Engine Misfire and Weak Acceleration
I recently had my 2001 Jeep Grand Cherokee serviced and tuned. About a month later the engine light came on and the Jeep began to vibrate in the idle position. I checked the gas cap and the dipstick first. Then I brought it to Auto Zone and had it scanned. The first cylinder was misfiring. The car was stalling at low speeds. AZ suggested gas cleansing fluid in the event of bad gas. The condition didn't improve and I brought it in to a mechanic- a different one. He confirmed the cylinder misfire and several other random misfires. He ruled out the spark plugs and other common misfire causes. He replaced the crank shaft sensor and the coil pack, which he said was malfunctioning and not properly installed the first time around. $400 later, the car was driving fine again. Three weeks later, I start the car and the light is back on. The vibrations are gone but now the acceleration is very weak. The engine is very quiet and lacks power it seems. I'm confused because I'm not sure if the mechanic fixed the problem and a new one arose or if he failed to properly diagnose the problem the first time. I don't want to spend another $400 replacing parts if it's going to fix the problem. I would also add that the weather has been very cold. I let the car heat up a long time but the performance didn't improve. Can someone give me some advice on how to proceed before having it looked at again? Possible causes or what to look for to spot the root cause?