Shadetreemechanic
Full Access Member
I need help diagnosing a knock. I bought a 89 superduty with a rebuilt 7.3. It was not running. Supposedly it quit going down the interstate and never started again. I got it started easily but it knocks loud out of the intake. The PO had been running WVO in it straight, so suspecting bad injectors i changed them with new Uhaul ones I had laying around.
Still knocks.
Compression check showed 450+ on 7 of the 8 cylinders. # 8 would only go to 320.
I pulled the valve covers and ran it that way and the valves seem to be doing what they are supposed to, nothing loose that I could see.
The knock is loud with the breather open, when I put my hand over the intake it seems quieter, though it is still pronounced. Their are no obvious weak cylinders when I loosen injector lines with it running. It does seem way out of balance when I rev it up, like a weak cylinder.
I am just about convinced something is loose in the bottom end. The only other thing I can think of is the DMF which does seem to have about 3/4 inches of play in it.
The truck knocks constantly throughout the RPM range and nothing changes when i push in the clutch.
So, Is there some way to isolate a knock to either the DMF or the bottom end? Would a bad DMF cause the whole truck to shake at idle?
Thanks for any help.
Still knocks.
Compression check showed 450+ on 7 of the 8 cylinders. # 8 would only go to 320.
I pulled the valve covers and ran it that way and the valves seem to be doing what they are supposed to, nothing loose that I could see.
The knock is loud with the breather open, when I put my hand over the intake it seems quieter, though it is still pronounced. Their are no obvious weak cylinders when I loosen injector lines with it running. It does seem way out of balance when I rev it up, like a weak cylinder.
I am just about convinced something is loose in the bottom end. The only other thing I can think of is the DMF which does seem to have about 3/4 inches of play in it.
The truck knocks constantly throughout the RPM range and nothing changes when i push in the clutch.
So, Is there some way to isolate a knock to either the DMF or the bottom end? Would a bad DMF cause the whole truck to shake at idle?
Thanks for any help.