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Troubleshooting Knocking noise

 
Old 04-02-2016, 10:23 AM
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Default Troubleshooting Knocking noise

Have a Knocking noise that matches the rotation speed of the crankshaft/drivetrain. Audibly (with the screwdriver to ear method) the loudest point of the noise on the entire vehicle appears to be the bottom of the bell housing of the transmission where it mates with the back of the engine (louder on the transmission side). It doesn't appear to be from the upper part of the engine (valve train, phasers, exhaust manifolds, etc.). It also is present in all gears.
I changed out transmission fluid - no change.
Have MIL for catalyst efficiency (no misfires), driver's catalytic converter appears to be spent based on inlet and outlet temperatures being equal. (so this seems irrelevant at the moment)

My thoughts are to disconnect the transmission from the engine and run the engine to determine if noise is coming from the engine or from the transmission.

Soliciting thoughts or advice on my idea and the easiest way to do it?

2006 F150 FX4 SCREW shortbed, auto tranny 4R75E
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Old 04-02-2016, 11:50 AM
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If you do disconnect the trans from the engine, check the flex plate for cracks, that can cause noise as well. You may be able to find a youtube video on that topic to see if the sound is what you are describing. Last year, I had a noise that I was concerned about that for a couple days, I thought was going to be the flex plate but after further diagnosis and actually crawling around for about an hour under my truck, found a gaping hole in the exhaust manifold between cylinders 3 and 4 on the engine side and was not visible from just looking around, had to feel for it. It made everything sound like it was coming apart. May want to check around there as well.
Good luck, let us know what you find, hopefully someone else will chime in with their knowledge or opinions as well.
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Old 04-02-2016, 12:26 PM
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Originally Posted by FixiTTilBroken
Have a Knocking noise that matches the rotation speed of the crankshaft/drivetrain. Audibly (with the screwdriver to ear method) the loudest point of the noise on the entire vehicle appears to be the bottom of the bell housing of the transmission where it mates with the back of the engine (louder on the transmission side). It doesn't appear to be from the upper part of the engine (valve train, phasers, exhaust manifolds, etc.). It also is present in all gears.
I changed out transmission fluid - no change.
Have MIL for catalyst efficiency (no misfires), driver's catalytic converter appears to be spent based on inlet and outlet temperatures being equal. (so this seems irrelevant at the moment)

My thoughts are to disconnect the transmission from the engine and run the engine to determine if noise is coming from the engine or from the transmission.

Soliciting thoughts or advice on my idea and the easiest way to do it?

2006 F150 FX4 SCREW shortbed, auto tranny 4R75E
Just run the motor with the inspection plate pulled off on the tranny (2 bolts).. You can also turn the engine over by hand at the harmonic balancer bolt with a socket and wrench while watching the 4 converter studs.. Watch for any cracks at the center of the flex plate..

Just tossing this out there..
Sometimes a stuck valve or stuck valve lash adjuster will sound like a rod knock, even though it's up top.. Try getting a stethoscope on it before splitting the engine and Trans apart which is not exactly what I'd call easy. IMHO your not done diagnosing, and the cat code would have me thinking valve train, probably drivers a side stuck lash adjuster..
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Old 04-02-2016, 02:44 PM
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That's pretty good advice, I think your criticism is spot on concerning inadequate diagnosis. I'm going to do what you mentioned.

I was originally thinking the source had to be in the valve train or something attached to it due to the nature of the rhythm.... but since I redneck-stethoscoped it ( read long Phillips screwdriver), I convinced myself that the source was where it was heard loudest (the bell housing)... but that may not be true depending on how the vibration travels... it is possible the bell housing is acting like a "speaker" and broadcasting the vibration over its large surface area at a resonant frequency and in effect "amplifying" the sound. Its also possible I'm just an idiot.
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Old 04-08-2016, 10:45 PM
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Well... son of a B.

Upon further investigation, I found some youtube videos of similar knocking sounds... one of which had something to do with spark.

So I decided to eliminate a bad spark plug and/or ignition coil as a possiblity before I started pulling her apart...

The cylinder on the driver's side closest the firewall had a plug that I would guess was never threaded in. I would not believe it could back out as much as it did, but maybe it could. In any case, see the photo. The plug AND coil boot are covered in soot. The noise was general misfire noise... or maybe the plug bouncing around in the few threads it had in common with my head. Changed out the driver's side coils and plug. Tick is gone. Never got a misfire code. Waiting to see if cat code 0420 clears out or if cat or O2 sensor are too dirty to save. Ran Seafoam into intake manifold. No issues getting spark plugs out (followed TSB procedure soaking them in carb cleaner).

Moral of story... a rythmic tick that is not clearly coming from the valve train, cam phasor, or exhaust manifold (gasket)... (especially if you are fortunate enough to get a cat code or a misfire code) ensure you've got air, fuel, and spark getting to each cylinder. (and start with spark!)

Last edited by FixiTTilBroken; 04-08-2016 at 10:49 PM.
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Old 04-08-2016, 10:47 PM
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Last edited by FixiTTilBroken; 04-08-2016 at 10:51 PM.
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Old 04-23-2016, 09:03 AM
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Just to close out...
Cat code P0420 never cleared. My scanner indicated the O2 sensors were responding to the exhaust, indicating they were working - this lead me to suspect the catalyst. Changed out driver side and passenger side cats and all O2 sensors (parts from rockauto). Drivers catalyst (the side with the knock/misfire due to the plug not being fully threaded in) ruined. Came out the O2 sensor hole in small dense charcoal looking clumps. I think it melted and ate itself, very similarly to this photo https://www.f150forum.com/f2/previou...334376/index2/
My guess is that the un-burned fuel passing through the malfunctioning cylinder deposited fuel on the cats inlet, consequently combusting from the heat and O2 present (the exact function of the second structure inside the cat)... this caused temperatures too high for the ceramic substrate and melted that bitch.
I learned: an unfixed misfire will cost you a catalytic converter too... gotta hunt down and kill any indication you may have a misfire (my knocking / ticking was one of those indications).
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