far I have:

1) Confirmed the motor will clack to each side with 12V applied to the middle connector and then a ground applied briefly to each outer pin.
2) Sprayed some WD40 into the motor
3) Checked ohms on the pins - I'm getting 10 ohm between the left outer and inner, 10 ohm between the right outer and inner, and 20 ohm between the two outer pins. I found a thread that said I should be reading about double those numbers. Can anyone confirm?
4) Cleaning the contacts
5) Plugged in the IAC but it's disconnected from the in/out hoses. Turned the key to RUN and observed the motor is opening and then slowly closing over the next 30 seconds or so. Seems to be OK.

So my ohms are maybe low if the other thread is to be trusted.

2 more questions as I check stuff -

1) Any advice as to how tight to make the throttle cable going around the throttle spool? My first thought was to make it tight so that the throttle is snappy but I'm thinking that can be preventing it from returning to a closed position. I might try just smartly wrapping the cable around without trying to take all the slack out.

2) Advice on setting the length of the linkage between the throttle spool and the arm? Again, my thought was to remove all slack but that might also be working against me. Maybe it's better to a bit of slop so that the three parts can't risk binding.

[82DMC12]

Thanks Dave. Unfortunately we got a cold front and it's far too cold to be standing in the garage with the door open messing with idle. So, I'm just doing some basic checks for now. So far I have:

1) Confirmed the motor will clack to each side with 12V applied to the middle connector and then a ground applied briefly to each outer pin.
2) Sprayed some WD40 into the motor
3) Checked ohms on the pins - I'm getting 10 ohm between the left outer and inner, 10 ohm between the right outer and inner, and 20 ohm between the two outer pins. I found a thread that said I should be reading about double those numbers. Can anyone confirm?
4) Cleaning the contacts
5) Plugged in the IAC but it's disconnected from the in/out hoses. Turned the key to RUN and observed the motor is opening and then slowly closing over the next 30 seconds or so. Seems to be OK.

So my ohms are maybe low if the other thread is to be trusted.

2 more questions as I check stuff -

1) Any advice as to how tight to make the throttle cable going around the throttle spool? My first thought was to make it tight so that the throttle is snappy but I'm thinking that can be preventing it from returning to a closed position. I might try just smartly wrapping the cable around without trying to take all the slack out.

2) Advice on setting the length of the linkage between the throttle spool and the arm? Again, my thought was to remove all slack but that might also be working against me. Maybe it's better to a bit of slop so that the three parts can't risk binding.

All right, this problem is really kicking my butt. This weekend I pulled the intake manifold off again to very carefully rebuild everything from the bottom up starting with the six o-rings. Everything is spotless and this time I re-assembled with red rubber grease on the o-rings to make sure they aren't slipping around. I'm very confident I have zero vacuum leaks to the outside at this point in time. I'm to the point now where this is my list of symptoms:

1) Idle is starting at about 900 on a cold engine, dwell starts at 22 on 8 cyl scale which confirms open loop
2) Within a few minutes, idle starts to very slowly creep up, around this time we go to closed loop and I'm swinging back and forth over 20 deg, as expected, in closed loop
3) By the time the fans kick on, I'm at 1,400 RPM and I haven't touched the throttle. The engine sounds good and isn't missing or anything. Timing confirmed at exactly 13 deg.

I adjusted the throttle as explained in post #3 of this thread - http://dmctalk.org/showthread.php?72...-lever-at-idle

Now I have zero play in the linkage when my throttle spool is fully relaxed/ on the stop so my linkage is helping to push the arm into a closed position. There is no further play in the arm, so I can't make the throttle close any further by pressing on the arm (throttle is not hanging up).

Three air screws in throttle body are all closed. If I unplug the inner harness from the idle ECU, my RPMs go up to about 2,600, and drop back down to wherever they were before once I plug it back in.

This is very strange since I used to sit at 750 when warm and never had any throttle or linkage problems previously. During this rebuild, I painted the throttle body and ended up sanding the paint off where the throttle arm and spring goes just to make sure that wasn't causing it to bind.

My decel springs look normal and don't look "collapsed" or crushed like I've seen photos of bad decel springs. However I did do a lot of cleaning of the butterfly plates including the springs so maybe I somehow weakened them?

Is there any other test I can do short of replacing my throttle body?

