'94 4G64 idle issues

Hello All! I'm wrestling with some funky idle issues on my '94 4G64 MM and hoping someone will know what's going on. Here's what the truck is doing:

- I started noticing idle issues when the AC was running, where the truck would idle down very low and almost felt like it would stall out. This got me wondering about the idle control solenoid and the problem continued to get more noticeable over the course of a few weeks.
- Without the AC running, sometimes the truck would idle way up. Sometimes, it would pulsate and rev up and down at idle.
- Apart from the idle issues, the truck runs just great. Plenty of power and revs up normally.

So, I tested the ohms on the idle control and it was outside the range in my manual. Not waaaaaay outside but I enough that I thought maybe that was the problem. I found a good used unit and put that in but I'm still having idle problems.

I tested the ohms on the throttle position sensor and it's in spec so I'm not suspicious there.

Edit - Forgot to mention that I cleaned the MAF with MAF cleaner, it looked spotless already and there was no change afterward.

I went to start the truck yesterday and heard a click from behind the glove box (this is a US-spec truck) and absolutely everything electrical stopped working. Interior lights, the whole deal. Disconnected and reconnected the battery and it came back. Huh...

This morning I pulled the glove box to listen for that relay and look for anything out of sorts. I can hear it popping open and closed and the truck started up fine. At first, it idled normally. Here's what has me puzzled. When I closed the passenger side door (not super gentle but not slamming it either), the truck immediately idled up high. I know the ECU is sitting right on the other side of that door hinge, is it possible that the ECU is the source of the issues here and the impact of closing the door made a difference?

It looks like a pain to pull and rather than cross my fingers and fire the parts cannon again, I figured I'd check with the experts here.

The original owner kept service records and the original ECU was replaced at a Mitsu dealer in 2007, about 27k miles ago. What do you all think, are these symptoms of ECU issues or am I headed in the wrong direction?

I've read some posts where the ECU was the issue, and they mention bad capacitors. I pulled the ECU to take a peek (way easier than I thought it was) but didn't see anything that stood out to me. But I've also never looked at the guts of one of these. Took a few photos just in case anyone sees something amiss here.

http://www.mightyram50.net/vbulletin...id=27466&stc=1
http://www.mightyram50.net/vbulletin...id=27467&stc=1

The tell tales signs of a bad ECU - swollen capacitors or stains/signs of corrosion around the bases of them and a bad smell. It's been described as smelling like fish? You said you heard a click and lost all on board power. Is there a main power relay in that corner of the truck?

The caps in my ECU looked like new and I didn't notice any kind of leakage or smell so I'm hoping that is good to go. The main power relay is mounted right below the ECU in my truck, and that's where the clicking is coming from. Since disconnecting and reconnecting the battery, it's working normally but I have to wonder if a poor connection or bad ground there could be causing the erratic behavior. Especially when shutting that door (it's right on the other side of the hinges) resulting in the truck idling way up. Maybe a coincidence, maybe not. Not sure if there's a way to test that relay or grounding, I didn't think about that...

Bad grounds are notorious for causing electrical faults. It's always a good idea to go around the truck and disconnect each ground and clean them one at a time if you know it hasn't been done previously. When the truck idled up, did you hear a 'sucking' sound from the intake/throttle body? The disconnection from the ground circuit may have caused the IAC to open, bypassing the throttle body :shrug:

When the idle is fluctuating, you can actually watch the throttle mechanism bounce back and forth. Something is telling that solenoid to move in and out, just not sure what.

This weekend I checked the grounds, and the body one was a bit gritty. Wire wheeled and reassembled with some dielectric grease. The resistance from the throttle body to the negative post on the battery is very low, so that connection would seem to be good. The problem still seems to be there, but I noticed some of the jacketing on the throttle position sensor wiring is cracking. I'm going to hit it with a bit of liquid electrical tape and try to make sure everything is sealed up as best it can be there. The truck is also showing a bit of a miss now at idle, not sure if it's related or not. It got a fresh distributor cap and rotor during maintenance not long ago, might try pulling the plugs and see if there's a smoking gun there.

Gotta get ol' Frank in top shape soon!

Hello all, still wrestling with this issue. I took the truck to my local shop and he wasn't sure what to make of it, and he said replacing the ECU was worth a try. Unfortunately, that didn't get me anywhere. I keep hunting for bad connections, etc. and noticed something this afternoon.

