Low idle speed even in park, accessories lowing it further

So here's my current plan of action:
1) Replace cam shaft sensor (already have a new one)
2) Replace PCV Valve ($2 part)
3) Clean EGR Valve (just need some throttle body cleaner)

I can't say for sure any of these are causing the issue. But I know the PCV and EGR valves have never been touched. And I've had a cam shaft sensor go bad on me before, and I noticed it had oil in it again. So not unlikely it is failing.

Got some freeze frame data from my rudimentary code reader.

P0506 IAC RPM Lower Than Expected
Fuel System 1: CL
Fuel System 2: N/A
Load Percentage: 22.4
ETC: 203
SHRTFT1 (%) 3.1
LONGFT1 (%) 0.8
RPM: 683
VSS (mph): 0

Alright here's an update from the dealership: The IAC is definitely malfunctioning. However it could be caused by the ECU. Evidently when IAC's go out, they can send a voltage spike to the ECU and damage it. Given how many times I've replaced the IAC to have the problem keep coming back, I'm thinking your hunch about the computer could be right. The problem is the dealership can't repair ECU's, only flash them, and Mitsubishi doesn't supply the computers anymore. Know of any good mail-in ECU repair services? Evidently there's a local ECU repair place in Denver, although only has 3.3/5 reviews. I still sent them a message.

I've had my code reader sometimes temporarily not connect to the computer for hours. Evidently the way you narrow that down is to start pulling all sensors, and trying the code reader again. After you have removed all sensors, if the code reader still won't connect, it's probably the computer. Anyway I disconnected the IAC and connected the code reader when this was happening, and it didn't get it to connect again. But disconnecting all 4 connectors on the ECU and plugging them back in got it to work again.

The other option is to get a used computer and try that. I can't seem to get a straight answer though. Will it be plug and play or will it need to be recoded to my VIN? The Mitsubishi dealership said I should try and get the immobilizer from the same car so they perfectly match if I do that. Only problem is nobody properly tests these used ECU's. They just say "Came from running vehicle". Well, my car is running, but that doesn't mean some of the parts aren't properly functioning. So I'm trying to get it repaired before going down the replacement route.

$250.00 for a refurbished ECU programmed to my VIN.
$135.00 repair of the actual ECU but about 2 weeks without my car.

Looking over their notes again, they said the OEM used IAC that I put in there was testing for the proper ohms. So I think I'm going to get the $250.00 refurbished computer that's coded to my VIN and should be good to go finally.

I had decided I'd rather send my ECU for repair rather than get a refurbished one, that way I can get it diagnosed and know for sure there was an issue. I haven't had a chance to do that, however. I did take out the ECU and visually inspect it and thoroughly clean it with some WD40 Contact Cleaner. It had a lot of yellow gunk on it and I got that off. There were no burn marks and no exploded or loose chips. So unfortunately I'm still unsure about the computer.

Recently I started having really bad stuttering/hesitating/stumbling acceleration issues. With the throttle pushed in up to half way it accelerates fine. Anything past that and it starts to stumble badly, sometimes having RPMs rise and drop 3-4 times before finally catching and accelerating fast as it should. I've had this maybe a week and finally started getting P0121 Throttle Pedal Position Sensor/Switch (TPS) A Circuit Range Performance Problem. Now it keeps coming back nonstop. Only issue here is I've been dealing with this code for years, ever since I switched out the TPS yearss ago trying to fix a different problem. I never properly calibrated the TPS so it has triggered this code forever. When the mechanic switched the throttle body a few weeks ago we manually calibrated the TPS to the .535 - .735 Volt range which is spec. The strange thing though is that the voltage was jumping around a lot, between .624-.735 or even moreso. From watching a video I'll link below, it seems the voltage should hold way more steady than that at idle. Also the TPS was showing 12.9% when it was completely closed. I read that if it's above 5% when the pedal isn't pressed, this can cause problems. I'm not sure what % it was at when completely open, but that's supposed to be above 95%, and 100% ideally, which I doubt it is. Again with the video below, he mentions that the voltage issues are intermittent and that generally to recreate intermittent issues, it involves vibration and heat. Well, my car has bad vibration problems due to worn out engine, timing case, and transmission mounts, which I know I need to replace. I've also noticed the throttle body gets VERY hot. I know that when my camshaft sensor went out 2 years ago, it was on a 100 degree day when I was driving at 80 MPH or so. So I'm thinking that heat and vibration is causing my sensors to get knocked out intermittently. Ever since I replaced the IAC with the OEM one, cleaned it thoroughly, as well as cleaned the ECU, it seems to be at least partially functional. So I'm thinking of doing the same with the TPS, getting a known working OEM one and replacing it and calibrating it properly, adjusting the throttle cable to ensure it's below 5% when the pedal isn't pressed, etc. From doing some reading it sounds like my car may be in fail safe / limp mode, and is reducing the amount of the effect the TPS can have since it's out of range. Although I fully reset the computer and noticed the issue was there again right away when I started it back up, which says to me the TPS is actually malfunctioning.

