89 Dodge Raider 3.0 V6

Symptoms:
High idle - it stays at around 2300rpm at startup, and will stay there even after a "warmup", sitting idle for several minutes, driving around a few blocks. When the engine gets "hot" or driven for several miles, the idle will drop considerably, and will stall at idle. From there on out, the idle will be super low, and will even stall out while car is rolling as soon as you are in neutral and off the clutch. Low idle will manifest until you reset the ECU by disconnecting the battery for awhile. Then it will go back to a high idle state and stay high until engine gets hot again.

Tested:
ECU = good per Haynes (pulsing analog volt-meter reading)
IAC = good per Haynes (resistance within spec, vibrates when testing with voltage per manual)
Also tried an aftermarket IAC from Standard Motor Products, same symptoms, so I just put back the original.


I think I've read all pertinent threads re: high idle, including this one: http://www.4x4wire.com/forums/showflat.php?Cat=0&Number=1234868, but I'm not sure if it explains both high and low idle.

----

As an example today, I drove ~8 miles/~20 minutes out for lunch with the idle running high until I parked. Engine sat for a good 30/40 minutes, and when I started it up again (heat soak?), idle was around 1200... as soon as you drive off, when you put the gear in idle, it will stall. You'll need to drive with the right foot on the gas the entire time or else it'll stall anytime it's in neutral without additional gas.

I took off the negative cable at the battery for an hour and let it sit, and when I reconnected and started the motor (it is still pretty warm), it started around 1300rpm, i rev the motor, it idled at 1500, rev, 1800, rev, 2000, rev, 2300, rev rev rev, and stayed there (2300rpm).

What should I look at next? Should I try a known-good ECU to 110% make sure the ECU is not bad even if it is returning no codes? Should I take my chances at the junk yard with another throttle body? Anything I'm missing?

wiring MM

Also wondering if any of these would cause my symptoms:
1) engine coolant temp sensor
2) air conditioner switch / engine coolant temp switch?

I am assuming this is the engine coolant temp switch? It is forward of the thermostat housing, right above the engine coolant temp sensor.




The Raider Service Manual electrical diagram only shows a single Black/Yellow wire to the switch, but there is another green/red wire that is not in the diagram and looks to be wired incorrectly. The switch has three terminals for NO (normally open), NC (normally closed), and (closed). Which wire should be where?

Not sure if this is related, but my AC stopped working during a long trip from SF to LA a few months ago, when the high/low idle problem started.

Welcome to the wire. Try this link to check your ISC. http://www.stealth316.com/2-isc-iac.htm
And download the FSM from mitsubishi links. No offense but Haynes gets no respect around here.
If it tests ok there is a specific way to adjust it in the fsm. Not too tough since you are familiar with a meter.

Hoping to revisit this 7 years later LOL. I actually sold the truck after having a baby and immediately regretted it. I bought it back from the guy I sold it to last year, and I'm trying to fix this issue correctly.

The PO taped up the FIAV/ISC bypass passage inlet in the throttle body, just to get it driveable. I took out the tape, and want to fix it correctly. At cold start, the engine is at 1500RPM but will eventually climb up to 2200RPM and will stay there. I have checked:
- Engine Coolant Temp sensor tested within spec
- Idle Position Switch adjusted per FSM
- TPS tested and adjusted per FSM
- FIAV port sealed off
- Smoke tested, and no vacuum leaks
- Idle screw on throttle body closed all the way
- ISC replaced with new

Tried to adjust base idle speed per FSM by grounding ignition timing adjusting connector on firewall and #10 on the diagnosis connector in the glovebox (per FSM procedures) but adjusting screw is all the way in and RPM has no discernable difference.

I replaced the original ISC that tested out of spec, with a new Standard AC99 (black housing) that tests within spec (35-40s ohm). I also replaced the ISC harness that was cracking, and tested 0.2ohm resistance all the way to the ECU for all 6 wires. I opened up the ECU and no caps are leaking, but went ahead and preemptively replaced two caps, and there are no traces of leaks or burns of any kind anywhere in the ECU. I am certain the ISC is either not working or not getting the correct signal because I tested with the old ISC manually rotated it to the closed (furthest) position with the harness disconnected, and the RPMs were super low, which is basically how the PO had it with both ISC and FIAV ports blocked off manually. So basically, the ISC is allowing too much air through, but everything checks out.

I gave up and went to C&A in Van Nuys to get an opinion and he mentioned to get the throttle body rebuilt. Would rebuilding the throttle body address my high idle issue?? What would you do in my position? Can anyone enlighten me if I missed anything before I start throwing more money at it?

As long as the Throttle plate is closing down ass the way, the base idle adjustment is not opened up from the minimum.I would then consider the isc
The isc is not driven or related to the caps,
I used to know this circuit but it has been a long time since soldering part inside an ECU.
The row of dual relays has or had something to do with idle but mostly control thing like fuel pump and other devices.
Not having these fresh in mind and with the dumming down online searches to paid advertisers only I am not really sure how to assist.
I will try to get this back in my mind since 15 years ago I have been there.

I was thinking since I was able to drop the RPM by essentially plugging the ISC hole, that's where my problem is.. I guess rebuilding the throttle body is nice to do, but I don't understand how it would solve my problem

Humm, I can not get back in my post and change the SS to LL, sorry about that.
The idle speed control is a stepper motor. If plugging the passage lowers the speed either the stepper motor has failed or the drivers on the circuit board in the ECU are bad. Could always be a few wires are bad in the harness.

The winter is not a great time for it, but an FIAV Delete will probly solve the issue. That same forum Oso mentioned is where I found the solution my high idle problem. There is one line in the FSM that if you do all the tests and adjustments per the FSM and it doesn't solve your issues, you should replace the throttle body. 🤦‍♂️🤦‍♂️🤦‍♂️ it took me a couple months of trying and testing all the different pieces to sole that. Found the FIAV Delete info,made one, put it on, and immediately had 800rpm. Kinda rough on cold mornings, but I'm in Florida so it's not too bad. Something to contemplate that is completely reversible.

I actually tried purchasing those fancy FIAV delete plates, but my throttle body looks different:

I ended up sealing one of the FIAV ports with JB-Weld instead, and keeping the ISC port open. I was really hoping that would be the end of it, but it looks like the ISC is not cooperating for some reason.



So since my last posting, I had the throttle body rebuilt (TPS and ISC tested to spec), and the ECU checked out by an ECU shop (they could not find any problems). My idle is still high ~2800 with temp gauge in the middle after all this high idling. Before I shut off the engine, I unplugged the ISC harness to "capture" the state of the ISC, shut off the engine, removed the ISC and saw the pintle is not extended at all, as if the engine was cold. So it's either the stepper motor isn't working right (but specced OK??), or the engine thinks it's cold?? The ISC vibrates when the key is switched on (also the harness to the ECU tested around 0.1 ohm resistance for all wires, so it looks like the ECU can talk to the ISC no problem).


Old Bad ISC, manually extended


New tested-good ISC, unplugged harness while warm engine running

Tomorrow I am planning to install the old ISC (manually extended) into the throttle body, and connect the harness to the tested-good ISC motor hanging free outside the throttle body, so I can see if it does anything at all when I rev the engine.

Not sure what else this could be, I am thinking maybe test with a known good ECU would be the last final step?