When trying to move along on the highway I get a engine rattle....sounds like marbles. I also notice my speedo needle jumping slightly.

Any ideas...The sound is similiar to when you are going up a hill in a higher gear.

BTW....truck is an A/t.

Thank you.

 

what year? carb or FI? any mods?
Maybe lean fuel mix, vacuum leak, off timing, a missfire, marbles in engine.

 

All my info is in the signature. No mods.

 

I agree with Sam.

Big vacuum leaks, faulty sensors, clogged fuel filters, bad fuel pumps, bad pressure regulator, bad EGR.

Any computer codes?

I had the same problem with my truck. Clogged fuel filter, charcoal canister was broken (vacuum leak), EGR not working.

 

thikn about u joints?

 

good thinkin - speedo jump doesn't usually relate to engine problems.

 

Sonds like detonation. Very common with the 300 I-6. If you explore older ford tuning procedures you will find that they want it set just at the point where it has detonation under heavy load acceleration like up hill, etc.

I just retired an 81 300 f-250/350 that I have used for nearly 350,000 miles and have been listening to this noise for years. A little maddening. If it is set wrong, it will hurt the tops of the pistons, usually collapsing the top ring land. If set properly, it will not hurt anything except the anoying meter..

On the other hand, lugging the venerable 300 too long will also damage the pistons in much the same way.

If this truck has been converted from CNG to gas, is it injected? THere may be some problems due to the fuel change. Not sure if cam would be the same.

Sam is listing what may be some possibilities to look at and correct. Still should have some detonation if tuned properly. Sounds funny, but according to a manual I read years ago, that is the way it was designed.

Still probably the best truck engine ever. No wonder UPS bought 1000's when they went ouf of production. All of the Brown gas boxes were 300 powered.

I have pulled 80,000ib semis withmine at an idle backwards with 4x4 in low range and in granny low with the T-18 trans. Must mention that truck weighs nearly 9000 lbs.

 

Is there a test for the EGR?

Also what is the best way to find any vacuum leaks?