QuestionQUESTION: When I am travelling on the highway between 50 - 70 mph my 2004 Hyundai Sonata accelerates and decelerates as the RPM's go up then down. It starts fine and runs fine at slow speeds. What do you think this problem may be ?
ANSWER: Hi, Larry. I'm not sure what the problem is here. It would be normal for the vehicle to accelerate as the rpm's increase and decelerate as they decrease. Can you explain further what is abnormal about what the vehicle is doing?
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QUESTION: Thanks for the response and I will try to explain...I travel to work around 45 miles each day mostly on highways and after traveling around 20 miles when I am going a consistent speed 70 MPH all of a sudden the RPMs will jump from 2.5 to 3.5 then back down to 2.5. This happens again after I have travelled about half of my trip. The car has 140,000 miles and the jumping of the RPMs when I am travelling at a consistent speed has me mystified as to what it is.
AnswerCan you tell whether the transmission is downshifting to third gear or still in fourth? You can test this by manually shifting to third and checking to see if the rpms raise.
It would certainly appear as though the transmission control module (TCM) is turning off the torque converter clutch or is downshifting to 3rd. Even so, if this occurs as you increase throttle application to maintain speed, it could still be normal. On the other hand, if your throttle application is steady when it does this, I think that's indicative of a problem.
Potential issues that could cause such a change are:
-- Faulty throttle position sensor
-- Transmission fluid overheating
-- Faulty transmission fluid temperature sensor
-- Defective input speed sensor
-- Any problem which would result in reduced engine performance (requiring added accelerator application to maintain 70 MPH).
Is your check engine lamp on or has it come on recently? If so, any stored diagnostic trouble code may relate to the issue you're having.
