this engine hadn't been started in about 4 years, the owner said he parked it when it started making the woffing noise. i hooked up a electric fuel pump and a 5 gallon jug, turned it over several times and it fired up no woffing at first start. after running for several minutes, the woffing started again, if it is valve train, that should be pretty straight forward, pull the heads and rebuild them. i can buy this for like $100, it shows 86,000 on the speedo. are the 6.9 as good a engine as the 7.3? and should i plan on a complete rebuild?