why does my g.marquise putter from the muffeler,almost feels like a miss on 1994 Mercury Grand Marquis

ive already gave it atune up, checked injectors feul press at 31psi plus installed a maf,egr and cleaned t-body. allso tried to retrieve codes but scanner just wouldnt pullthem

Asked by for the 1994 Mercury Grand Marquis
You may have a stretched timing chain causing cam timing issues. Could also have valve probs causing probs. Have a qualified tech look at it and advise.