Engine Performance problem
1997 Toyota 4Runner 6 cyl Four Wheel Drive Automatic 204000 miles
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For the last 2 weeks, my truck has been spitting and sputtering right when I first start it up and get going. After it has had some time to get it out of it's system, it drives fine. This usually happens after it sits for a day without driving it, but this morning it happened and I drove it yesterday. The other day when it did it, I put it in neutral, and the engine cleared back up. When I put it into drive or reverse, it acted like it wanted to bog down. We were thinking it might be the fuel filter, but we don't know where it's located to change it. A couple months ago, our check engine light came on, we put it on the computer and it said it need the O2 sensor replaced. We heard that didn't do much, so we just erased the code. Could that have anything to do with it? We really haven't had any problems with this truck. We've had it for almost 7 years now. We've had to replace the starter and the timing belt. We would appreciate your help with this!

Check the oxygen sensor voltage switching activity and also test the coolant temperature sensor.