Rough Idle and TERRIBLE fuel mileage

If the IACV has coolant lines running through it, you might have some air in the system and it might be affecting it. If this is true, let car cool off, turn heat all the way up and take radiator cap off. Let the car reach opoerating temp and add coolant. Also look for foam in coolant as that is indication of air inssytem. Repeat process once if you note foaming or coolant level is not full or if idle is still low.
You might also need to clean the ports out that the IACV opens to control the idle. They start on the outside of the throttle body just in front of the butterfly valve and end just inside the intake manifold near entrance. Spray cleaner from outside in towards direction of air flow as doing the sopposite can make deposits go deeper into ports.
I am not sure what else could affect idle except TPS.

Yes even if it moved it does not mean it is moving as it should. I forget, did you ever get an IACV code?
You never know about the EGR. I don't know if it is possible for it to have a vacuum leak there or allow too much flow at the wrong time but it is worth trying at this point since you are getting down to the wire on options for the problem source. Daignosing can be really tough sometimes. All you can do is logically eliminate possibilities and you have certainly done that. The only thing I have not done is give you our link to a diagnostic walk through for engines with rough idle issues. There are several of them that are based on; "Performance loss", "Surging Idle", etc. So I will give you the rough idle just for the sake of having it to look through to see if there is something that has been overlooked as a possibility. If you want any of the others, just let me know.

Here is the link;

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Looking back through posts I thought you might want, "surging Idle"

https://www.2carpros.com/articles/engine-surges

I Also was thinking about the fact you ran a can of SEA FOAM which is good. But I reccomend if you have never run a fuel system cleaner to use two cans of BG44K. It is literraly a miracle worker and it removes so many deposits, which end up in the oil, that you have to change the poil after using it. Use One can per tank and then run a third tank without it before changing oil as it still takes some time for all depostits to work there way out. Sometimes the car may even run a little funny until the deposits are out and the computer adjusts. I used it on an olde motor that had a ton of crud on the pistons and was running on after it was turned off or dieseling. It cleaned the pistons off so you could see the tops of them. It made the oil black within three tanks of gas and it ran like crap at first and then it was better than ever. You have to go to a car dealership to find it and most dealerships carry it. You might call first. They also offer a direct injector clean but I think it is better to run through gas to get the whole system. It runs $20 per can or bottle but is worth it. You can also dump it in the vacuum hose of the brake booster to do a quick clean. But you end up smoking out your neighbors for five minutes or so and I don't think it works as well but techas have told me that it gets into other areas that adding to gas won't.
Anyway, just a thought. SEA FOAM is good but BG is truly better. It is caustic so be careful and have a rag and some water and soap if you think you might get it on your paint. I hold a rag under the bottle when I do it.

Yes, I don't really go with the method of dumping cleaner in the vacuum line or in any other way where it is not diluted with gasonline.
BG44K says that you at least need 10 gallons, or something like that, to treat fuel.
So, the method for using the two cans of BG44K is like you said;
Get a full tank of gas and add one bottle/can, they recently started putting it in bottles used to be in cans only.
Run that tank until near empty.
Fill the tank again and put in the second bottle of BG44K and at one or two points, drive the car a little high in RPMs. Stomping on it to where it kicks down a gear and changes RPMs quickly is good for high vacuum and the quick change helps to remove deposits. Don't rag on it just get on it once or twice. I got that advice a long time ago from a guy who builds some of the fastest drag motors in the country George Reggio. He said that any kind of performance motor does not do so well lugging it around all the time. They are meant to run, not be beaten or ragged on but run, and it helps keep them clean.
Run the second tank to near empty.
Fill the tank again without any cleaner.
When the tank is near empty or soon there after, change the oil as it will get dirty and even if it does not look dirty it is good to get the cleaner remains out of there just to make sure the oil is still up to its needed capabilities as additives can make it less capable of holding viscosity.
So, that is the full procedure with a little extra bit of advice I got from a pro. As I said the oil is compromised some with additives, so only get on it once or twice, some people think the car should be driven hard for the whole tank and that is not the best idea.
I have never heard of anyone who has usedB-12, but I have seen it but not to the extent that I come across BG44K in performance, Like Goerge Reggio swares by it, and almost every car dealership I have ever bought parts from has had it for sale at parts counter. I have paid up to $25 a bottle but $20 is a reasonable price.
Once you have cleaned it out good once, you can follow the reccomendations on the can which is every other tank. Of course they are trying to sell the stuff. I always run a can just before I change my oil or every other oil change and I have always found it keeps the combustion chambers really clean as well as valves and I have never had injector issues when using it.
So, it is something you should do anyway and it might just help the car performance issue as it can clean places you can't get to without tearing the motor apart.
Let me know the latest as I am keeping an eye on your replies.
If you want to get good aftermarket stuff for cheap, www. Rockauto. Com is unbeatable. They sell closeout stuff and you can literrally get $100 for $20 or even better from reman companys like Beck Arnley and Four Seasons. You have to for go the warranty, but it is so cheap and it is the real stuff. You can return stuff if it does not work or is the wrong part, but the long term warranty does not apply to parts they sell as they are closeouts. I got a windshield wiper motor from Beck Arnley for $15, $20 shipped. I have bought from them for years and always been happy with products. They have a great on line catalog too.

