My '77 400 engine makes a clattering sound (also could be described as a rattling sound) when under the load of acceleration. It starts right up, idles okay, and revs up in neutral normally. Only when under load does it make this noise. It does it whether cold or warmed up. I have already tried all the obvious stuff: new egr valve, new distributor, plugs, wires, cap, rotor. Timing seems spot on. What else could it be: please help me save my venerable F-350 from the junk yard.

 

sounds like you got some bad gas? Possible pinging, preigniton. wondering if something has recently changed or has done it all the time. another possible issues piston clearance issue. pistons rattling around in cylinder. If this is the issue change engine oil and use a heavier oil and if this changes the rattling around issue could definetly be a piston clearance issue. Also is this problem just recently started could be a weak fuel pump and engine going lean under load. check fuel pressure should be around 5 to 7 psi with fuel pump. what is your timing at???

 

Thanks for the suggestions. I forgot to mention that I had previously drained the gas tank and tried a full tank of the highest octane gas available around here--made no difference. Haven't checked the fuel pump pressure yet; I will do that this weekend. Is that 5 to 7 psi after both filters?
Timing is set to 12 degrees BTDC.

As far as thickness of oil, the sound happens even before the engine is warmed up so that the oil is still cold and relatively thick. Since it does seem to bog down powerwise, it seems more likely a fuel or ignition problem than a mechanical problem.

 

yes around 5 to 7 after filters. If the engine is clattering cold this making me guess the pistons have excessive wear. I know the sound when I hear it. also sometimes what happens is the skirts on pistons break off some times and causes this noise. caused from excessive wear in engine. have you compression tested the engine? might tell the story also.

 

Have you had a timing light on it with the vac advance line plugged off ? If so, what is your inital timing and total timing ? It sounds like your total centrifugal advance may be set too far.

 

Yes Mark, I timed it with a timing light and the vacuum line removed and plugged. Set it to the factory spec of 12 degrees BTDC. Not sure what you mean by initial timing vs. total timing.

WY, I sure hope you're wrong about the piston slap hypothesis. I guess if the fuel pump pressure is adequate, I'll do a compression test to see how worn the engine is. It's a rebuilt engine with not that many miles on it, but I didn't do the rebuild myself, so there's no way to be sure of the quality of it.

 

Total centrifugal advance is how far it advances when reved up without the vacuum advance plugged in. Try that and get back to us with a figure and what rpm it reaches that figure.

 

checked the fuel pump pressure and it was 6psi. Haven't checked the total advance yet, but starting to think it's time to junk it.

 

Was there an identifiable change in anything you did to bring this on or has it just "crept" up on you, if you know what I mean? When you say under load, this is actual driving conditions or just blipping the throttle? Sounds odd considering you've recently had it rebuilt. Maybe bite the bullet and remove the sump and check to see if you've spun a main or something horrible like that.

 

my possible guess is piston clearance was machined wrong. what piston? and what clearance was required for proper fit? Hope i'm wrong but its possible.