Engine diagnosis, rod knock. - Ford Bronco Forum
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post #1 of 36 (permalink) Old 02-11-2013, 10:52 PM Thread Starter
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Engine diagnosis, rod knock.

Seeking your advice in diagnosing my engine. The truck has a new battery, cap and rotor, spark plugs and wires and an oil change. The engine turns over fine, sometimes I need to hold the gas a little when I start her in the morning. It idles fairly smooth expect for a repeating knock in the engine. My buddy said it was rod knock. No vacuum leaks that I was able to detect and it is not throwing any codes. How would I go about in diagnosing the problem? The oil levels are good.

95' 5.0/E4OD
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post #2 of 36 (permalink) Old 02-11-2013, 11:06 PM
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Install a Mech oil pressure guage and check oil pressure if you suspect any mech knocking. Can also get a 2' long heater hose, have motor running and see if you can isolate the sound with it. A stethescope is easier but i'm cheap and heater hose I have.
Have you pulled and checked plugs, see if one is discolored differently.
Look at manifolds for black soot if there is a leak or cracks.
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post #3 of 36 (permalink) Old 02-11-2013, 11:15 PM Thread Starter
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Not sure if this is related but I have a good amount of exhaust smell in the cabin. I was thinking it was an exhaust leak. Anyway, the plugs were installed 3 days ago ran maybe 20 miles, too early to pull them for discoloration?

95' 5.0/E4OD
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post #4 of 36 (permalink) Old 02-11-2013, 11:19 PM
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Check for cracked manfiolds for sure then. How many miles? Mine they where both cracked at the back near the collector and I was missing two bolts on each side as well.

Exhaust coming into cab? Do you have any holes in exhaust pipe, driving with rear window down sucks all that sleepy gas inside if any leaks.
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post #5 of 36 (permalink) Old 02-12-2013, 12:18 AM
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Originally Posted by musclegto View Post
Not sure if this is related but I have a good amount of exhaust smell in the cabin. I was thinking it was an exhaust leak. Anyway, the plugs were installed 3 days ago ran maybe 20 miles, too early to pull them for discoloration?
exhaust leak will sound more like a tick than a knock. unless you have a portion of the exhaust physically banging against something with the vibration of the motor.
x2 on the hose to isolate the sound. you can also use wooden rod like a broom handle.
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post #6 of 36 (permalink) Old 02-12-2013, 12:20 AM
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Check for cracked manfiolds for sure then. How many miles? Mine they where both cracked at the back near the collector and I was missing two bolts on each side as well.

Exhaust coming into cab? Do you have any holes in exhaust pipe, driving with rear window down sucks all that sleepy gas inside if any leaks.
good way to check intake cracks is to spray starter fluid or carb spray on the area. if there is a crack in the intake you will hear the motor jump rpms.
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post #7 of 36 (permalink) Old 02-12-2013, 01:37 AM
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X3 on the hose.
Garden hose works just fine too.
Great way to isolate where the knock is coming from.
Sometimes the old school ways are still the best.

I would recommend you isolate the noise first,
May save you a lot of unnecessary time and effort.

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post #8 of 36 (permalink) Old 02-12-2013, 01:42 AM Thread Starter
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will do on the garden hose

95' 5.0/E4OD
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post #9 of 36 (permalink) Old 02-12-2013, 01:43 AM Thread Starter
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might actually have access to a stethoscope

95' 5.0/E4OD
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post #10 of 36 (permalink) Old 02-13-2013, 06:50 PM
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post #11 of 36 (permalink) Old 02-13-2013, 08:17 PM
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being an 95 you have efi. pull codes should be your first step. if your hitting the gas peddle to get it started that also is an issue being you shouldnt need to do that. check your fuel pressure as well and timing

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post #12 of 36 (permalink) Old 02-13-2013, 10:04 PM Thread Starter
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noted

95' 5.0/E4OD
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post #13 of 36 (permalink) Old 02-13-2013, 11:39 PM
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the hose is a good method

also if it comes rite down to it and you cant isolate the rod/crank knock youll have to drop the oil pan and try to wiggle each rod journal to see which one is knocking

if a rod is found to be knocking hopefully it wont be slaping the head and all youll have to do is replace that rod/bearing/piston rings for that cylinder......hone the cylinder before reinstalling the piston so the rings can seat properly

parts needed would be a complete engine gasket set, connecting rod, rod bearings, piston rings, fresh oil, and a cylinder hone with instructions on how to hone a cylinder properly


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post #14 of 36 (permalink) Old 02-14-2013, 12:07 AM
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I've heard manifold leaks that sound like knocks. the fact that you have exhaust IN the cab, and this sound, i'd thoroughly inspect the exhaust system.

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post #15 of 36 (permalink) Old 02-14-2013, 02:29 AM
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The classic method of finding a rod knock is to run the engine up to about 1000 RPM, and then pull one plug wire at a time. The knock will go away when you pull the plug for the offending cylinder.

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post #16 of 36 (permalink) Old 02-17-2013, 06:38 PM
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The classic method of finding a rod knock is to run the engine up to about 1000 RPM, and then pull one plug wire at a time. The knock will go away when you pull the plug for the offending cylinder.
So the rod wont knock since theres no ignition in the cylinder? The compression alone wouldnt be enough to make the rod move?
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post #17 of 36 (permalink) Old 02-17-2013, 08:29 PM
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Yes, it takes the load off the piston and the rod.
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post #18 of 36 (permalink) Old 02-17-2013, 08:48 PM
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Thats awesome,im guna have to remember that

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post #19 of 36 (permalink) Old 03-01-2013, 10:20 AM
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Rod knock/crank bearings

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post #20 of 36 (permalink) Old 03-03-2013, 08:14 PM
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The classic method of finding a rod knock is to run the engine up to about 1000 RPM, and then pull one plug wire at a time. The knock will go away when you pull the plug for the offending cylinder.
I don't feel like that is a reliable method. And how is that easier than a stethoscope?

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