2009 Expidition 4x4 107k miles. Good shape. No rust. The mechanic says my timing chain needs to be replaced. Then since he is there, he might as well replace the water pump and spark plugs. Total appr: $ 4000 This makes sense to me after researching it. Kelly's blue book has a 4K trade value. The question is, should I? Or cut my loss and move on? I fear that some other major repair could develop... Also, the engine runs fine, only once and awhile does it act up...