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!

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.

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!

It doesn't have headers - just the original cast style manifold.
Whilst inspecting last night I noticed that the air pump has been removed, and as such it looks like any other ports on the exhaust have been plugged (although I'm not quite sure what the original setup would've been).

In case it was missed in my previous post, I've also got a video of some play in the water pump shaft. This is with the belts slackened off:

Do you think this could cause a knock that would only be audible when warm? I'm wondering if it could get worse once the viscous fan is locking-up, putting more force on the shaft?

There definitely seems to be some play.

I'll report back to confirm whether that was the problem!

After an eventful week in the shop (many snapped bolts), the water pump has successfully been changed.

This has got rid of the severe play in the shaft, although the viscous fan clutch has a little play too.

At first it seemed like the knock was gone, but a couple of days later I noticed that it's back. 

The Jeep drives beautifully (mechanic said the same - I forgot to ask him to listen for the knock but he evidently didn't notice it) and I've put a few hundred miles on it with no change in sound. The knock still isn't really audible when the engine is cold (this could be because of all the other noise), or at anything much above idle. 

I've not got a video of my Jeep, but the one in this video sounds the same to me in both sound and speed:

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.

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???

  

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!

You should be able to hear the knock - I'd say it's at about half engine speed, so maybe 300-350bpm.

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.

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.

  

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?

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 

I took the belts off tonight and started it cold - turns out the noise does seem to be there when cold after all.

Have a listen, it sounds a bit more ticky today:

If it is exhaust there should be no change, belts on or off.

That stated, now it does sound higher up and more like a valve noise but the video is just from one side and before the knock was just as loud at the bottom of the engine?

I would be pulling the heads in place and inspect valves/valve train and pistons. Still just guess work at this point.

Lyle, my thinking is that the change in pitch and frequency is probably more to do with the fact that this was a cold start rather than warm, although I've not ruled out the possibility that I have a knock when warm and a lifter issue when cold!

Maybe it's just rockers, but, it's a nice Grand, freshen it up. 

Rod knocks tend to get louder as the engine warms up, and under load and/or with more throttle. The fact that yours didn't do that, plus the excellent oil pressure, tells you that it probably wasn't a rod knocking.

Now that it's fixed, go drive and enjoy it.