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I need some help as I have exhausted all my mechanical knowledge on this 2013 Sonata GLS with 4-cyl GDI motor and only 69k miles...

Car turns over strong and motor "starts" almost right away, but RPM's fluctuate between 300-600 rpm (fights to stay alive, maybe 2 cylinders?) and eventually stalls. BUT, sometimes it starts and idles fine at 1500 rpm after attempting to restart the car 10-15 times. Once running, it will drive and perform perfect all day (no knocking, no hesitation, no rpm fluctuation, etc.) Turn off the car, it becomes a roll of the dice whether it starts or not. No check engine light illuminated besides in AUX position, and no codes appear when connected to scanner.

Things I've checked:
-I can hear the fuel pump turn on (and no issues while driving suggest its working properly)
-Cleaned throttle body (made no difference)
-Pulled spark plugs and they look good.
-Disconnected and reconnected every plug connector I could reach
-Battery and connections solid, reads 12 volts on meter.
-Car has no MAF sensor so that cant be it.
-High pressure fuel pump has fuel (how can I check that it isn't faulty?)

What else should I look for before throwing money at expensive parts? What makes this strange is this just began happening, but once it starts it runs perfectly. Here's a few things I've thought about: Fuel pump- but wouldn't that show signs while driving too? Crankshaft position sensor- the RPMs raise when turning over and car never stalls while driving. Timing belt- cranks normal, not rapid.. and wouldn't it run crappy once started? Purge valve- usually to blame after fueling, but this does it randomly after car is turned off at any time. Fuel filter- again, wouldn't I notice hesitation while driving? Dirty valves- I know GDI motors are notorious for carbon buildup, but would that prevent a car from starting? Then why run prefect when started? I'm so lost..

I'm a decent DIY mechanic so throw any knowledge you have at me, I'll be able to troubleshoot it at least!


All modern vehicles has even the computer for a brain and fuel system for a heart. Forget the brain functions as those toss codes . Any of your sensors dying toss codes. What’s left , the old standby...FUEL FILTER , FUEL PUMPS.
First thing I do with any issues after a car has 50 k is toss on a new fuel filter. Why, Just because! Remember while most of the old school maintenance items run great right to 100 k the gas you use is scrungier than its ever been. Your cars are far more sensitive to fuel issues than ever as well. With that in mind it’s the usual suspect. It’s been that way since the 90’s.
When a regular rail injection system starts to die it? usually begins by starting very hard. That’s because though it runs on 45- 55 psi it needs 60ish to light off. What a DI needs I can’t say and I can’t even tell you how to check mine. One thing for sure , with a dual pump system it’s got twice as many pumps to fail, that’s a given.
Here’s another thought. Have you cleaned that power valve using the aerosol spray can with the tiny straw yet? Being it’s a 13 you, ??*♂ like me ( can sitting on shelf 9 months now) might want to do that first. Still along with that $8 from Wallymart I’d put a new filter on before seeing a Stealership. Good luck!

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