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If it is running very bad there is a good chance the intake valves are bent. They will come in contact with the pistons if the chain guides break bad enough. You need to run a compression test or even better a cylinder leakage test on the right side and compare it to the reading son the left side.
To set the cam timing on this engine you need a 4.0 sohc timing kit.
Autozone.com has good info on your vehicle. Use a 2000 year model explorer.
You can also rent more in dept manuals on a web site called all data.
With the year and milage of your vehicle it would probably be a good idea to remove the engine and replace the back tensioner and cassette also. This is a difficult engine to time. You may want to consider having a shop make the repairs for you if you are unsure about it after reviewing the repair information.
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If it runs bad and the compression is low it probably did bend the intake valve. That is a common problem. The only way to know for sure is to pull the cam followers out of the head and do a cylinder leakage test.
It wouldn't be cost effective for the dealer to dissassemble the engine and replace the valves, that is why they priced you out an engine. It would be cheaper and easier for them to do that.
You have to remove the valve covers and front cover to inspect the front cassette, the back one is hard to see without removing the engine. You can't replace the back one with out removing the engine. You can't just do the tensioner, you have to do the chain cassett and tensioner. You also have to have the special tool kit I showed you to lock the cams and crank in place to set the timing. There are no timing marks to set the chains.
Its not as simple as just replacing a tensioner.
Good luck to you. I ll opt out.