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Knocking noise in motor?

There seems to be a slight knocking noise coming from my motor after I start it up. This occurs when it is idling slowly before warming up. It almost sounds like a faint rod knock, but goes away when the motor is revved up. It sounds kind of like a pinging or knocking noise like you would get from pre-ignition. After the motor is warmed up it is not there at all. It is also can only be heard when the motor is idling slowly especially if it is about to stall before being able to idle on its own just after start-up.

I used a stethoscope and it sounds like it is coming from the right cam drive area at the rear of the motor. Weird that the stethoscope does not hear anything after the motor is warmed and has been driven. Also the sound is not noticeable when being driven.

The motor is from an SC with JE pistons and Webers. It has about 3K miles since a complete rebuild. New John D. cams, and new Carrera tensioners.

I will be doing an oil change and thought I would pull the cam cover on that side, but wanted to hear opinions as to what to look for.
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Last edited by Trackrash; 02-08-2018 at 03:00 AM..
Old 02-08-2018, 02:54 AM
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hi Gordon make a sound recording i will gladly listen to it

Ivan
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Old 02-08-2018, 03:55 AM
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Sounds like an exhaust leak to me that seals itself once the engine is warm. Check your exhaust system.
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Old 02-08-2018, 04:49 AM
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Gordon, it would seem your engine has been talking to mine. The reason mine is apart this winter is due to a very similar noise as you have described. For mine, on cold start up, quiet for the first 20 seconds then that slight knock you describe. It was present if I revved it and once warmed up, zero noise.
Some history, a case split rebuild 7k miles ago. 964 cams from John D, Craig Garret did the heads and rockers including bushings. New rod bearings with mains being perfect so left alone. Mahle cylinders light hone and new rings. Timing chains replaced as were all ramps. Runs like a champ with negligible oil consumption. Figuring I had a slightly collapsed piston, I jumped on a set of 98 P&C that came up for sale and they are going in this winter. But I have yet to find anything that would account for the noise. I have replaced the small end rod bushings as they were on the high end of OK. Tensioners are hard. 95mm P&C still check out as good clearance. I would really like to find the cause as it would be a downer to hear the same noise after all this work. Hopefully someone can provide a clue for both of us.

Interesting point jlex made about an exhaust leak. My noise is a very definite metallic sound so I don't think (but I wish) it was that. No evidence of exhaust leaks around the gaskets either and the system was tight at the heads.

Dave
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Old 02-08-2018, 05:16 AM
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piston slap from the JE pistons.

this is what I have heard about it in the past:
the mahle pistons have a slight offset of the piston pin perhaps to eliminate this, the JE pistons do not have this offset.
just what I heard a long time ago from a mech friend that does not like the JE's.


I have not heard the Porsche piston slap but the older jeeps did it when you used synthetic oil, sounds kind of like a diesel truck idling
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Old 02-08-2018, 05:54 AM
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Could it be a loose rocker? As it warms up it would swell a little to take up some of the gap.
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Old 02-08-2018, 05:55 AM
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The piston slap theory is something to consider. I had thought of that. Listening to the valve covers, does not seem to be the source.

However, I just went out, started it up and listened again with my scope. I listened on the bottom of the motor even on the cylinders, and those were not making the noise. Not an exhaust leak, I am familiar with those.

The only place I hear it with my scope is at the center of the right side cam chain cover.
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Old 02-08-2018, 09:44 AM
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hi Gordon..if you can make a video...with sound..i did actually asked already ..do not know if you are able?

Ivan
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Old 02-08-2018, 10:04 AM
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Center of the chain box cover would seem like the chain tensioner. I forget Gordon, do you have regular self-contained tensioners or the (in)famous Carrera tensioners? I know you participated in the lengthy discussion about the functionality of the Carrera tensioners. So i'm confident you'll know what to do if that's the problem location!
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Old 02-08-2018, 10:09 AM
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This motor came with the Carrera tensioners already installed. The one on the right failed a while back and I replaced it with a new OE tensioner.

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Old 02-08-2018, 10:22 AM
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My theory would be no. 4 piston pin a little loose...can you hear it on cyl no 4 intake mani...?


Ivan
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Old 02-08-2018, 10:35 AM
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That noise sounds familiar to me.........

Gordon,

I had the similar noise from my driver side engine when the hydraulic tensiner failed. Depending on the degree of which the hydraulic tensioner collapsed, the tapping or knocking could be faint to alarmingly loud. Remove the chain covers and do a compression test to the tensioner/s.

Tony
Old 02-08-2018, 10:43 AM
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Hi Tony, do you think when the chain tensioner gets warm this noise is gone...?That is what Gordon is saying..

Ivan
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Old 02-08-2018, 10:47 AM
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chain tensioners do not get better when warm, the noise does go away when you rev it.

mine collapsed no a trip on my 77. sounded like an open exhaust port, kind of made me think of a blown head gasket and the exhaust was coming out between the head and cylinder.
it was fine driving home though. 250 miles. really had nothing to lose at that point.
oh, I did not realize it was the tensioner until I got home.

if its only one side I would lean towards the tensioner
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Old 02-08-2018, 10:58 AM
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Sounds like a tensioner. It goes away with higher revs and returns when lower rpm?Easy to take off rear metal and covers to check.
Get it repaired sooner rather than later .
Old 02-08-2018, 11:06 AM
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Hi t77..i had similar problem with my tensioner..but for me it was to take a trip back home 1400 km
knowing it was the bad tensioner i had to make sure i keep the rpm high so it does not have the time to go to idle and keep the oil in....i made it back home and replace that beast....happy it lasted that long...
Ivan
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Old 02-08-2018, 11:07 AM
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also Gordon said..." After the motor is warmed up it is not there at all. "
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Old 02-08-2018, 11:08 AM
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Gordon i have a question..This is the 1st time you have noticed it or was it there longer?

Ivan
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Old 02-08-2018, 11:25 AM
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I didn't notice it until recently. I listened on the head and cylinder and did not hear it. Since these are Carrera pressure fed tensioners, they could get better when the oil pressure is up.

I agree, it's time to pull the cover. I'll be doing my 1K mile service soon, in the next week or so. I'll take a close look at it then.

So I will report back when I get a chance to tear into it.

Thanks for all the feed back. I just wanted to be able to anticipate any possible issue.

Thanks all.
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Old 02-08-2018, 04:00 PM
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Does it sound like this?

Saving a 73 from the crusher...
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