Engine knock while accelerating but not at idle?
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Laughing Gas
Joined: Oct 2014
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Engine knock while accelerating but not at idle?
Ive got a 95 jeep with lots of miles on it. It starts and drives really good but.... Its got this loud knocking. When I first start the car the knocking appears for only a few seconds then goes away. It does not do it at idle no matter how much I rev the engine then From about 35 mph while accelerating the knocking starts and is very loud, If I brake or take my foot off the gas the knocking goes away. On the jeep forums they said it could be a cracked flex plate, the torque converter or a rod. I inpected the flexplate by removing the inspection plate and made sure the bolts were tight and did not see any cracks. I am having a hard time diagnosing this. If it were a rod, wouldnt it do it all the time, especially at idle? Thanks.
Postmaster
Joined: Mar 2013
Posts: 3,066
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From: Over There
4.0? Cherokee?
"When I first start the car the knocking appears for only a few seconds then goes away"
Does it run rough for those few seconds?
"From about 35 mph while accelerating the knocking starts and is very loud"
Automatic transmission? Is "about 35" a shift point?
"If I brake or take my foot off the gas the knocking goes away"
Do you not always take your foot off the gas before braking?
You have a point about the timing of the knock, not happening at idle or when revving, a rod knock would very likely be audible when revving. Also that you say it's "very loud" internal engine knocks are rarely loud enough to be called very loud while driving, generally they are "very loud? for just a couple seconds before it blows. My best guess, a physical knocking outside of the engine from something like a broken or worn engine, transmission, or exhaust mount. When the engine shakes during start up or has enough torque on it in a high gear something like the exhaust is banging against the body or steering box or something.
"When I first start the car the knocking appears for only a few seconds then goes away"
Does it run rough for those few seconds?
"From about 35 mph while accelerating the knocking starts and is very loud"
Automatic transmission? Is "about 35" a shift point?
"If I brake or take my foot off the gas the knocking goes away"
Do you not always take your foot off the gas before braking?
You have a point about the timing of the knock, not happening at idle or when revving, a rod knock would very likely be audible when revving. Also that you say it's "very loud" internal engine knocks are rarely loud enough to be called very loud while driving, generally they are "very loud? for just a couple seconds before it blows. My best guess, a physical knocking outside of the engine from something like a broken or worn engine, transmission, or exhaust mount. When the engine shakes during start up or has enough torque on it in a high gear something like the exhaust is banging against the body or steering box or something.
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Laughing Gas
Joined: Oct 2014
Posts: 793
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Yea, 4.0 inline 6
No. it doesnt run rough just after starting it or while it makes the noise.
Automatic, dont know if 35 is a shift point.
I have replaced one of the the engine mounts, the transmission mount, and the muffler. I have inspected everything but cannot see anything out of the ordinary.
Yea, its so loud while driving I think the engine would have blown up by now, LOL.
I guess I will have to look a little harder.....
No. it doesnt run rough just after starting it or while it makes the noise.
Automatic, dont know if 35 is a shift point.
I have replaced one of the the engine mounts, the transmission mount, and the muffler. I have inspected everything but cannot see anything out of the ordinary.
Yea, its so loud while driving I think the engine would have blown up by now, LOL.
I guess I will have to look a little harder.....
Hotshot
Joined: Oct 2002
Posts: 18,488
Likes: 22
From: The Big, Oregon
Sounds like a bad catalytic converter. I have seen over a dozen of these, it's the main cat next the transmission, the ceramic brick gets loose and rolls around inside the housing. In extreme cases it can flip around and plug the exhaust pretty well.
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