Hi, I have a 1979 F-150 with a built 351w in it, and notice I have a slight knock. I have had this truck since march, and when I bought it the mufllers were cut under the cab, so I noticed no engine knock at the time. Well I just had my pipes ran out the back of truck, so now I can hear the motor more. Come to find out, I have a slight engine knock coming from the rear passenger side of engine. I have 60Lbs of oil pressure, It still has all of its "Ford Power", but has that annoying knock. And just so everyone knows it's not a ping, its more of a knock. When I start it up cold, the knock is a little louder. As it warms up the knock starts to die down, but is always there. It's not loud enough for most non car guys to hear. I would like some help here. The knock also sounds like a bottom end knock. But it doesn't really sound like a rod knock or main bearing. It's a steady slight knock sound.

Oh and when the rpms get past 2000 the knock is completly gone. And if you get on it, there is no knock. It's mainly in the low rpm, and idle. Any help would be nice. I've changed the oil to Valvoline 10w 40, and noticed NO metal shavings in metal.

Thanks, Tom

 

Probably piston slap. Forged pistons make a knock or slapping sound cold, but it usually goes away after a few moments. Piston rings or pin could be shot as well on the cylinder.

Hard to diagnose without being there is person or at least having a Youtube video or something.

Josh

 

I'm with Bullitt, it sounds like piston slap. Doesn't hurt anything just annoying.

 

I have also noticed that the knock doesnt start till about 10-20 seconds after the vehicle starts up. It runs nice and smooth till about that 10-20 second mark then starts knocking

 

Are ya using an auto tranny? Could it be a cracked flex plate?

 

Does it sound like this?

YouTube - 2010 Hummer H3 Alpha Piston Slap?

Josh

 

actually yes it does kind of sound like that but much much quiter. and after about 5-10min of running it gets so quite that you have to listen to the bottom of the motor, or be inside the cab to hear the knock. And mine does kind of go away when reved to a certain rpm. I also noticed today when i was trying to figure the knock out, that if i hold rpm at 2500 there is a slight knock, but it isn't a steady knock.

one more bit of advice, is when starting cold, the back side of the passenger part of the engine, has a tap coming from the top of the engine. And that tap comes from the same area i hear the knock at. But the tap always goes away after about a minute.

 

The tapping is likely a lifter on cold start. Not typically a problem, are you using Fram or other cheap oil filters?

With a performance rebuild using forged pistons you can encounter piston slap until the pistons heat up. If one cylinder is noisier than the others you may have excessive wall clearance that the piston isn't taking up causing even more piston slap.

Josh