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post #1 of (permalink) Old 09-18-2003, 10:47 AM Thread Starter
 
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Name that engine noise...Please help!

Didnít really find an answer doing a search, so here it goesÖ

I have a 1990 YJ with the carter carbed 258 motor and 5 speed trans. Motor has 115k miles on it. Lately, itís been making an awful racket. The noise is NOT present at normal idle speeds hot or cold. Noise presents itself when the motor is revved above 1500 rpms whether in neutral or in gear; whether coasting or accelerating. Iíve run the engine removing one spark plug wire at a time and it made no difference. Noise remained. Iíve removed the fan, and the fan belts from the alternator and power steering pump and again, the noise remains. I recently replaced the distributor, timing chain, and water pump. I previously had the timing chain slap noise (chain hitting the front cover) which is now gone only to be replaced by this new noise (which for all I know could have already been present and I just couldnít hear it over the timing chain slap noise). The best I can describe the noise is a rapid chit-chit-chit, almost like a diesel sound. Like I mentioned earlier, itís not present at engine speeds below 1500 rpms. I canít really pinpoint the area of the motor where itís coming from either. Itís pretty loud though as even some people have commented on hearing it as I passed them by. My oil pressure is good and driveability/performance doesnít seem affected. I doubt itís an exhaust leak as Iíve checked the manifold to front pipe and thereís no apparent leak. I even ran some Marvel Mystery Oil the last oil change interval with no benefit in eliminating the noise.

Anyone have any ideas on what it could be or common ailments with the 258 motor that would produce such a noise? Without pulling the motor apart, how do you know if itís rod knock or piston slap or wrist pin slap, etc. Itís really beginning to drive me crazy not to mention how embarrassing it soundsÖlike Iím driving some hunk-of-junk.

Thanks
Mike
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post #2 of (permalink) Old 09-18-2003, 11:09 AM
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Re: Name that engine noise...Please help!

1. Worn Cam?
2. Valve Float?
3. Bad Lifter(s)?
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post #3 of (permalink) Old 09-18-2003, 11:19 AM
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Re: Name that engine noise...Please help!

If it doesn't change between load and no-load, it really rules out all of the common serious problems. Rods get louder under load, wristpins when backing down, and piston slap is louder with speed but diminishes quickly as the engine warms. Mains are too low-pitched to be described as chit chit.

My first guess would be an exhaust leak between the head and manifold, which is a common problem with the 258s, but it will change significantly between load and no-load. You should be able to close in on it by free-revving the engine while you listen under the hood. Also try revving it in the dark and looking for light escaping around the manifold.

A very good description, by the way.
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post #4 of (permalink) Old 09-18-2003, 11:37 AM
 
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Re: Name that engine noise...Please help!

You may *think* you don't have an exauhst leak, but from what you're describing (which you did very well, btw,) it sounds like one. The intake/exauhst bolts on the 258 are semi-notorious for loosening themselves up... check them for tightness...In 2 years I've replaced my exauhst gasket 2 times, by the way, so make sure yours isn't torn anywhere....

-pablo
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post #5 of (permalink) Old 09-18-2003, 11:39 AM
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Re: Name that engine noise...Please help!

Did this start right after you replaced the timing chain? If so, could it be a bolt head rubbing on the crank pulley?
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post #6 of (permalink) Old 09-18-2003, 11:56 AM
 
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Re: Name that engine noise...Please help!

I have a hunch that it is fan related. At idle no noise then as rpm increase the fan starts to pull making the shaft move.
Also since you replaced the timing chain, water pump, etc., look for the wrong length bolt in a hole. I would bet some of the bolts you removed were different lengths and they did not get back in the same holes.
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post #7 of (permalink) Old 09-18-2003, 12:09 PM Thread Starter
 
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Re: Name that engine noise...Please help!

All good tips. Thanks.

tincan...what do you mean "wrong length bolt in a hole"? Do you mean that when I put the front cover back on I possibly mixed up the position of the bolts? I didn't pay too close attention, but do some of these bolts protrude thru in such a way that some rotating part makes contact with them if they are not in the right position?

BTW, I removed the fan and ran the engine and the noise is still there. That's why I ruled out the fan as the source.
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post #8 of (permalink) Old 09-18-2003, 01:17 PM
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Re: Name that engine noise...Please help!

Pull the valve cover off and check for a broken valve spring. If the spring is only broken in one spot and hasn't destroyed itself it may still capable of handling the valve motion at low engine speeds but when you rev it up it will not be able to control the valve causing significant noise due to valve float or the rocker slaping the valve due to excessive lash at speed.







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post #9 of (permalink) Old 09-23-2003, 10:28 AM Thread Starter
 
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Re: Name that engine noise...Please help!

Good call Pablo. After looking closely, 4 of my manifold bolts were loose. Tightened them up and the noise considerably lightened. Didn't go away completely, but I think I'm on the right track. I guess I had convinced myself it wasn't an exhaust leak but I was only checking the manifold to front pipe connection.

When I bought this YJ 8 years ago, the front most manifold bolt was broken off in the manifold. I just ignored it. When I place a mechanic's stethoscope in that area now, I can almost pinpoint the noise I'm hearing right there. (That can also explain why I thought I always heard this noise and it took a buddy of mine to say that something ain't right with the way it sounds.) Unfortunately, I'll probably have to pull the manifolds to get the broken bolt out to replace it since I can't really drill it out since there's no room to work the drill.

Now I'm contemplating doing the swap of a head from a 4.0L motor if I'm going to go thru the trouble of removing the manifolds. Knowing myself, I'll probably just create more problems. [img]images/graemlins/grin.gif[/img]

Thanks for all the tips.
Mike
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post #10 of (permalink) Old 09-24-2003, 03:39 PM
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Re: Name that engine noise...Please help!

You say it sounds like a diesel!!! have you checked your spark pluggs. Maybe its getting a little spark knock. In the past, the only time Ive gotten spark knock is under acceleration. It does kind of sound like chit,chit,chit but at the same time it has a little tang,tang,tang mixed in there giving it a diesel sound. But then again what do I know!!!! Just a thought!!!
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