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Strange Sucking Sound

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ghcoe View Drop Down
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    Posted: Sep/29/2018 at 11:51am
Hello, I am writing because I need some help figuring out a strange sucking sound that my engine makes. 360 stock in a 1977 Jeep Cherokee.

The sucking/miss sound has always been there since I installed the motor in my '72 Jeep in '94. Since then the motor moved to the '77 Cherokee in '99. The motor has about 80,000 miles on it now, but after the last trip the noise is much more noticeable now. It sounds as if it is sucking in air on the left exhaust pipe like a misfire or a burnt valve. The strange thing is that it only does it when not under a load. Such as in neutral or park. Once you drop the transmission into drive the sound slowly goes away and the engine sounds normal. Usually it is most noticeable at cold startup and dissipates once the motor comes up to operating temp. Since it is worst now I am trying to figure it out again. Really was not a issue before. Anyone have a idea of what this could be?

Thanks, George.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXFSTBK390 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 12:28pm
I have always associated sucking sounds with a vacuum leak. Used Berryman b12 and sprayed it everywhere a vacuum leak could occur. If the RPM dropped I knew I located the vacuum leak.  
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ghcoe View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ghcoe Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 1:25pm
Originally posted by AMXFSTBK390 AMXFSTBK390 wrote:


I have always associated sucking sounds with a vacuum leak. Used Berryman b12 and sprayed it everywhere a vacuum leak could occur. If the RPM dropped I knew I located the vacuum leak.  


alas, I have tried that. No difference in engine RPM. Sprayed along bottom of carb, along intake manifold and all vacuum related devices. No effect. Unless it is a internal vacuum leak. Sounds like valves though. Can't figure out why it would quit once in gear though....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 2:44pm
I have no experience with the later AMC V8's but I'll throw this out there.  Does your engine have an aspirator tube (about 5/8") that runs from the air cleaner to the exhaust manifold through a check valve?  The Mopars that have this feature produce a gurgling sound if the upper hose becomes dislodged from the air cleaner.
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ghcoe View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ghcoe Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 2:56pm
Thanks, but no aspirator tube.

It does have a EGR and a heat riser valve. I am not sure if the heat riser valve is installed properly. Anyone know it the heat riser valve can be installed upside down? To me it looks like the spring load and the weight are working in the same direction. Not sure if this would cause the issue or not though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 6:51pm
Just to make it easier to see and hear, here's the VIDEO of the sound ghcoe posted:



Something that might help find the noise, about a 3 ft or so long piece of say 1" or 1-1/2" PVC or something, even a Christmas Paper Roll, hold it to your ear and use the far end to try and "zoom in" on the noise / locate the noise when it's happening... maybe...

Has the Passenger  Side Intake Choke Stove been checked out ?
It kinda works with the Exhaust Heat Riser Valve...
Choke Stove Internal Tube Hoop maybe has a hole in it ?

Heat Riser, this is only a 67-72 Parts Manual Drawing, but may provide orientation clues...
The Intake Choke Stove Opening is also visible in this drawing.
I have little to no knowledge about EGR function & how / if it might cause the noise.




Edited by PHAT69AMX - Sep/30/2018 at 12:15am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmaher94087 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 6:57pm
Power brakes? Check the rubber grommet on the PB booster.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 7:01pm
Good call, Mike!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 7:48pm
Also a rusted out air emissions valve will make a hiss, one one each side, mounted on the injection pipe.
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ghcoe View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ghcoe Quote  Post ReplyReply Direct Link To This Post Posted: Sep/29/2018 at 8:37pm
Great responses to what I described. I knew this was going to be hard one to describe properly.

I made a short video of the sound coming out of the exhaust. In the video it sounds more like air escaping rather than sucking. You will hear it come on loud right after I let up on the throttle and slowly dissipate. There is no sound coming from under the hood.

This is a cold engine. Usually when it is most noticeable.

https://youtu.be/jbzPMUfX6e4

Once warm it goes away for the most part.

Hope this helps because I am stumped to what it could be. George.

Edited by ghcoe - Sep/29/2018 at 8:45pm
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