My truck is idling rough sometimes, and the belt seem to squeak. I had major tune-up done (plugs, wires, fuel filter, oil, oil filter, power-steering flush) but the mechanic couldn't fix the belt/idling because he didn't have the electronic tools to fix it?

What else should be done by 142k? How can I fix this problem to get it to run better, I checked the belt, there is no cracks or marks in it, it just squeaks sometimes on start up and idling, as well as a rough idle (seems the timing is out?)

This is a 99 ranger, 2WD, auto, 3.0L

Perhaps a good cleaning of your MAF sensor.

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ɹǝʌo ǝɯ dılɟ

How do you do that? I'm new to rangers.

Any Auto haus "MAF cleaner" too easy.
But if I may offer a diff approach.
Your motor is apparenly talking to you, but you have got to listen along with the belt chirping do you hear any ghostly noises or ratty bearing noises? Just listen to a new car then listen to yours you'll know if they are there. There are a couple of bearing failures that can lead you down that road. A big one is the tensioner bearing, secondly is the water pump if it begins to rotate out of round then it will drag and cause the symptoms you described, but usually you see and hear it crying, unless you have no exhaust and you have never cleaned your motor. to pre check you MAF simply disconnect it and run the idle up to 2,-2,500rpm you should notice it stumble and sputter, then reconnect and it should clear up withing 5 seconds, if you disconnect and notice nothing by all means clean it. Another thing to wonder about is failing coils, failing fuel pump, coils- check spark on each wire end, fuel-if the truck starts perfectly normal and runs fine for 2 minutes or so and then nose dives then you fuel pump has failed.
G/L