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Discussion Starter #1
just purchased my first foreign car ever. it is a 2002 Elantra it idles very rough changed plugs using NGK and wires new fuel filter and ran 2 full tanks of high grade fuel with STP fuel injector cleaner. but had no effect on idle also have no check or service engine light. any suggestions on what to check next? Thanks
 

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the plugs and wires were a nice try given the age of the car, but the injector cleaner never fixes anything. i'd suggest saving your money there.

usually a rough idle is caused by an air intake leak somewhere along the system...air filter is the first place to start to see if its clogged. check the intake pipe up to the throttle body for cracks, breaks and failed clamps. next the throttle body should be checked for excessive carbon buildup. best way to clean it is to pull it off and use a can of carb/throttle body cleaner and a soft bristle brush. an old toothbrush works nicely. use the proper torque spec for reinstalling. the gasket can be reused unless its damaged. in that case you have a couple choices: 1. reenforce with RTC silicone. 2. cut a new gasket out of stock gasket material 3. buy a new gasket...

next check all your vacuum lines for cracking and breaking, especially around the ends. after that, time to check the intake manifold gasket. these are a known issue on the LC accents, i'm unsure about the elantras of the same era but its quite possible. test is easy, there's two ways to do it. if you have access to welding equipment this is the better test: simply spray some argon gas around the intake manifold where it mates to the engine head. the idle with smooth out almost instantly if you have a bad gasket. the second is the poor man's test, and does not have as high an accuracy rate but it can work. get some of the starting spray fluid, spray it around the intake manifold gasket (same area). if it sucks some in, the idle will pick up briefly again proving a failed gasket. this test can also work for vacuum lines but is a bit more delayed and the result is not as obvious. the ultra poor man's test is to replace the start fluid with WD-40. can work, but again less accuracy.

the good news is all of these items are very cheap and easy to fix. i did my intake gasket last spring for $32 at the stealership for the part and about 45 minutes of my time.

just be sure to adhere to any and all torque specs. they're there for a reason.

and yes if you want to be super cheap the intake gasket can be fixed/replaced with the appropriate RTV silicone...

only question i have: does it run rough at higher RPMs or does it smooth out? if it smooths out (which i'm guessing so from your post) then it is not a fueling issue and a sensor issue is very unlikely. rough idle is almost always an intake leak.
 

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Discussion Starter #3
I had 2 bad vac, lines and changed them but I did not check intake I will do that tomorrow. thank you for the info.
 
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