That is great news. Isn't that the best feeling!
I am very glad to hear it is running well. Sounds like you got it. Sometimes the smallest things can cause so much trouble.
Really glad you finally got it.
Let me know if anything else comes up from it.
Otherwise, CONGRATULATIONS!

Your very welcome. Thanks for your kind words. Not everyone thanks me and it is really cool when they do as I like doing this to help people. It is a really cool things the owners have going and the team of Techs are just great and they are from all over the world.
Come back any time you need help or advice.
Take care.

Dr. C

It could be a hort, but it works and doesn't blow a fuse. Still could be a bad ground though. The IACV might be bad or you might need to clean out the throttle body better and maybe the intake manifold. Check that port to see if it clogged back up.
As far as the cat, I would think that the O2 sensor is the only way to detect the catylitic converters performance.
I hope it goes well as I can't run this thread any longer than I have. I have to get to other threads as the long line of customers grows if I don't have a limit to thread length. Most Techs only go back and forth once or twice, but I try to solve every issue I can. But, I always have a load of people that I have to get too and it all depends on how many people are in line for respones.
You can always re-post with new issue, or the same one and it might even be good for someone else to pick it up with new view.
I think I did ask a tech to look at it and he didn't have time because we have really grown and don't have enoguh techs for the workload. It comes and goes like that.
I wish I could help you more but I am actually out of ideas at this point anyway.
Good luck with everything and I don't think the diagnosis is out of your capability. You have the aptitude you just have never done it before and that is always something that holds me back. Just go with your instinct as you have and I bet you will get it figured out if you haven't already.
Take Care.
Dr. C

I have looked through our correspondence and their is so much that I do not see if you did a compression test or not.
I think with oil getting past the rings the O2 sensors are bad or failing and that will affect gas mileage a lot when they go bad.
Do a compression test to see how bad your ring seal is. If it is really bad you probably need to look into a used motor or a rebuild.
The best bet for a used motor is to get a JDM, you are fortunate you have a car that the engine will be available, engine. In Japan their is a law that requires a new engine or a new car at 30,000 miles. So, you can get used Japanses spec motors online easily. You can poften get them with a whole drivetrain for less than $3000. You can even change to a turbo motor as they will give you a wiring harness and ECU. You would then go to a place like, www. Swapshop. Com or www. Rywiores. Com to get your harnes fdor your engine converted and you will have a motor that is essentially new and they run better than the American spec engines.
Don't go to Panther motorpsorts in Canada. They are the only place with a bad rep that I have found. You can try to get Hmotororsonline. Com to special order you one as they only do Hondas, but they are the best place. I got a GSR motor and drivetraing to my door for $3500 with LSD tranny and all wiring and ECU. It was like brand new. The owners name is Steve and might order you one when he has buyers in Japan get a batch. No one else has the reputation he has. But, if you research you can find other JDM places that have Mistu stuff. Try going to a mistu enthusiats forum, I believe there is one called 3gen. Com or something like that.
If you have compression the valve seals are bad and that will get to your O2 sensros too and cause bad MPG.
So, Do the test, get new O2 sensors in either case for after the repair. If the valve seals are bad I would consider a JDM anyway as the head repair will cost half as much as a motor. If you do a straight swap you won't have to convert harness.
Don't install new sonsors until you install or fix the cars engine.
That is my best guess at this unless I missed something. We have gone back and forth a lot. But, that is about all that is left.