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8N Knocks after changing manifold

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Greg Hall

03-09-2000 12:11:51

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I recently bought an 8N, it had a leak in the manifold. This weekend I replaced the manifold and gasket, and changed the engine and tran/hydrolic/gear oil. It sounds better and runs smoother, but now there is a Knock at low RPMs only. The knock sounds like someone is hitting the engine with a hammer. The engine idles well and runs well at high rpms. I understand from the archives that a large screwdriver as a stethescope can help pinpoint the trouble, and a diesel flush might help clean out gunk. Any other advice would be appreciated.

Greg

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Chris

11-06-2000 14:31:16

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Re: 8N Knocks after changing manifold in reply to Greg Hall, 03-09-2000 12:11:51

I agree with Nolan, most likely you need to replace and reseat the manifold gasket. I just had the same problem in my volkswagen car, it sounded similar to valve clatter, but when i used a long tube to find the problem, it turned out that it was a leak between the cylinder head and the manifold. Firstly I removed the oil filler cap to hear if it was the valves, (I got so much oil in my ear I went through a 500 swab box of Q-Tips, dual ended for a grand total of a thou. actually, thats a joke) Anyway, then I suspected the gasket. Sure enough.....that was it. I would specifically check that out.

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Charlie in TX

03-10-2000 12:15:32

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Re: 8N Knocks after changing manifold in reply to Greg Hall, 03-09-2000 12:11:51

Whats your oil pressure. If you have good to high, 25 lbs or more, it is not likely a rod. However, low pressure does not mean bad rod. In car motors I have had bad main berrings that behaived as you described.

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EricG.NH

03-10-2000 04:44:25

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Re: 8N Knocks after changing manifold in reply to Greg Hall, 03-09-2000 12:11:51

My guess is that you have a true knock, likely caused by a worn rod bearing, etc., and that changing the oil made it surface, rather than changing the manifold. Since you don't know the history of the tractor, it's possible a less than honest seller/owner used a thicker oil and/or treatment to hide the problem. Bottom line is you need to play detective and figure out where it's coming from. The big screwdriver trick and/or a cheapy stethoscope are good ways to help locate the problem. You could also see if the knock changes when the clutch is pressed. You could also remove the oil fill cap and listen there. Don't forget to clean the oil out of your ear! :o)
You could try a thicker grade oil as well.
Just another point of view,
Eric

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Greg Hall

03-10-2000 06:58:54

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Re: Re: 8N Knocks after changing manifold in reply to EricG.NH, 03-10-2000 04:44:25

The oil I took out was thin, looked like they had thinned it with Kerosine (sp?). I put in 10-30. I am going to play detective this weekend, I will let you know how it turns out.

Greg

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Nolan

03-09-2000 12:31:10

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Re: 8N Knocks after changing manifold in reply to Greg Hall, 03-09-2000 12:11:51

I doubt you really have a knock, I bet you actually have an exhaust leak between the manifold and the block. The big clues are that you didn't knock before, and you just replaced the manifold.

You can isolate the cylinder easily enough by grounding out the spark plugs one at a time. When you ground a plug and the noise goes away, that's the cylinder. Now pay close attention to the manifold/block joint, and I'll bet you find a leak.

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Greg Hall

03-10-2000 07:01:56

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Re: Re: 8N Knocks after changing manifold in reply to Nolan, 03-09-2000 12:31:10

I hope you are right. Funny thing is that I replaced the manifold because it had a half inch hole on the rear bottom. I am going to play detective this weekend, I will let you know how it turns out.

Greg

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Nolan

03-10-2000 07:44:18

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Re: Re: Re: 8N Knocks after changing manifold in reply to Greg Hall, 03-10-2000 07:01:56

A big hole in the exhaust just produces a lot of exhaust noise. A small hole, like a blown out section of the exhaust gasket, produces a sharp rap sound, very much like a rod knock or wrist pin knock. My ear isn't senstitive enough to differentiate between those two.

You say the knock sound goes away at higher rpms. A true rod knock or wrist pin knock will not, it will just get faster. There is a faint chance that the higher oil pressure at higher rpms can help fade it, but in that case, it would barely be audable at lower rpms with the lower oil pressure. I'd also doubt that you wouldn't have heard a true knocking before even with a broken manifold. It's a nasty distinctive sound that will come through a loud exhaust. Look at the number of hot rods with loud exhausts that you can still hear a rod knocking.

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RICK

03-09-2000 16:02:21

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Re: Re: 8N Knocks after changing manifold in reply to Nolan, 03-09-2000 12:31:10

I think you have a knock that you didn't hear before because of the manifold leak. Piston slap?

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Jay

03-09-2000 21:03:51

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Re: Re: Re: 8N Knocks after changing manifold in reply to RICK, 03-09-2000 16:02:21

Could be the hydraulic pump?
Shift it out of gear and see if the knocking stops.

Jay

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