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Scott, ASE Certified Technician
Category: Car
Satisfied Customers: 1638
Experience:  Ase certified, 24 years, bumper to bumper all makes
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2004 KIA Optima: The problem happens only intermittenly..shift..MPH.

Customer Question

I have had our 2004 KIA Optima in several times for analysis. The problem happens only intermittenly. The car seems to want to shift up or down while traveling at between 35 and 50 MPH. The engine races and the tackometer jumps from normal speed to between 2000 and 3000. The dealer claims that nothing shows on their testing equipment and that the mechanics have driven the car several miles and nothing happens. I experienced this problem again today. I don't do anything but hold my foot steady on the accelerator during the experience. The car is a 6 cylinder with 104,00 miles. The service center has checked all the fluid levels and they are normal.

Also, at times the speedometer drops to zero and stays there until I stop, then resumes as normal. Sometimes this happens several times during one trip. Again, the mechanics haven't had this experience nor does anything show during analysis testing.

Please e-mail your answer [email protected]

Thank you, XXXXX XXXXX
Submitted: 7 years ago.
Category: Car
Expert:  Scott replied 7 years ago.

Hello Ron,


From what you described, it sounds like the VSS {vehicle speed sensor} is dropping out. With your foot steady on the accelerator and moving at 35-50 mph, if the VSS dropped out to zero, the computer would think that you were at an idle; thus, causing the engine to rev and drop gear.


Now, the problem is the shop has not been able to duplicate the concern. Is it a VSS, PCM or the PCM loosing the signal due to a wire issue? I can understand the shops not wanting to replace a part with not knowing what the issue actually is.


I am guessing there are no trouble codes. P0500 is the VSS. All I can do is take your info and guess at this point, but if it is the VSS, I am surprised it would not have set a code. The only other thing could be a internal slipping problem, but once again, why had the shop not seen it.


You did not mention if it happens on extended drives or short trips. I would see if it is extended drive, operating for a while, keep a mental diary of what the conditions are and give the shop another try.