[Bitsyncmaster]

Try pulling the vacuum advance hose going to the ignition distributor and see if that reduces the idle RPM.

Keep backing out that curb idle screw.

RPM will increase because the oil gets thinner as it warms up. So if the idle system is not working that would be the result. Does your idle motor hum (vibrate)?

[82DMC12]

Try pulling the vacuum advance hose going to the ignition distributor and see if that reduces the idle RPM.

Keep backing out that curb idle screw.

RPM will increase because the oil gets thinner as it warms up. So if the idle system is not working that would be the result. Does your idle motor hum (vibrate)?

I'll check the vacuum advance hose. I know I'm NOT getting vacuum advance at cold idle because I double-checked it while setting timing yesterday morning.

Will have to keep checking the curb idle screw but if I let it out any further I start to get that stickiness in the butterfly plates where they start to contact the inside of the throttle body. I have it adjust just a hair open so there is no stickiness when operating the arm manually. I followed that other dmctalk post I referenced in the above post which supposedly is a good way to set it. But I don't know.

My idle motor hums with key in RUN position. I applied +12 to the center pin and gounded either side pin and it snaps into place. I also see that when I turn the key to RUN, the motor opens and then slowly closes over the next 30 seconds or so. I have about 10 ohms between left pin and center pin, 10 ohms between right pin and center pin, and 20 ohms between outer pins with the motor removed the car. I cannot really tell if the motor is humming while the engine is running due to other vibrations. Is there anything else I can check on the ISM, or is that pretty definitive that it's working?

[Bitsyncmaster]

I'll check the vacuum advance hose. I know I'm NOT getting vacuum advance at cold idle because I double-checked it while setting timing yesterday morning.

Will have to keep checking the curb idle screw but if I let it out any further I start to get that stickiness in the butterfly plates where they start to contact the inside of the throttle body. I have it adjust just a hair open so there is no stickiness when operating the arm manually. I followed that other dmctalk post I referenced in the above post which supposedly is a good way to set it. But I don't know.

My idle motor hums with key in RUN position. I applied +12 to the center pin and gounded either side pin and it snaps into place. I also see that when I turn the key to RUN, the motor opens and then slowly closes over the next 30 seconds or so. I have about 10 ohms between left pin and center pin, 10 ohms between right pin and center pin, and 20 ohms between outer pins with the motor removed the car. I cannot really tell if the motor is humming while the engine is running due to other vibrations. Is there anything else I can check on the ISM, or is that pretty definitive that it's working?

I've not seen bad deceleration springs that don't look bad but maybe that is your problem. Can you press on the springs to make sure they have some tension? But since the problem came on suddenly that probably is not the problem.

[82DMC12]

I've not seen bad deceleration springs that don't look bad but maybe that is your problem. Can you press on the springs to make sure they have some tension? But since the problem came on suddenly that probably is not the problem.

Here's a picture of my decel springs. I can squeeze them and they move freely. The plate that gets sucked open by vacuum on each butterfly is fully seated on both sides. Hard to imagine this would be my problem unless it's possible for the springs to crap out without collapsing. Not sure how to measure that.

PXL_20220119_131015287.jpg

[Bitsyncmaster]

The springs look good.

[82DMC12]

I worked on this some more last night and can report the following:

1) There is no ignition advance at idle either hot or cold, confirmed by pulling the hose off the advance
2) I put my K-Jet gauge on this time while idling to make sure my control pressure is not going wild at the request of another owner - confirmed my control pressure starts at 1.8 bar (55F in my garage) and maxes out at 3.4 bar
3) Confirmed my idle switch is indeed closed and detected closed at the idle ECU by checking voltage between battery and black/green on the inner ECU plug
4) Confirmed the following readings from my thermistor at the ECU plug - when cold, 13.3 on the 20K scale, and when hot, 2.35 on the 20K scale.
5) Also tried jumping out the thermistor at the white church window plug by jumping black/slate and black/yellow. This made no difference.
6) Tried resetting idle screws one more time while the engine is hot - it dropped slightly but still elevated. Even by pushing on the throttle arm with the curb idle backed out, and ECU screw screwed in, it won't idle at 775.

Somehow it just keeps raising as it gets hot to about 1,300 RPM.

Is it possible the idle ECU is flaking out?