On the 96 to 00 Elantra the Idle Air Control valve has a tendency to go bad, producing symptoms like the ones you describe. To test the IAC, just undo the bolts holding it to the intake manifold, and remove it. Then start the engine. If it idles ok with the IAC removed, you'll know it was a problem related to the IAC, that needs to be corrected. You can also use an ohmmeter to test for continuity between the three prongs of the IAC connector. The center prong should have continuity with both side prongs. Also check the connector harness, and check for voltage between the center plug and the two side plugs. Refer to the website hmaservice.com for the complete troubleshooting procedure of the IAC.
Also, check the vacuum system for any leaks or malfunctions. With the engine running, listen for any tell-tale hissing sounds. Make sure that all vacuum hoses are tightly secured. Disconnect the PCV valve hose and make sure that with the engine running it does draw a vacuum. Then with the engine off remove the PCV valve and check that it is not obstructed and you can blow air through it from the crankcase side.
If the IAC and the vacuum system turn out to be ok, you'll need to
check the fuel system. If you have not done so, replace the fuel filter.
Check the individual injectors. Refer to detailed troubleshooting procedures at hmaservice.com.
There are many possibilities...but first reset your CPU by removing the negative terminal from the battery. Wait a minute or so, thern re-fit it. If this doesn't improve the problem, get back here and we can go from there.
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