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  #1  
Old 08-23-2011, 02:48 PM
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Knock On Startup

I noticed sometimes on a cold start I will get a little knock for a second or two. Anyone else have this issue? I have a 3.7 with 106K and It does not burn any oil? Weird....
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Old 08-23-2011, 03:22 PM
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Re: Knock On Startup

Does it sound like sound like a wrench hitting a pole?
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Old 08-23-2011, 03:29 PM
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Re: Knock On Startup

not really sounds like a little bit of valve train noise
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Old 08-23-2011, 03:36 PM
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Re: Knock On Startup

Like bad timing?
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Old 08-23-2011, 03:38 PM
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Re: Knock On Startup

3.7 and 4.7 do have some piston slap that's prevalent on cold start up. Nothing to be worried about as the noise will subside as the piston expand and warm up.
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Old 08-23-2011, 03:43 PM
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Re: Knock On Startup

Yeah. I have that noise too. I hear it in a lot of autos on start up. I mostly hear it in Chevy Blazers and Tahoes.
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Old 08-24-2011, 01:00 AM
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Re: Knock On Startup

Quote:
Originally Posted by mrtosh View Post
3.7 and 4.7 do have some piston slap that's prevalent on cold start up. Nothing to be worried about as the noise will subside as the piston expand and warm up.
Yep piston slap can happen, they are 0 tolerance pistons that have a coating on the side of the piston and when that coating wears it can give the piston room to rattle a little till things expand.

Typically some clatter on startup comes from the valvetrain. perfectly normal on hydraulic valvetrain with some miles on it, the lifters can bleed down and have to pump back up after sitting.

Either way, its pretty common and normal. Keep fresh oil in her and so long as one of these is legitamently what it is it will continue plugin along for another 100k.
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Old 08-24-2011, 09:16 AM
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Re: Knock On Startup

Quote:
Originally Posted by 2005JGC View Post
Yep piston slap can happen, they are 0 tolerance pistons that have a coating on the side of the piston and when that coating wears it can give the piston room to rattle a little till things expand.

Typically some clatter on startup comes from the valvetrain. perfectly normal on hydraulic valvetrain with some miles on it, the lifters can bleed down and have to pump back up after sitting.

Either way, its pretty common and normal. Keep fresh oil in her and so long as one of these is legitamently what it is it will continue plugin along for another 100k.
Yupp, You guys pretty much nailed it. Was gonna reply and say it was like piston slap. It doesn't happen all the time either like y'all said, so I think ill be fine. The 3.7 is a pretty stout motor from what I've experienced. It uses hardly no oil after 3500 miles so that's a good thing for me coming from a Subaru Legacy.

Always keep on top of my oil and change it regularly. Thanks for the help!
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Old 08-24-2011, 10:05 AM
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Re: Knock On Startup

Same here. The oil is clean every time when it's changed. And I don't get the noise all the time.
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Old 08-24-2011, 11:32 AM
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Re: Knock On Startup

You should hear a Hemi on startup after sitting over night. Sounds like a sewing machine for about 30 seconds. The liters bleed down pretty quick on the 5.7's. I agree the 3.7/4.7's are pretty stout motors and essentially the same, minus two cylinders. Only thing you want to watch is NEVER let them overheat. Very weak head gaskets.
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