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The weather has warmed, and we're not using the block heaters. (They provide approximately 140-degrees F coolant temperature at start-up.) When starting the Jeep XJ Cherokee's 4.0L engine on a relatively cold morning this week, I could hear a subtle misfire. This was during the warm-up/enrichment cycle, and by the time the engine came off the warm-up cycle (around 140-degrees F), the idle stabilized. At highway speeds, the engine seemed to run smoothly in terms of cylinder firing, though there had developed an unusual downshifting habit on grades and strong headwinds. Concerns came up about the TPS or oxygen sensor, maybe even a fuel supply issue. Miles earlier, however, the TPS had been replaced, and the O2 sensor as well. Giving this some thought, I pulled the air filter out. Holding it to sunlight, there was little light showing! The filter was neatly and uniformly clogged. This was the burbling at idle and sluggish on-highway at cruise trouble spot. So why didn't I replace the filter earlier? I change oil and the oil filter on cue, watching the oil color constantly. The air filter, however, gets dismissed when our mileage is mostly highway without dust conditions. In looking at the clogged filter, it was clear that winter "road film" was the culprit, and this can be just as impacting as dust and dirt particles. How did the engine continue to run in this state? Because the marvel of modern EFI has the O2 sensor compensating for air/fuel ratio constantly. Unlike a carbureted engine, which would show symptoms of an over-rich running condition much like running with the choke on, the A/F remains normal with EFI. At highway cruise, especially with an engine that chugs along at only 2,000-2,100 rpm most of the time, there is limited CFM (cubic feet per minute) air flow needed to maintain the A/F at that throttle setting. However, when load demands raise the throttle opening and drop the manifold vacuum at the same time, notably with the cruise control operating, the engine requires a transmission (AW4 in this case) downshift to maintain the A/F ratio constant. This creates the needed power at the expense of fuel efficiency. The dirty filter explained the downshifting on hills and subtle misfire at cold idle during the enrichment cycle. The moral: Check your air filter regardless of the season and whether or not you're driving off-pavement. Of course, when driving off pavement, one trip down a desert alkaline road can clog a brand new air filter. On highway, this does take longer, but with winter road debris and salted roads, the dirt accumulation is equally impacting. It just takes a bit longer to produce symptoms. A good rule-of-thumb for replacing the filter is the factory service interval. In our Jeep 4.0L engine's case, I'm targeting every 3rd or 4th oil change when driving strictly on-highway. For dusty off-pavement conditions, inspect the filter regularly and replace it as soon as necessary. Moses