Has over 200,ooo miles. replaced idler tensioner two months ago, car running better than ever. three weeks ago, rattling sound from engine compartment, than two days later -loss of power. will still drive, just takes a while to get up to 40mph-about 1/2 mile. slow and sluggish, but still drives and seems to shift fine. rattling is consistent whether idling or accelerating-no change. have heard timing belt, valves, transmission. don't know what to look for-already took to mechanic-couldn't pinpoint w/out pulling apart. said not worth fixing-but i don't want to scrap yet.

Have you checked the oil level with the dipstick? What's the year, make, model, engine?

Well,a drive belt needs to be replaced every 50,000 mi. or so,and a weak tensioner can allow slack which causes sqeak even if the belt is good.
The idler pulley on those is easy to replace also. If the tensioner isn't holding the drive belt very tighly,replace it and the belt.
Auto Zone will let you put down a deposit and borrow a tool to work the tensioner so the belt can be easily removed. don-ohio

The rattle is caused because the rpm is lower. When stopped in drive with one foot on the brake, press the throttle down slowly until the vibration stops and the noise goes away. Cause? Weak or bad motor mounts, loose bolts or parts.

Could be the timing belt tensioner. They are known to make a rattling noise when worn and/or the belt has stretched....When is the last time you had the belt serviced? They should be replaced and pulleys inspected at 100,000 miles. Gates recommends 120,000 miles on their belts. Good luck, i hope this helps.

REPLACE THE TENSIONER IDLER PULLEY.THE TENSIONER IDLER PULLEY MAINTAINS THE BELT TENSION WHEN IT GOES BAD.THAT WILL LEAD TO LOOSE BELT TENSION CAUSING BELT TO BREAK.

Pull the codes with a obd2 scan tool and see what faults are stored in the system computer. After that you will hopefully have a better idea of what you are dealing with and can focus your repairs more easily.

It sounds like the Timing Belt has broken causing major and expensive engine damage. Have it checked out by a mechanic to confirm this.

could be just something as simple as the idler/tension pulley damper. Take a look at the belt and pulleys when the car is idling. When normal everything runs relatively smoothly. Identify the tensioner pulley, it has a smooth track edge and the belt runs around it with its back to it (the v belt 'face' runs over driven pulleys) and the bolt in the middle of the pulley remains stationary (it does not spin with the pulley as with others). If this pulley is spring loaded to apply tension to the v belt. If it shimmying around and the sound coincides with its action it is most likely that the damper, which normally smooths out vibration, needs to be examined. I recently sorted a car that was chattering quite loudly and all that was wrong was that the rubber bushing on the damper end had split and this was causing it it rattle. With regards to the belt, if the back of the belt is glazed and the front is cracked (you will see this plainly where it is exposed over the tensioner pulley) then it would be a good time to change it. They are not expensive and only take a few minutes to swap out. Have a mechanic give it a diagnostic look and for him to give an opinion. My guess is that the engine light is down to a duff oxygen sensor but without more details this is no more than just a guess.

have you check you motor monts they go out a lot good luck let me know if that helps

this sounds similar to a problem i had. as the rpm went over 2000 the engine would stop for a split second and then kick back in.
after a few days looking my garage finally found the problem was a faulty air mass meter.
this cost me £200 in total to be repaired.