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Help diagnose this noise 2

Hi group,

I own a 2000 Jeep Wrangler 4.0 Automatic with about 70K miles on it. After a period of driving at slightly heavier than normal engine load on hot days, a knocking sound starts coming from the engine compartment.

I think that the noise I've been experiencing is most likely coming from within the engine or maybe the transmission. The first time I'd experienced it I was driving through some sandy washes in Mojave. As the engine and transmission load in the sand increased the engine temperature went up (but well within the safe zone) and I started hearing at first a tapping noise that gradually grew louder to a pronounced knocking (like a small hammer hitting metal) in sync with the engine RPMs. I stopped and put the transmission in park with the engine idling, but the knocking continued. I opened the hood and tried to listen where the knocking was coming from. It was pretty loud and hard to tell where in the engine compartment it was coming from. It almost felt like I was hearing it louder from under the vehicle when I leaned down and listened to it from the side while looking at the transmission. At that time, I shut the engine off and waited for about 15 minutes. Then I started the engine and the knocking was gone. I continued driving slowly at lower gear in order to reduce the load on the engine, which seemed to help. But still the knocking would come back at times and I had to really slow down or stop for it to go away.

The best guess, I figured from the frequency of the knock (about

8/sec), would point to a something occurring every other engine revolution like a valve.

When I first got the Jeep a year ago, I had no trouble driving up a long hill on a paved road at a fairly normal rate of speed. Now a couple months ago, I experienced the knocking again as I drove on a road like that (paved, no sand) to the mountains on a hot day. Most recently, I started hearing the knocking slightly after a period of stop-and-go driving on pavement in rather benign hilly area.

So the knocking seems to start after a period of heavier than normal engine load on hot days (though the engine temperature gauge always remains well within the safe zone).

I would appreciate any ideas on what this might be and what if anything can be done about this.

Thanks.

Reply to
jeepeeyes
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My first guess would be a crack in the exhaust manifold.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

I'd also suspect a bad plug or carbon buildup in one cyl that is causing detonation. Time to pull the plugs and check. I'd think if it was manifold the noise usually changes over temperature. Some cracks close some open all depends on what side of the pipe they are on. Not saying it couldn't be though.

Reply to
DougW

Had a neighbor across the street when I was growing who owned a parts shop. He used to take a large screwdriver and hold the handle to his ear, touching the blade to various parts of the engine. Could tell very accurately what was amiss. It does take a little practice. D

Reply to
D

"screwdriver is touching the fan blades" :-)

Just jokin'. You can get proper engine stethoscopes as well.

Dave

Reply to
Dave Milne

Ayup, had a neighbor like that once out west of Amargosa. Called him Old Screwdriver Head after he got a bit too close to the power steering pump one day. Not an uncommon tactic, works best on old iron blocks that tend not to ring as much as alloy ones. Lot safer to use a stethoscope with a contact sensor.

D proclaimed:

Reply to
Lon

Thanks for the responses. I think that I will take DougW's advice first and look at the spark plugs. I replaced all of them last year, but that does not mean one couldn't fail. Then, if they all look good, I will buy the stethoscope and carry it in the Jeep ('cause it always happens out in the boondies) and probe for the source of the knock the next time I hear it. Oh, by the way, I've already inspected the exhaust manifold and there seem to be no signs of cracks.

Reply to
jeepeeyes

jeepeeyes proclaimed:

There are two major categories of mechanic stethoscopes. The inexpensive ones are between $10 to $20 and are purely mechanical with just air from the probe up to your ears--the earpieces are typically earbuds. e.g.

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about $150, you can get the amplified version with the larger ear mikes like a set of stereo headphones. e.g.
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Reply to
Lon

Reply to
philthy

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