Hey Guys My friends dually 6.2l engine started a knock on one side of the engine a couple of weeks ago. To give some history, it's an 85 with 150k, he had just put some Restore in the oil a couple of weeks earlier and added some fuel additives that day. After checking, he found that the dose was doubled, and when the knocking started it was right after a big bang and a black puff out the exhaust. I told him to check the injectors and he did, tuns out they were opening at a too low pressure, so he gets a new set. Still the same knocking. He pulls the valve covers, doesn't have any abnormal symptoms, drops the pan, no shavings or signs of wear, checks rod bearings for play, nothing. Now he is going to check the compression, and I was thinking maybe a collapsed or malfunctionning hydraulic lifter. Before we tear off the heads and check everything, does anyone have any opinions? The sound is comming from the passenger side front top area of the engine, that is why I suspect valve train. ANy thoughts? Thanks guys. UPDATE: 03-26-03 Well After changing the injectors, my friend started it and heard the knock again so he stopped it right away. After checking everything we discussed, and last checking the lifters and finding nothing wrong with them, he decided to put it back together, and ran it, and it's fine, smooth, no knock. I am guessing that there was some air left in some of the lines or the lifters were stuck and when he took them out and tested them they loosened up. What do you guys think of this? For now he's happy that it's back on the road, but he's like me, he wants to know what was wrong. Any guesses? My vote is on the injectors and air in the fuel system. The injectors that he replaced were opening at a very low pressure so the knock could have been from them.