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Hi guys i have a 52 plymouth cranbrook with a p26 motor . Ive had it since December last year just working on it to get a warrent ( New Zealand's road laws ) ive had it running and driving round but its got an knock at idle this knock disappears over idle it doesn't clatter or knock when you rev it or under load ? I have 40 pound oil pressure minimum when hot . Worryingly it does go away if i short the rear most cylinder plug .Any ideas?

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Bryce,

If it happens when cold it may be Piston Slap (When a Piston Pin was fitted a tad tight during engine work).

As the Engine warms up it goes away. Other than that try a compression or Vacuum check ????

Good Luck.

Tom

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Also check for exhaust leaks around the manifolds. Much more noticeable at idle rpms.

 

A cheap check, with paper held above and below exhaust and possibly try soapy water when still colder temps?  Also check the manifold to exhaust pipe connection, often a problem area.

 

DJ

 

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Several years ago my Dodge developed a noise that at first I thought was a knock.  It would tick at idle when cold,but would quiet down when warmed up.  I finally figured out that it had a crack in one of the exhaust manifold  'ears'  where it bolted to the block. When cold there was a leak causing the tick, but when warmed up the crac closrd and all was quiet.  

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Bryce,

I personally would drop the Oil Pan and have a look see. Check the Connecting Rod Bearings. A $20 Oil Pan Gasket will give you total piece of mind.

Otherwise revving the Engine and guessing at it could lead to more damage than you might otherwise be inclined to deal with.

The part about checking your reading with a Vacuum Gauge was to try to determine the Timing - Late perhaps, Etc. - Just the general Health of the Engine.

It is a quick check I use - just for piece of mind before I take a long trip or return from one. It can't hurt.

Good Luck!

Tom

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4 hours ago, Bryce Mcclintock said:

Ill try that why would depressing the clutch change or stop the knock

Crank thrust bearing, or something clutch related. You just never know 

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I've heard of the mechanical fuel pumps being a source of a knocking type noise. I think before I started tearing the engine apart I'd pull the fuel pump and run it off a temporary fuel source. 

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To answer Young Ed's post:

The Fuel pump noise is more like a louder click or tick noise than a knock, use a long handle screw driver and place it the handle to your ear and press the tip in the vicinity of the fuel pump.

Try placing the tip of the long screw driver else where around the engine and listen. As stated earlier revving the Engine with a possible bearing knock is not advisable. But hey its your car, do as you wish with it.

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Excuse me for being a old carpenter & not a mechanic, & lazy for not going back and read previous post.

 

My 2 cents.

A knock is not good. At idle & goes away, kinda makes me wonder if it is wrist pin issues?

A engine gasket set cost about $100 a few years ago, with today's inflation may be more now.

 

If it was a wrist pin, it will be digging into the cylinder wall and leaving scars. Will be obvious the issue with the head removed.

Your oil pressure sounds good which makes you think your bearings are good.

If you got piston slap because of worn wrist pins. When is it too late? Can you just slap a old used piston in there to replace the bad one?

 

These old girls learned some tricks along the years .... Will cost you $100 to peek inside , may cost you more if you do not peek.

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22 minutes ago, Los_Control said:

Excuse me for being a old carpenter & not a mechanic, & lazy for not going back and read previous post.

 

My 2 cents.

A knock is not good. At idle & goes away, kinda makes me wonder if it is wrist pin issues?

A engine gasket set cost about $100 a few years ago, with today's inflation may be more now.

 

If it was a wrist pin, it will be digging into the cylinder wall and leaving scars. Will be obvious the issue with the head removed.

Your oil pressure sounds good which makes you think your bearings are good.

If you got piston slap because of worn wrist pins. When is it too late? Can you just slap a old used piston in there to replace the bad one?

 

These old girls learned some tricks along the years .... Will cost you $100 to peek inside , may cost you more if you do not peek.

Makes good sense. 
in my case, i would pretend to not hear it. Still have to look at engine in my parts truck, i think that one will be easy to refresh. A rebuild tag on the block tells me it has probably more life it it. 

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I had a 54 ford when I was a teenager.  Got that car for 25 bucks.  After awhile it had a knock.  So I jacked it up in the driveway, pulled the pan and replaced the bearings.  When I got it running again the knock was still there.  Turns out the knock was the exhaust pipe hitting the frame.  After awhile the engine really started to knock.  My repairs ruined that engine.  Keep in mind that I was 16 and I did most of my work with a screw driver, a hammer and a pair of pliers.  I have the proper tools now.  We live and learn.

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