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Discussion Starter · #1 · (Edited)
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...

Symptoms:
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!

Thanks!
 

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Discussion Starter · #3 ·
sbr711,

Thanks for the thorough response. I'll try taking it to the dealer and have them do a full diagnostic scan and see if they find anything hidden. I can hear the low pressure fuel pump in the tank turning on. Is there an easy way to check fuel pressure leading up to the high pressure pump? Timing chain- good to know! ;)
 

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Discussion Starter · #7 ·
I'm planning to borrow a fuel pressure tester later this afternoon. Does the Sonata have a schrader valve or something I can connect the gauge to? Otherwise, what is the best method for testing the fuel pressure between the primary pump and high pressure pump on this car?

Thanks!
 

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Discussion Starter · #14 ·
UPDATE: Fixed the problem! Turned out to be the primary fuel pump having low fuel pressure, so I replaced the fuel pump and now the car starts perfectly. Thank you to everyone who helped me troubleshoot.
 
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