Q: My 2000 Jeep Cherokee Classic has 90,000 miles. It is 4WD with the 4.0-liter inline-six. I've had no major issues other than a leaking compressor in the AC system, which I haven't fixed. Lately I've noticed that, when I'm driving with the cruise control on, after a period of time the engine starts to run rough, as if it's misfiring. If I cancel the cruise control, the Jeep runs fine again. If I reengage the cruise control, sometimes the Jeep runs normally and sometimes the same symptoms recur. Someone suggested the speed sensor could be to blame. However there are no other symptoms, such as an erratic speedometer, and the MIL doesn't light.
A: Good observation on the speedometer. The Vehicle Speed Sensor is located at the transmission and supplies a common speed signal to the speedometer and the Engine Control Module. The cruise control operates by means of a vacuum servo and three electronic solenoids controlled by the ECM--basing its commands on the signal retrieved from the lone VSS. If the VSS was acting up, chances are you'd also see the speedometer fluctuating. Plus, cruise control is a slow reacting system and symptoms from a VSS failure would be more of a surging condition, or the cruise shutting off altogether. You may be feeling a good old-fashioned misfire, only under high-load conditions experienced while the cruise control is engaged. The next time you're out on the highway and the problem occurs, try turning off the cruise and attempt to duplicate the high-load condition--very light on the gas while in overdrive/lockup (trying to bog down the engine). If the condition still exists, it's just a question of tracking down the misfire. And there may be information stored in the ECM--viewable with a scan tool--that can help narrow down the ailing cylinder.
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