How to get rid of fast idle when hot?

[mrdance123]

In the driveway, my idle is what I want, 600 rpm when warmed up.
Idle speed screw can be adjusted even lower, say <400 - (no big vacuum leaks).
In the driveway, idle always returns to 600 in a few quick seconds after prolonged revving at 3000.
BUT...when driving, the idle remains way too high for too long when stopping for a light,
keeping 1200 - 1800 rpm 30 seconds or longer, huge waste of fuel in city driving.
Will not reduce to 600rpm, unless shut off for a few minutes.
I don't care if it's to reduce emmissions, I want it to stop doing this.
I removed the TP throttle opener screw from it's pattle, so it's not that.
The throttle valve is not sticking, and the cable has almost 1" of slack, so it's not that either.
I put in stronger springs on the throttle lever, no help there.
I pulled over with this fast idle many times,
to find the throttle not physically stuck above idle, and choke is fully open.
I removed all vacuum lines from carb, capping both ends, with no change in behavior.
Replaced PCV valve & grommet, no change.

[dlb]

does it get up to regular operating temp in the driveway? it doesn't make sense to me that the symptoms would occur only when the car is moving so it seems it must be temp related.

it sounds like a vacuum leak to me, or maybe a stuck choke, but might also be a leaking AAP. disconnect the hose that goes to the AAP and plug it and see if that helps your high idle.

[xirdneh]

I have similar problem
When driving stop and go in town
On warm days it idles high at stop lights (1000-1200)
On cold days 600-700

[irowiki]

Hot idle compensation (the HIC valve) might not be working right.

[mrdance123]

I tried disconnecting HIC valve, by unhooking the line under the air cleaner and plugging it. No change in hot idle speed behavior. If this vacuum line is loose and unplugged, it makes the engine slow and die, it does not speed up the idle. So I don't understand why HIC might be causing this (help explain your theory please?) I also disconnected and plugged vacuum line to AAP, no change in behavior. And in the driveway, the engine is fully warmed to the point the radiator top gets hot, the fan cycles on and off, coolant at 180F (I have add-on digital heat gauge), and the idle still remains NORMAL like I want it, even after prolonged 3000rpm revving in driveway. The idle remains slow, until AFTER I start driving. It will idle too fast until the engine has been shut off for a few minutes, I've never seen idle slow down to normal again unless turned off first. If it was not left turned off long enough at least 5 minutes, idle resumes being too high after restart. Coolant is still 170 or more after 5 or 10 minutes, then Idle slows when restarted. I also tried unhooking and plugging the small line from the charcoal canister, and sealing off the carb port it attaches to, this did not help. It will cause idling engine to slow or die if unplugged without sealing the port, and it has an electrical vacuum switch in it's line, that's why I tried this. I used both propane and carb cleaner searching thoroughly for leaks, nothing found. Propane also directed to underneath side of intake thru a hose, no change in engine speed, no explosions either. If I hold the throttle exactly at right point in the driveway, it does the cycle speed up and down thing with the fuel cutoff solenoid, about once per second, about 1700 to 2300rpm... I don't know why, that never happens while driving. I have indicator light hooked onto cutoff solenoid, it lights briefly only when decelerating from mid-high rpms, or when doing the cycle thing just mentioned.

[xirdneh]

Make sure the throttle cable is not too tight and that the big spring is there (pulls throttle mechanism back to idle)

[xirdneh]

When all else fails swap carb with a known good one, that's what I did.

[tercel4wdrules]

Check the TP screw on the front of the carb. If the TP system is not working properly it will cause the idle to go up. You can back off the screw and lower the idle if this is the case.

[Petros]

Just a guess: use some spray carb cleaner to see if the throttle shaft is worn and allowing air in past the throttle plate because of a worn carb throttle body. It will act like a vacuum leak. it may behave normal when sitting still because the throttle shaft id in its "normal" position, but is pulled out of line when driving on a warm day and stays that when when you come to a stop, but drops back into its worn spot after a few min of fast idle.

I have seen worn throttle shafts, or throttle shaft housing, make it act odd, and cause a fast idle at random intervals. the fix it to either replace the carb with a good one, or there are shops that can over bore the throttle shaft housing and put bushing in it.

good luck.

[mrdance123]

Thanks guys for your ideas! I removed the TP screw long ago.... checked and wiggled the throttle lever while idling fast many times, definitely not sticking. I'll try checking vacuum leak at a possibly worn throttle shaft with brake cleaner..... and I have the same fast idle behavior with 2 different carbs if that means anything I don't know. I put in a stronger than stock spring on the throttle lever, and have added second spring and that did not help, and have almost 1" of slack on the petal cable.

[mrdance123]

Now I've done it. Big mistake.
I was testing the carb solenoids activating them manually with 12v,
forgetting to disconnect the wire harness from carb to car.
Burned out the ECU under dash over glove box, most likely.
Now the fuel cut solenoid on primary is always on,
(commanded by ECU) it runs terrible.
Cutting the solenoid wires does not help it to run any better.
Now I'm looking for the creamy yellow colored ECU. (some are blue)
MP-663 89550-16090 1984 Federal.
I have spare ECU in blue box, MP-657, the harness connector is identical.
Any ideas if it's wise to try that one as replacement?

[irowiki]

I would say give it a try.

[xirdneh]

I have swapped different colors a few times and noticed no difference

[mrdance123]

I swapped in ECU from my other running car, no difference, problem remains,
1st fuel cut solenoid remains OFF at all times, engine runs very poorly won't idle,
very hard to start unless I give it a squirt of starting fluid.
There is 12v on the positive lead to both 1st and second solenoid.
If I'm reading the schematic correctly (page EC3 in online 1985 shop manual)
The 1st solenoid is activated by the ECU completing it's circuit to ground,
and the 2nd solenoid is always on with ignition.
Normally, the ECU turns off the 1st solenoid momentarily upon deceleration,
which I have observed on both my T4's, while monitoring voltage to the solenoids,
but not now after I blew something out.
I can ground the 1st solenoid ground wire myself directly and it turns on, car will idle,
but that's not a good fix, car still runs very poorly.
I'm thinking something disabled the ECU, disconnecting it's power source, or it's ground.
Anyone have a better wiring diagram?
Which ECU harness leads are power and ground?
Are there hidden fuses, other than main fuse block under hood,
the driver kickpanel, and the one fuse behind passenger speaker next to glove box?
All known fuses test good.

[irowiki]

I am pretty sure the ECU does not control the solenoid but one of these vacuum switches does there are two vacuum switches