351m knocking on decelleration
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Senior User
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From: southern Indiana
351m knocking on decelleration
I have a 351 m that has a fast type of klunk or knock to it when you let up on the acellerator.It also runs rough and lacks some power.It happens around 2000-3000 rpm.my chiltons manual said it was an ignition problem.So far I have changed the spark plug wires,spark plugs,ignition module,distributer,fuel filter,fuel pump,and rebuilt the carburator.I pulled the oil pan off and all the rods and bearings looked good.NO one knows what the problem is.I dont know what else to do except tear the engine apart.which is my very last resort.
thanks,
tim
thanks,
tim
Elder User
Joined: Feb 2002
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From: East Bend, Carolina, U.S
I had a 351C that had the same knock you described and it was a rod bearing. It didn't knock when cold, but It would knock when letting off of the gas pedal at highway speeds. After it warmed up it would knock even at an idle.
Later I disassembled the engine and found that the number eight rod bearing was all but gone on the bottom (cap) side and the crankshaft rod journal was worn down on one side also. Rod bearings tend to have a double knock where a main bearing has a single knock.
Later I disassembled the engine and found that the number eight rod bearing was all but gone on the bottom (cap) side and the crankshaft rod journal was worn down on one side also. Rod bearings tend to have a double knock where a main bearing has a single knock.
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From: southern Indiana
Also at about 2000 rpms it seems to knock about every 3 seconds.It does Knock constantly under load driving.It sound like someone taping a hammer on the block.
Quick question.If its a rod knocking ,wouldnt the engine still have power?
Quick question.If its a rod knocking ,wouldnt the engine still have power?
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From: southern Indiana
what would cause a rod to start knocking?
What would cause a rod to knock on a long block engine with 45,000 miles on it that has never been starved of oil? There is no chance it could be a rebuild error just showing up is there? This was a new auto zone engine about 5 years ago.
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From: PacNW
Unfortunately the quality of many high volume engine rebuilders is not what it should be. Mixed & matched parts from different engines, poor quality control, inferior aftermarket parts, low wage workers, etc. all contribute to this. I wouldn't be surprised at all to see a long block from <insert major partshaus name here> go bad after 45K.
The best way to try to start diagnosing the problem is get the engine to the RPM where it's making the noise and pull the plug wires 1 at a time until the noise quits. Then pull the valve cover and check to see if the rockers are loose. From the symptoms you're describing it sounds like a bad wrist pin. It could however be a flat lobe on the camshaft or bad lifter.
The best way to try to start diagnosing the problem is get the engine to the RPM where it's making the noise and pull the plug wires 1 at a time until the noise quits. Then pull the valve cover and check to see if the rockers are loose. From the symptoms you're describing it sounds like a bad wrist pin. It could however be a flat lobe on the camshaft or bad lifter.
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just a possibility?
Well, I had a newer engine that I ran into this sort of problem, so I pulled the engine to repair it, I found that the front input shaft in the c6 trans had a busted snap ring and the input shaft was knocking or moving to much without the snap ring only on decel. Just a thought, I would think if it were serious wouldn't you get knock on accel.?
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Joined: Aug 2001
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From: Delaware
On acceleration the cylinder pressure is pushing the piston & pin & Rod and bearing, when that cylinder is on the power stroke.
On deceleration, the crank throw is pulling the bearing & rod & & pin & piston against closed valves on the power stroke.
On the intake and exhaust strokes the valves are open and there is less pressure on those parts.
It may be a piston pin or rod bearing.
Before you rip it apart.
Are you sure that it is not a spark knock?
Do you have a working EGR valve?
On deceleration, the crank throw is pulling the bearing & rod & & pin & piston against closed valves on the power stroke.
On the intake and exhaust strokes the valves are open and there is less pressure on those parts.
It may be a piston pin or rod bearing.
Before you rip it apart.
Are you sure that it is not a spark knock?
Do you have a working EGR valve?
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From: chicago burbs
when you lift your foot off the gas the oil pressure will drop and could let a rod or even a main bearing sound off. do you have a mechanical oil pressure gauge?
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From: southern Indiana
I have a mecanical oil gauge.It reads around 45-50 psi highway and 20psi idleing. I do not have a working egr.The knock started out as a spark knock then it got worse. I have been driving it without any emissions controlls on it since I got it.The guy I got it from never hooked up the emissions controls since the engine was new.
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