When the truck is idling high and misfiring, unplugging the MAF brings it back to normal. Idle comes back down and even the miss goes away. Now, I can't run it like that since the check engine light comes on and I lose the ability to run A/C... but it's a very consistent cause/effect when I plug and unplug that connector.

The question is... is this indicative of a bad MAF? Would a truck with a good working MAF idle down in that situation? Replacements for these trucks sure seem hard to come by, might be impossible to find new anymore. I cleaned it with MAF-specific cleaner at the beginning of this wild goose chase, any experts know if trying to find a replacement unit is a good bet?

All I can suggest is IMHO if you can get an original Mitsubishi part it should out perform aftermarket stuff !?!

Usually disconnecting a sensor will indicate if you are on the right track to diagnosing a fault. Like most cars with a crank/start issue, if you unplug sensors one at a time you'll eventually find the one that is non functioning. IMO you have found the culprit.

*check error codes!!!

Unfortunately, no change with a different MAF. This really has me stumped.

I'm going to put a new grounding cable on this weekend, but I'm not convinced that will get me anywhere. Any other ideas out there?

I picked up an analog voltmeter and I'm going to see if the ECU tells me anything.

While I wait for the analog voltmeter to show up, the next clue. Disconnecting the coolant temperature sensor does the same thing as disconnecting the MAF. Truck idles down and sounds just fine. I keep looking at the schematic for the EFI system and wondering if something has failed in a way where a common ground is getting noise or signal that shouldn't be there? Maybe I'm overthinking it.

I LIKE THIS SITE IT HAS COLOR PICS VIDS ETC:
https://www.worldwidefaqs.com/what-causes-a-high-idle/

...hope you don't have a broken wire in the harness. It's a common issue with DSM cars that the harness ends up failing somewhere and breaking the circuit. It's due to age.

Hey everyone, this week I finally sorted out the idle issue and wanted to post in case anyone else is wrestling with something like this.

It's taken a long time to chase this down, and I spent a lot of time scrutinizing the schematics for the EFI system on this truck. I purchased the old OEM service manuals and scrutinized just about everything. Tried replacing more parts than I'd care to admit as well. You name it, I've probably looked at it, pulled it apart, tested it, cleaned it, replaced it, etc. Two mechanics took a look at this truck and shrugged their shoulders.

The biggest clue came when I realized that disconnecting the AC coolant temp sensor would return the truck to mostly normal without a check engine light. MPGs were normal too, so I figure it wasn't running the "dummy" ECU map. That was good enough to get the truck through its last NC state inspection (it turns 30 next year and will be exempt). So, I ran it over the winter with the AC electricals disconnected but of course warmer weather is approaching so I've been back on this over the past couple of weeks.

I spent a ton of time on the EFI schematic and chasing anything that looked suspect at all. The big change was when I started focusing on the AC system schematic. I realized there's a small electrical controller module for the air conditioning system that pipes two wires directly into the ECU. The schematic showed that module has two grounding wires in the pigtail that are bonded together and have one discrete ground. I located that module, it's on the left side of the black AC box under the dashboard behind the glove box. (Mine is a LHD US-spec truck) Pulled that connector and tested the black leads. They had continuity to each other but not to ground. They both terminate to a o-ring terminal that fits under a screw at the top of the AC unit. That screw is in a small L-shaped bit of metal that bolts on one end to the AC box and the other to the firewall. I started the truck with the AC electricals connected and of course it ran poorly. I connected a jumper wire from that L-bracket to ground and... ta-da everything was immediately normal. Disconnect it... broken. Reconnect it... normal. I finally found the smoking gun after 6 months of chasing my tail. That ungrounded module must've been sending some wild signals to the ECU causing it to idle super high and the timing was all over the place.

It also explains why early on the truck would sometimes idle up if you shut the passenger door with some force. That ground is located only about 18 or so inches from the door and it was iffy. The impact would sometimes break continuity and the truck would start to act strangely. After a while, the door thing didn't make a difference anymore because the ground point had lost continuity entirely.

Anyhow, cleaned and tightened the relevant points and everything is back to normal. Running like an absolute champ and the AC works great just in time for warmer weather. An obscure little ground point for the AC system located way up under the dash had me banging my head against the wall for months. I suppose the silver lining is that I now have a big box full of good spares for the EFI system if I ever need them. If you're wrestling with something similar please don't hesitate to reach out. I'd be glad to help however I can.

Thank you to all who offered help both in the thread and in private messages and even on the phone. So glad ol' Frank is back to normal!

It turned out to be a bad ground. And in a hard to find place too... Bad grounds can cause all kinds of weird faults :shakehead: