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360 Knock at Idle

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Brynjaminjones View Drop Down
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    Posted: Sep/27/2017 at 5:57am
Hi Guys,

I'm new to this place and the world of V8s, so please be gentle! I have had a search but struggled to find anything.

I've recently bought a 1991 Grand Wagoneer with a stock 360 V8, albeit running on LPG/propane. The clock shows 106,XXX (about 104,500 when I bought it). 

About 10 days/400 miles ago I gave it an oil change. The oil that came out was quite black, and the (Mopar) filter was well seized on. 
Prior to the oil change, oil pressure when warm was at about 35-40 at 60mph on the highway, with it dropping slightly below the first bar (so about ~8) at warm idle, but would jump up the moment I got on the gas. 
I know these gauges aren't known for their accuracy, but it seems like a decent representation. I'd also like to say that I have low voltage at idle, which I think is causing the gauge to drop ever so slightly lower than it should. 

Anyway, I replaced the filter with a Wix 51626 and filled her up with Valvoline VR-1 20W50. The oil level is a bit above the full line, as I've read that these like a bit extra.

With this new oil the pressure is much improved. When cold it sits at 60 or slightly above. When warm it's about 40-45 on the highway, 50ish when accelerating and about 10 at warm idle.

I've done a few hundred miles like this with no problems.
However, I noticed a few days ago whilst stuck in traffic that it has developed a new knocking noise at warm idle.
I can't hear it at all when the engine is cold, but once warmed up and let to idle there's a definite dull knock (sounds a bit like knocking your fist gently on a desk). It's quite a slow, consistent knock - I'd say once per engine revolution. 

I first noticed it inside the car, coming through the firewall. On inspection last night you can also easily hear it in the engine bay, although I wasn't able to pinpoint further. 
The sound is inaudible once the engine speeds up, although of course it's hard to tell if this is because it stops or because it gets drowned out. 

For those of you familiar with the 4.0 in a Jeep XJ, inside the car it sounds much like when the EVAP purge solenoid starts clicking.

I'm so used to the 4.0 engines which have been making noise for years, but I can't help but worry about this. 
The oil pressure hasn't changed its behaviour. 

From experience, where would you guys say I should start looking?

Apologies for the long read and whiny first proper post, but I'd rather give too much information than too little!
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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: Sep/27/2017 at 12:24pm
Knock at idle can be from simple to difficult to just all out bad.
The list, and it won't be complete:
bent dip stick -  hits the crank
distributor cap or rotor interference
fan blade hitting shroud - usually a bad water pump bearing or bent pulley
starter not completely disengaged and hitting a spot on the ring gear or broken tooth
bad/sticking lifter
bent or broken valve
bent or broken crankshaft - yes I have seen an engine run with the crank completely broken at the connecting rod bearing.
piston hitting valve or head - bad bearing, bent rod, broken rod, bad piston ...
bad connecting rod - see piston
connecting rod hitting block - see piston

Your going to have to narrow down were the knock is coming from to determine a better diagnosis.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote vinny Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 12:26pm
First I would do a compression check to make sure the pistons are OK. I have seen it before with propane or natural gas conversions that they can run lean and destroy pistons or do other damage. Burning through pistons was on Grumman delivery vans with the GM350 running on LNG. I've also heard a pickup backfire through the carb to the point of blowing off the plastic intake manifold. That was also on a GM V8 converted to propane.

The dirty oil surprises me though because usually when on propane it comes out very clean.

Placing an iron bar to the ear helps in locating from where the noise is coming from.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 12:39pm
You could pull one plug wire at a time when it's knocking. If the knock goes away with the cylinder not firing, it's almost always a rod knock. Not good, but at least it would isolate it.

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brynjaminjones Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 1:59pm
Thanks for the help guys. I'll use this advice to start narrowing down.
I was of course aware I'd need to investigate further, but didn't really know where to start!

Dummies' question, but is it okay to pull the spark led off whilst the engine is running? I'm sure I've done it in the past but just want to check!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote A0M190Y Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 4:40pm
I bought a Grand Wagoneer years ago from a dealer that had the same issue. Worked good for the first couple of thousand miles then and then started to knock. Turns out the crank was shot and the dealer just put new bearings in to get it to last a little longer. Ended up needing a new engine.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 4:57pm
Originally posted by Brynjaminjones Brynjaminjones wrote:

Dummies' question, but is it okay to pull the spark led off whilst the engine is running? I'm sure I've done it in the past but just want to check!


It's OK, just use insulated tools. Quite a jolt, and lots of them if you become the ground point.

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurst390 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2017 at 5:42pm
I've had stock fuel pumps make a knocking noise years ago..use a piece of hose held to you ear as a stethoscope and try to pin point the location.
If you think it is a rod bearing I would pull the plugs and spin it over and listen to each plug hole..that will tell you if you have a bad bearing
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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: Sep/28/2017 at 11:13am
You stated "original 1991" and that brought to mind another knock or loud tick at idle that is hard to find. Take a look at the exhaust manifolds and especial the dilution tubes. A leak there can also produce this noise but is often overlooked. The tubes tend to rot or crack near the manifold connection.
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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: Sep/28/2017 at 5:05pm
Thanks again for all the help guys. I've been having a play today.
I tightened all the manifold bolts and found some were loose. Having been for a drive though I don't think that's helped.

I've been out in her this evening and I really have a hard time believing that it's an unhealthy engine. The oil pressure is good and at anything other than idle she's perfect. Even at idle, the noise isn't immediately noticeable.

I did also loosen the belts, and I noticed that there is some play in the water pump. I've uploaded a video:


I assume that's not normal - do you think that could be it? The fan doesn't noticeably wobble, and as I said the knock is only when the engine is warm.

Another thing I was wondering - when I'm on LPG the gas supply is shut off. Could this wear out the fuel pump and cause it to knock?   https://youtu.be/j5JUpXT7M-w - https://youtu.be/j5JUpXT7M-w

Edited by Brynjaminjones - Sep/29/2017 at 3:16am
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