One other thing I've been curious about is the TPS actually causing the IAC to not function properly. Since the IAC won't kick on until the TPS is at or near 0%, so if it's sitting at 12.9% at idle, I'm wondering if that can mess with the IAC. Certainly if the voltages are out of range, that can cause confusion. Anyway I'm having my mechanic look at it again tomorrow. I have 2 different cheap TPS's but I think they're probably both bad. I'm thinking I'll use a known good OEM TPS, properly calibrate it and adjust the throttle cable properly, etc. Also I'll try and sort out the heat and vibration issues, I may need to replace the water pump since my heat gauge is going slightly over 50% while idling sometimes, and I have a nearly new radiator which has no leaks, and the fan is working fine. The fuel pump is also new and fully functional, which I've read can also cause overheating issues at idle. I'll report with more specifics tomorrow, going to have the mechanic fully test the IAC and TPS tomorrow.

https://youtu.be/8RB7BlLnTiw
Hesitation and idle problems from a bad TPS (OBD1 Eclipse)

I got a basic ELM327 OBD2 Bluetooth scanner and checked out the TPS. From looking at the notes from the dealership, they reset my throttle cable, and I do see the throttle % as 0% when stationary with the engine off. With the car running, the throttle % is bouncing around between 0-1.5% or so. I would imagine this is definitely causing part of my vibration problems. 2 shops have been blaming the vibration on my engine mounts being worn out. Does it seem likely that vibration from worn out engine mounts is translating to the TPS voltage / TPS % bouncing around slightly like it is? I know the TPS being worn out itself also causes vibration and rough idle, so it's a bit of a chicken and egg scenario. Any idea if it's normal for this to happen with a TPS? If it's this sensitive to vibration, I don't see how it could ever sit at perfectly 0% with the engine running.

BTW as for my horrible bucking acceleration, the graph for the TPS showed this perfectly. It's a jagged mess full of dead zones, and won't even go above 70%. I didn't realize how bad this TPS really is.

Anyway I have a tested working used OEM TPS that my mechanic will be putting in for me soon. I was going to do it myself, but all the Android OBD2 scanner apps I looked at don't have TPS voltage, only throttle %. So I can't properly calibrate it between.535 -.735 Volts, which means I'd get an CEL code. At least I know the TPS is definitely very bad though. I just wish I'd feel confident that the new TPS wouldn't have the voltage bouncing between.5-.7 Volts, and throttle % between 0-1.5% while idling. But it seems vibration is causing that since with the engine off, the throttle % sits firmly at 0%.

Sounds like the TPS voltage bouncing around could be a grounding issue? Not sure if this could be related or not, but ever since I got the car in 2004 I've had a dim Fuel gauge, while the rest of the instrument panel is bright. More recently I noticed half of my Heat/A/C/Defrost lights are dim on one side too. I didn't start out knowing anything about cars, always figured there was a bad bulb in there. Now that I know a bit more, that seems like a grounding issue. Not sure if it could be related to the TPS voltage bouncing around or not. Was also reading that a bad alternator diode could cause that. Pretty sure the alternator voltage was fine though when we checked it.

Anyway, I guess we'd just need to test the ground pin on the TPS connector to know if it's a grounding issue?

This post is about a motorcycle I think, but sounds like a possible solution is just wiring in a more direct ground to the TPS ground wire. Obviously I'll also have to see if the new TPS fixes this issue. My gut tells me it won't though since it's staying stable with the engine off, which means it isn't sending out fluctuating voltages all on its own. Something else must be causing it, my suspicions being grounding issue or vibrations.

"To stabilize a TPS signal that may be picking up noise from the bike’s electrical system, or aftermarket electrical components such as plug wires, exotic high-end racing style coils, gapless spark plugs etc, we can reroute what we call the digital ground wire. Rerouting from the main ground location directly to the ground at the TPS sensor itself provides a shorter ground path and less chance of picking up the noise that may be causing the jumping TPS reading problem. Rerouting the wire in this manner often stabilizes the TPS from fluctuating."

https://dynojet.zendesk.com/hc/en-us/articles/360003719854-Unstable-TPS-Signal-Power-Commander-V

From doing more research, it seems to point to either the TPS itself or a grounding issue. Didn't find anyone saying that engine vibration had any effect on the TPS voltage. So these will be the next steps when my mechanic is available. From reading about the fluctuating TPS voltages, I saw a lot of people having similar idle problems to me. So I'm feeling confident that with a good TPS and with the fluctuating issue fixed, that the low idle shouldn't resurface.

So I had my mechanic swap the TPS with a good tested OEM used one and calibrate it. I had him show me the voltage on his snap on tools scanner, which was set to.58 I believe and was fluctuating much less than the old TPS, it was going between.58-.60 Volts and 0.0-0.4% throttle. Whereas the old one was doing this between.6-.724 or so and 0.0-1.5% throttle. He didn't get a chance to ground it out so I still want to try that to keep it completely steady. How should we do this? There are engine ground cables that are 10" long or so, wasn't sure if I should have brought him one to use for that.

Anyway, the bucking acceleration problems were fixed by the TPS. It seems the low idle and stalling at low speeds (especially while decelerating to a stop) were caused by a bad TPS. And the bucking idle and problems with the throttle plate getting stuck closed and not accelerating from a stop / not moving, were caused by a bad throttle body. I did replace the cheap China knockoff IAC with a good used OEM one and the IAC seems to be working fine now. It's difficult to separate what was caused by IAC and TPS, but I do know the TPS was VERY worn out and had tons of dead spots.

As for the computer, I've had it tested before a long time ago (6 years ago) and they said it had no issues. I think it's probably fine as I don't think the IAC actually shorted out, I think it was issues with the TPS when it comes to the low idle and stalling from low RPM's.

Yes, the car is running perfectly now. I will just have to monitor the idle RPM's and make sure the low idle problem doesn't return. At which point I'd have to reconsider sending in my computer for diagnostic and repair. I'm pretty confident that the good TPS and IAC fixed all these issues though.

And now that I think about it more, yeah, the IAC functionality started working after cleaning and installing the OEM IAC. Now when I turn on the A/C, the RPM's kick up 100-200. So yeah I agree, both a bad IAC and TPS.