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Good afternoon everyone! This is my first post so ill keep it short. I drive a 2015 Kia Forte LX with 89k miles on it. For the past few months ive noticed a shake at idle that differs each day, sometimes its bad, sometimes its bearable. Here is what I have replaced so far


Replaced Spark Plugs
Replaced Coils
Replaced Belt
Replaced Motor Mounts
Used Fuel Injector Cleaner (Jectron)
Cleaned MAF & Butterfly Plate

So far the car still shakes, ive checked the fuel injectors, each seem to be appropriate timing, ive also noticed my catalytic converter/exhaust pipe is shaking underneath my car, could this be causing it? Most of the shaking i feel is in the seat, barely any in the steering wheel.
 

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In the same boat. You know one thing that it can be? Dirty fuel system. Meaning specifically, the throttle body may be dirty (common cause of rough idle) and the fuel filter may need replaced. Get the service manual for your particular car. You have a 2015 so kifomanual.com will probably be fine, Gen 2. I have a 2018, so basically the same car as you (gen 2, mine is also LX). The manual will show you that the throttle body is under the hood, I DO NOT recommend totally removing it (you have to redo engine coolant if you do and fuck that), DON'T remove the coolant lines, just clean the throttle body there with Q-Tips and shop rags using THROTTLE BODY CLEANER only (sold at advanced or autozone). Once that's nice and clean, put it all back together.

As for the fuel filter, ever replaced it after 89k miles? Most people don't, however it really needs replaced every 25k miles! The automaker doesn't want you to know this, or do it. They want your car to fail so you can buy another. For our car, the fuel filter is unfortunately (and purposefully) under the rear seat. You can see how to get to it in the service manual. A few bolts to remove the bottom part of the rear seat, seat comes out. I don't think you even need to remove the back part of the back seat. Under the bottom part is a black flap. Under that is the fuel pump. The manual shows how to remove wire clips, etc. Pull the fuel pump out, and find the filter inside. Replace it. New fuel filter is like $15-$30 or something on rockauto.com, and you need one at 89k if you haven't replaced it.

If you don't care to do any of that, then at least be using top-tier fuel (list of brands near you https://toptiergas.com/licensed-brands/), I have BP, marathon, and shell near me. Top tier gas (not high octane, just the regular fuel but from top-tier stations) keeps your engine clean. In addition, you could add some techron fuel cleaner to your gas for a few tanks (up to three gas tanks per oil change interval) and that may help to clean things out a bit. Only use Techron (or redline if you must). Other fuel additives are not guaranteed and can fuck things up (trust me, royal purple maxclean fucked up my car). So techron is the best though, it's safe, and mechanics recommend it. That with Top-tier fuel from now on is a good idea.

If you do all of that, clean throttle body, replace fuel filter, and use top-tier fuel stations from now on with techron cleaner for a few gas tanks every once in awhile (don't use it every gas tank forever, it has limits), I think it would likely make a big difference if you haven't tried this yet. Throttle bodies get gunked up, so do fuel filters. Engine runs rougher without good stable fuel being sent to it and runs rough, causing the car to vibrate at idle.

My advice: do all of the above. Total cost would be not much, between a new filter, throttle body cleaner, and some techron here and there (also top-tier gas is just as cheap usually). Are you good with tools? Do you work on your car? Make sure you torque everything back to spec with a torque wrench. The seats, everything. FOLLOW the service manual as stated.

Could also be something else, torque converter going bad. Have you changed your transmission fluid ever? Due at 89k miles. I recommend ravenol SP-IV, double check the service manual to see what it calls for. Or you can get official Kia Hyundai SP-IV. If your transmission is dirty it can cause problems like this. If you don't change your transmission fluid now (just change, NOT flush), you are asking for a dead car past 100k miles. Would not expect the car to last to 150k. That is, if this is the first time you are changing the fluid.

Manufacturers lie and say "transmissions never need fluid changed, lasts forever" but funny how every transmission shop technician, owner, etc requires you to change transmission fluid every 25k miles after service. And they live by that. Sounds like the automaker is lying and the guys who work on transmissions know what they are talking about!

I just changed my tranny fluid. Bottom cover comes off the bottom of the car. Drain bolt on the transmission. Then torque that to spec back in, and then have to take air cleaner stuff out under the hood, and theres a cap where the transmission fluid needs to go in. If you have any questions message me. It can be tricky. Also a bolt to check fill level. It's all explained in the service manual but sometimes the manual leaves you hanging.

But if you do all that, you are looking at a few weekends of work, but for about $100 in new tranny fluid, $30 fuel filter, $8 throttle cleaner, some $6 bottles of techron fuel cleaner here and there, you are looking at your problem potentially fixed for $150 and some elbow grease.

Even if it doesn't solve the problem, you need these things done (tranny fluid and fuel filter, for example) or your car was going to die one day anyway.

But you might kill two birds with one stone. Let me know if you get stuck, Kia likes to make it hard to find things under the hood and make things hard to change, etc. Don't break anything, ask before you wrench away if you're not sure!
 
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