Hyundai Repair: SURGING, coolant temperature sensor, hyundia sonata

Question
QUESTION: I HAVE A 1999 HYUNDIA SONATA 2.6 V6 AUTOMATIC VEHICLE. MY CHECK ENGINE LIGHT JUST WENT ON AND I HAVE NOTICED THAT THE CAR AT A FULL STOP WOULD IDLE FINE AT ABOUT 500-600 RPM. BUT THEN ALL OF A SUDDEN THE CAR WOULD JUMP/SURGE UP TO 1200 RPM AND I NEED TO APPLY THE BRAKES HARDER TO STOP CAR. THEN SOMETIMES THE CAR WILL DROP DOWN TO NORMAL IDLE ON ITS OWN AT IDLE OR IT WILL CONTINUE TO IDLE HIGH WHILE DRIVING.

ANSWER: Many things can make the car idle abnormally with the symptoms you describe.  My first suspicion is that you may have some sort of vacuum leak.  But since there are many possibilities, there's only a low percentage that's your issue.  I'd recommend having your engine and transmission diagnostic trouble codes read to find out the nature of the problem causing the check engine lamp to come on.  AutoZone and AAMCO have advertised they'll read the codes for free.  

If you follow up with the trouble code numbers, I'll be able to give you better instructions for diagnosis and repair.

---------- FOLLOW-UP ----------

QUESTION: I DID A SCAN AND CODES P0115 AND P0125 WHERE GIVEN. WILL THAT HELP YOU. THANKS

Answer
I'd recommend replacing your coolant temperature sensor and thermostat.

P0115 indicates there's a problem with the coolant temperature readings and p0125 indicates that the coolant temperature doesn't reach proper operating temperature, implying a stuck closed thermostat.  There's a possibility that the coolant temperature sensor flakes out a little, causing the engine control module think the engine is cold and increase the idle speed.