knock in motor only after warmed up
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knock in motor only after warmed up - 9/15/2007 6:23:37 PM
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honda450
Posts: 14
Joined: 9/15/2007
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I have 88 chevy pickup 350 1/2 4x4 auto. They guy I bought it from said that the motor hade been replaced, I think it was rebuilt going by the fact that it doesn't have the stock heads on it, it has on a set of older heads. Anyways, start motor no noise, let warm up to operating temp and you can hear a light knock, rev it up in park to around 1200 to 1500 and gets a bit loader then slowly goes away when reving it farther. It shows on the gauge as having 60psi at idle cold or hot, not sure if gauge is reading right or not. My thought here is to take the pan off and snoop around, thinking possible crank bearing spun some what. The knock goes away when you put it in gear.
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RE: knock in motor only after warmed up - 9/16/2007 5:15:22 AM
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astroDZak
Posts: 84
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It sounds more like a classic wrist pin knock to me. The wrist pin is where the piston is connected to the connecting rod and looking into the pan won't see it.
To check for a bad wrist pin drive down a quiet paved road and while on a flat part let the engine freewheel at the rpms it takes to keep the truck moving but not slowing or speeding up. If your hearing the noise then let up on the gas and see if it goes away also it should go away by giving it gas if its a wrist pin.
A bad wrist pin is simply the hole in the piston is ovaled. It can be quite annoying but not usually a major blow up factor.
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RE: knock in motor only after warmed up - 9/16/2007 7:23:24 AM
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honda450
Posts: 14
Joined: 9/15/2007
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Ya that makes sense to me, someone else hade told me that also. I am a motorcycle tech and we generally don't run across problems like this. If a wrist pin is bad on a ATV they just make noise, mostly cause the piston lands are scored. So what about the high oil pressure? I forgot my oil pressure gauge at work but still plan on testing it. If it still reads high should I put a different spring in it to lower it. I have been told that there was a update on these year trucks the new motors, only running 30psi.
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RE: knock in motor only after warmed up - 9/18/2007 4:30:21 AM
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astroDZak
Posts: 84
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I have yet to see a motor hurt from to much oil pressure although I did have a 350 that would blow oil filters right out one after another.
I changed the pump to a stocker and still have 60 psi warm at an idle. I wouldn't worry about to much but not enough scares the hell out of me.
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RE: knock in motor only after warmed up - 9/18/2007 5:20:39 AM
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honda450
Posts: 14
Joined: 9/15/2007
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True I guess all the others that I have owned and drive ran less, my big thing was I was worried that it has a spun bearing.
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RE: knock in motor only after warmed up - 9/18/2007 5:32:46 PM
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astroDZak
Posts: 84
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If it had a spun bearing it would have most likely locked up on you by now. The part you said about putting it in gear was the part that makes me believe its just a wrist pin.
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RE: knock in motor only after warmed up - 9/18/2007 6:49:21 PM
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honda450
Posts: 14
Joined: 9/15/2007
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With a bad wrist pin is that going to make the computer think that the motor is going into detenation? I'm ok with taking off the oil pan and head to replace it, cause after all what would be the most it would cost me in parts
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RE: knock in motor only after warmed up - 9/20/2007 4:04:56 AM
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astroDZak
Posts: 84
Joined: 9/2/2007
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I can't see it effecting your computer since its not an electronic issue and shouldn't change emissions or manifold vacuum.
If your thinking about tearing it down for the piston then your going to have to buy a gasket set anyway so why not get an overhaul kit with rings and bearings too. The price difference is not very much and its more work pulling the motor apart in the hole than it is to pull it and go clear through it. Provided you have a way to lift it I guess.
Once the head is off and before you push the piston out make sure the ridge at the top of the cylinder is not so bad that pushing the piston past would break the rings.
Last I knew a basic gasket set was about 80 bucks. A rering kit includes rings and bearings and the gaskets for around 125 to 150 bucks
A master kit including rings, bearings, cam bearings, cam shaft, timing set, oil pump, and pistons are about 350 bucks.
Theres a lot more to putting a motor together than most people think and since I don't know your abilities let me say be sure you mark EVERYTHING so it goes back exactly where it goes and not turned around backwards.
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RE: knock in motor only after warmed up - 9/21/2007 5:23:03 AM
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honda450
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Joined: 9/15/2007
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I'm just going to leave it alone for now, was talken with someother techs and they also said that was mostlikly the wrist pin. They also meantioned that some that did deside to take apart and just replace the problem, later the motors came apart, so going to drive it and smile. Thanks too all for the advise.
By the way aleast it's cheaper to buy the part to repair this than the parts for my fourwheeler I just bought to repair. Even at my cost it's going to be 400 just for the motor and another 200 for the rest.
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RE: knock in motor only after warmed up - 9/21/2007 11:59:00 AM
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astroDZak
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Glad I could help. Enjoy 4 wheeling.
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