1979 Engine knock PLEASE HELP
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1979 Engine knock PLEASE HELP
I have a 79' f100 ranger with a 351m in it. I bought it from my brother in law and when I did my inspection I realized a spark wire wasn't connected to the #7 cylinder. I connected the spark wire and then I ran the engine, the #7 was knocking. I pulled the valve cover and couldn't see anything out of the ordinary.. What could this be? What would I have to do to find out what it is? I haven't done anything to the truck yet. I was planning on doing a full tune up. sparks, wires, cap & rotor, oil change, fuel filter, all that stuff. I really don't want to waste the money if it is something crazy. Any help is greatly appreciated. I will try to post a video of the noise in the morning
Tuned
Joined: Sep 2010
Posts: 279
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I hope I'm wrong in your case, but the knock noise going away after pulling the plug wire is a rod bearing or a wrist pin, when the cylinder pressure is gone (no combustion) the load downward into the piston and rod is gone and the noise is gone as well.
Good luck.
Good luck.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Actually, my guess would be piston slap, excess clearance between the piston and the cylinder.
It's been my experience in the past that wrist pin and rod knock are still present whether the plug is firing or not, just varying in how bad it sounds. Piston slap however tends to disappear when the plug doesn't fire and reappears when the plug fires. It also has the tendency to quiet down some as the engine warms up, as the piston expands a bit.
Decades ago I had a '70 F100 with a 302. When I bought it, it had piston slap, not bad at first but continued to get worse over the years untill it pretty much sounded like a rod was gone.
I was going down the expressway one day and the knocking disappeared. Looked in the rearview mirror and all I could see was a thick cloud of smoke. Called a buddy with a tow strap, got it home and pulled the engine.
After teardown you could see what happened. The excess wall clearance had allowed the piston to "bang" against the cylinder wall when the cylinder fired. This eventually weakened the piston's skirt and it collapsed. When this happened the piston saw fit to break up and the connecting rod decided to ventilate the side of the block.
Catastrophic block damage but it took a long time, I had put a lot of miles on the truck, don't remember how many, but I drove it for over three years before the cylinder went south.
It's been my experience in the past that wrist pin and rod knock are still present whether the plug is firing or not, just varying in how bad it sounds. Piston slap however tends to disappear when the plug doesn't fire and reappears when the plug fires. It also has the tendency to quiet down some as the engine warms up, as the piston expands a bit.
Decades ago I had a '70 F100 with a 302. When I bought it, it had piston slap, not bad at first but continued to get worse over the years untill it pretty much sounded like a rod was gone.
I was going down the expressway one day and the knocking disappeared. Looked in the rearview mirror and all I could see was a thick cloud of smoke. Called a buddy with a tow strap, got it home and pulled the engine.
After teardown you could see what happened. The excess wall clearance had allowed the piston to "bang" against the cylinder wall when the cylinder fired. This eventually weakened the piston's skirt and it collapsed. When this happened the piston saw fit to break up and the connecting rod decided to ventilate the side of the block.
Catastrophic block damage but it took a long time, I had put a lot of miles on the truck, don't remember how many, but I drove it for over three years before the cylinder went south.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
The wrist pin, other than the knock is hard to diagnose without teardown. If the rod bearing is bad enough, when you pull the plug wire, the knocking gets worse because the piston is now hitting the cylinder head because there is no expanding cylinder pressure to cushion the stop at TDC.
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