86 F-150 302 FI SWB 4x4. I have noticed recently that when climbing a hill, pulling my trailer and sometimes taking off from a stop my engine had an odd rattle. It happens only when the motor is under a load and quits when i get up to speed. When i am driving around town and on the highway it doesnt happen unless i put my foot in it. Sounds like someone dragging a stick over an old washboard. Is this serious or just bad gas etc.. thanks for any info.

 

It is detonation (pinging). It can be caused by several things. Timing too high, carbon build-up, lean condition(not enough fuel). I would try to find the root cause. It can be very damaging to pistons and such. Basically, what happens is that you have two flame fronts inside the cylinder, one by the spark plug and one by something else such as a hot spot. What you are hearing is the two flame fronts smashing together causing a small explosion in the cylinder.

 

When the engine is under load, like climbing a hill or pulling a heavy load, the cylinder pressure increases beyond the optimal operating conditions the engine is tuned for, causing detonation. Not that it matters much, but I have to respectfully disagree with Jimmy about what causes the noise you're hearing. In a sense the detonating part of the mixture hits the slow burning part but that is not what makes the noise (in my opinion).
Studies done by NACA in the 1940's with ultra high speed photography, (40,000 frames per second, and 200,000 frames per second) clearly showed that the "theory" of colliding flame fronts was not supported by the evidence. They clearly show flame fronts passing through each other as if the other did not exist with no detonation taking place.

Those studies showed the detonation reaction originated in partially combusted gases following the initial burn reaction. (the flame front actually passes through the combustion gases mulitiple times with the fastest burning compounds burning off first followed by slower reacting compounds.)

The detonation reaction involved the supersonic combustion of partially reacted fuel air mix. The speed of combustion is so high in this reaction that it took a frame rate of 200,000 frames per second to determine the basic processes, frequently only capturing one or 2 frames showing the detonation in process.

This combustion speed is so fast that it causes a pressure spike several times the prevailing cylinder pressure, which slams the piston top like a sledge hammer blow

Interesting reading: http://www.streetrodstuff.com/Articl...ne/Detonation/

FWIW, det. starts in the coolest part of the chamber, by the intake valve. That's why when you see broken ring lands caused by det., it's almost always by the intake valve side of the piston. But that's a whole 'nother thread.


Pete

 

Sir, I am not a freaking scientist I just want it to go away. Like lxman1 said "find the root of the problem" right now i am in the tree branches. Thanks anyway you are one smart person, guess i need to take it to my buddy who does mech work for a living. This site is awesome.

 

Try retarding your timing, it may just be too far advanced.<hints id="hah_hints"></hints>

 

better fuel