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360 Knock at Idle

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Lyle View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lyle Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 7:47am
Definitely a knock and not what I would describe as an exhaust leak sound or valve train.
Try changing your oil up a grade or two, i.e. 10w30 to 15W40 or a straight 30 weight.
If that knock changes or goes away you will know it's related to bearings, rods or pistons.
I,m just guessing hear from a distance but being an AMC V8 I would suspect #7 or #8 rod bearing has spun/worn and the piston is just touching the head at idle. At speed the oil pressure is enough to form a wedge that fills the void and the knock is gone.
If correct it may not be that bad as a refresh and new bearings, cam, lifters ... may be all that is required.
Not what you want to hear but a nice looking truck and worth the effort.
Why does the text keep changing size on on this forum???
  
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Brynjaminjones View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brynjaminjones Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 8:36am
Thank you for the advice.
If that is indeed the case, how quickly might I expect it to get noticeably worse?

The truck in the video isn't actually mine, and it's owner says it's done 7,000+ miles since that video with no change. 

I'll try to get a video later of my Waggy later when I get back from work.

I don't know what oil this motor was using before but this noise only started recently, not long after a switch to 20W50. 
With that being the case, I guess switching to a heavier grade as diagnosis isn't really possible. 
This oil has bumped up my oil pressure. 

Is it also possible that this could be the fuel pump? My impression is that this noise is at half engine RPM, which to my understanding limits the number of things it's likely to be. 

Whatever it is, it will be fixed. This Wagoneer is my dream car, a final edition in Hunter Green, and I can't see there ever being a point I'd give up on her!

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Brynjaminjones View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brynjaminjones Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 12:55pm
I've just got back from work (25 miles there and back in the Wagoneer today) and taken a video of how it's sounding nice and warm after the drive.
You should be able to hear the knock - I'd say it's at about half engine speed, so maybe 300-350bpm.


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Lyle View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lyle Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 1:19pm
I still suspect rod bearings 7 & 8. The knock at rpm change and frequency would be about right and the distinct change at idle. The exhaust is not showing smoke so is just transferring noise. 
I've seen an AMC V8 go 10,000 miles like this but it was scrap metal at the end.
If required for a short time, put in heavier oil but if it was mine I'd be tearing it down while there is still a chance of relatively cheap rebuild.
That clean exhaust tells me the pistons, rings, valves and valve train are likely still in good condition.
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DaemonForce View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaemonForce Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 3:52pm
I'm willing to bet it's a main bearing as mentioned. Is it a good weekend to pull that thing? Ermm
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limachine View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote limachine Quote  Post ReplyReply Direct Link To This Post Posted: Oct/18/2017 at 8:31pm
Have you run the engine with the belts off? You can do this, but I'd recommend a cold motor and not for more than a minute. I have a buddy sit in the vehicle and start it while I poke around under the hood, have him shut it once I get the info. If there's no noise, its a belt or a pulley. Might be difficult to pinpoint if it only happens on a hot motor. Also, if you suspect a fuel pump, unbolt it and pull it out of the way, run it a minute and listen. I've had a 390 fuel pump sound like a knock and found it that way. There's enough fuel in the carb to keep it running a short time.
In my experience, gasoline engines converted to propane don't last long without mechanical problems. It's like running jet fuel. In the late 70's, early 80's, Ford in America experimented with propane conversions without much success. That was probably mostly due to technology. About 12 years ago, GM offered propane powered vehicles and we replaced ALOT of engines at the dealer from mechanical failures.
  
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Brynjaminjones View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brynjaminjones Quote  Post ReplyReply Direct Link To This Post Posted: Oct/19/2017 at 3:28am
Thanks guys. What's the likelihood of it being a flex-plate? I figure I should check before doing anything else. Is it just a case of removing an inspection cover?

As for running on propane, it's actually reasonably common over here in the UK. Our 4.0 ZJ has been on it with no engine problems for the last 75,000 miles (now at ~145k).
Lots of guys in Aus seem to run these AMC V8s on it too.
I think you're right though, that it can cause issues on a lot of engines.
On those where it does cause mechanical problems, my research shows that it generally seems to be top end though, such as causing valve seat recession.

If my problem was caused by a bad main, would I have noticed a change in oil pressure?
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DaemonForce View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaemonForce Quote  Post ReplyReply Direct Link To This Post Posted: Oct/19/2017 at 6:15am
Check your torque converter bolts and find out? Embarrassed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brynjaminjones Quote  Post ReplyReply Direct Link To This Post Posted: Oct/19/2017 at 6:24am
I'll have a look. 
Not much time for investigating this at the moment, so I just want to check feasibility before I waste time chasing something that wouldn't show these symptoms Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LakesideRamblin Quote  Post ReplyReply Direct Link To This Post Posted: Oct/19/2017 at 9:32am
Also could be a worn piston skirt. Won't get worse but Won't get better. Most noticeable at idle, not so much if at all under pressure. Several causes are possible for piston skirt wear. You Tube it for more information. Good luck!
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"If you could kick the person in the pants responsible for most of your trouble, you wouldn't sit for a month." T. Roosevelt
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