IAC ? and High Idle (3FE FJ80)

[rkuffel]

Well, I did the timing today. What a P.I.T.A.! Anyway, it was ever so slightly advanced; it may have dropped 100 rpm or so when I retarded it to factory spec. It is still revving way too high. It's about 1050 in neutral and about 750-800 in gear. I'm going insane trying to figure this out!
The only items left to check (according to the FSM) are the fuel pressure and the ECM.

Can someone tell me what a "normal" warm up cycle is like with the 3FE? When started cold, what are the revs in neutral and in gear? Is there an obvious jump/decrease in RPMs when it warms up? I'm not seeing any decrease in RPMs; it starts high and stays high, I better double check the new temperature sensor...maybe it's not working or the signal isn't getting to the ECM. I would think prolonged engine cold signal would throw a code, but who knows.

 

[rkuffel]

Tested continuity from the water temp sensor to ECU. It tested good. Now I'm starting to wonder about the AC switch and possibly fuel pressure. Hey, by the time I'm done figuring this out, I will have gone through everything on this car.

 

[rkuffel]

So I read a ton of posts last night on high idle and a lot of people seemed to fix their high idle by cleaning the throttle body. Can someone explain why this would fix a high idle? Is it because the butterfly valve would not completely shut? I don't get it...

 

[CJF]

Is it because the butterfly valve would not completely shut?

Yes. Simple as that.

 

[rkuffel]

Partial success! Last night, I replaced the motor and plunger of the new IAC with the old motor and plunger I got working again. I manually moved the plunger all the way closed (via grounding of the pins per the IAC test procedure). Then I unplugged the ECM. Turned the key and it revved to 1700 but dropped to 750 as it warmed up. I actually think the fix may have been unplugging the ECM. Which is hilarious after pulling my hair out trying to find anything wrong. I wonder if the 1992 ECM has a idle learning mode? I'm going to drive it now and see if it keeps dropping or the symptoms come back. But for now I will call it a partial success. I did notice that the PO unhooked the air-conditioner and that the idle doesn't move up when the switch is turned on...

 

[rkuffel]

At the risk of looking like a moron, I'm going to post this so that others can avoid what I did. After doing every test in the FSM and about 6 months later, I decided to look at my high idle issue with fresh eyes. I realized the dash pot screw was adjusted such that the butterfly valves were not completely closed and the throttle stop screw was ever so slightly above the stop. Visually, the throttle looked closed, lesson learned: it takes a very tiny amount of air to make the idle raise. So, when looking for high idle issues, start by checking that the throttle stop screw is hitting the throttle screw stop plate...don't get it confused with the dashpot screw. The throttle screw is next to the dashpot screw but more toward the fuel rail and set down a little. So, with an hour of adjusting the screw, the dashpot screw and then the throttle position sensor, I've got a perfect 650 RPM idle. By the way, this nice LC is for sale in Spokane WA now. Thanks for all the help and I hope this helps someone in the future.

 

[Radwelder]

I know this thread is old but it’s a new problem for me when I test my ISC valve with positive at B1 and negative at B2 only S1 and S3 move the plunger if I reversed polarity with negative at B1 and positive at B2 only S2 and S4 move the plunger. They do not work in the sequence of 1234 or 4321 as FMS shows

 

[ocinataS]

I know this thread is old but it’s a new problem for me when I test my ISC valve with positive at B1 and negative at B2 only S1 and S3 move the plunger if I reversed polarity with negative at B1 and positive at B2 only S2 and S4 move the plunger. They do not work in the sequence of 1234 or 4321 as FMS shows

Aren’t you supposed to connect positive to both B1 and B2?