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Hello: My 2008 Versa MT with 160K miles had the same issue. It would suddenly start slowing down at 65MPH (3000 RPM) and depressing the throttle had no effect. The slowing continued until the engine reached about 2500 RPM at which point I could increase speed to around 65 MPH then it would slow again. This sequence continued until I got into town. In town there were no issues. This occurred every day for 3 days in a row some in the morning and some in the evening and under different engine loading conditions. There were no visual indications of a problem, no check engine, no dash lights, nothing. I plugged in a data logger and monitored all ODBII data available. The data indicated that the alternator voltage was varying from approximately 14V (at 2500 RPM) to less than 12V (at 3000 RPM); at the same time both oxygen sensor voltages would vary as well as other related outputs. Even though a load test indicated the alternator was operating correctly, I replaced the alternator and the problem went away. Examination of the removed alternator revealed the slip clutch was excessively slipping at high RPM resulting in a low voltage condition. Data logger data is now steady and correctly indicates proper data changes correlated to throttle position. My theory is that the voltages in the ECM are well regulated whereas the sensors rely on alternator voltage which is normally regulated but in this case were allowed to vary outside of normal limits. The ecm was responding to the varying O2 sensor data and either leaning out the mixture thus starving the engine and reducing RPM and power or going into limp home mode (which limits engine to 2500 RPM). I rejected the limp home mode theory because there were no codes indicated and no pending codes stored; the shop manual indicates that if the ecm goes into limp home mode, it will throw a code that indicates what caused it to go into limp home mode. Summary: In my case of loss of power at highway speeds; replacing the alternator eliminated the problem. The problem has not happened again in over 2 weeks of driving over 100 miles per day.
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