My Trooper doesn't run smoothly unless I add fuel injector cleaner. I had a tune up and replaced the timing belt, but she hiccups around the 30 mph unless she has the additive. The check engine light came on yesterday, but cleared up when I filled up and added the cleaner. Do I need to check the fuel filter? I am the 2nd owner - she has 103,000 miles. I use premium gas. She really doesn't like reguluar. One more question. She has auto 4WD, but doesn't do well in the mud and snow unless I put her in manual 4WD. How fast can I go in manual? My husband says I should creep, but if I'm reading the manual correctly, I can go up to 35 mph in 3rd (automatic). TIA