Your mechanic sounds like a retard. The Explorer is an all American made vehicle which is designed to run on 87 octane, the standard fuel avaliable in the US. Since 91 is the lowest octane fuel avaliable in AU or NZ then 91 will have to do. I have run higher octane (98) in my Explorer before just to see what happens and to put it simply, in stock form they just don't like it. They need to be able to suck more air to "like" the higher octane fuel.
It sounds to me like your mechanic is trying to empty your wallet. That's not your engine "pinging" it's timing chain guide or spring or both rubbing on the edge of your timing chain or chains. There are 4 timing chains located inside the 4.0L SOHC V6. 3 are on the front and 1 is located on the back of the engine on the drivers side (right hand drive export models).
If the rear guide was busted your drivers side camshaft would be retarded by 10 degrees. you would notice that type of performance degredation unless you purchased the vehicle with that fault existing. A dead give away of the rear guide being busted is that you will hear a slight "hiss" when you turn the engine off about half of the time. The "hiss" sound will be air leaking into the exhaust through the exhaust valves on the drivers side (right hand drive export models).
Now, sounding by your first post I'd say that the guide in the front jackshaft to camshaft timing chain cassette has broken and dropped into where the crankshaft to jackshaft timing chain is located. That rattle you can hear is from not all the slack being taken out of the front Jackshaft to Camshaft timing chain by it's hydraulic tensioner, this can be remedied with a larger tensioner. That ticking sound you can hear is the Crankshaft to Balanceshaft tensioner broken. One of the metal springs gets exposed and rubs on the timing chain, this causes the ticking noise.
Is there any chance I could get you to take the fan belt off and shoot that video again so we can hear exactly what is happening without the noise from the accessories and belt driven fan?
I'm going to post some picures on here of the timing chains and there locations to try to give you a mental picture of how they are setup. I want you to take in as much of this information as possible so you have some ammunition to take to your mechanic. If I prevent just 1 person from being ripped off blindly by mechanics on the southern side of the equator I will be a happy man.
The following picture shows the location of all 4 timing chains installed into the engine block, without the heads of course. Please note: these are the upgraded timing chain cassettes with beefier springs and steel reinforcement. In your engine the rattle you speak of is cause by the upper right front timing chain cassette. On the original cassette there was a plastic guide that ran between the chain and had a habbit of snapping. I found peices of mine wedged between the lower Crankshaft to Jackshaft timing chain and the front timing cover.
The following picture shows the Balanceshaft timing chain. Notice the metal tensioner spring under the plastic curved peice on the under side of the chain. This can become dislodged from its position if the plastic peice above it breaks. When this happens the spring becomes jammed between the bracket and the chain causing the "ticking" noise you can hear.
The 3 timing chains on the front can be replaced without removing the engine, although the front diff must be lowered in order the remove the upper oil pan to replace the Balance shaft tensioner guide and chain if it is indeed broken. It would be easier, cheaper and less time consuming for a professional to remove the engine to replace the balanceshaft timing assembly. It would be even cheaper to remove the balance shaft from service by simply cutting the chain.
The engine must be removed to replace the rear Jackshaft to Camshaft timing chain cassette.
This is the updated rear timing chain cassette. For some reason they did not decide to reinforce the rear guide with steel.
FACTS AND FIGURES FOR THE SOUTHERN HEMISPHERE:
If you were to purchase an engine out of a wreck from a wreckers it would cost $2200 and you would have no way to tell if the timing chains were bad in that engine to begin with. The mechanic would most likely charge $5000 for the engine and installation. Sure you could have a mechanic install the timing chain componants into this engine, however, firstly they wouldn't want to do that and secondly they wouldn't have a clue how to time one of these engines anyway.
After months of searching you may be able to find one on ebay from $500 but that would be strictly on an as is basis with no guarantee on wether the crank would even spin freely.
Unfortunatly a new 4.0L SOHC V6 can not be found on a shelf down here anywhere, even at "Rare Spares" and would need to be imported from the US. I have not even considered the cost of this because it was going to be way out of my budget. Although, I would have really liked to import one.
You can leave it in the hands of your mechanic and he will change every sensor and non relevant componant and use every oil flush and other product on the market to try to please you all the while charging you several thousand dollars over the course of years until you arrive at the first option of a salvaged engine.
You could "man up" and change the parts yourself for like $600 and wind up with some awesome tools in the process.
You could try the 00m12 fix for $70 + postage. It would take a beginer about 5 hours to complete. But for someone who is fully tooled up and loves working on their truck, they could do it in under 2 hours.
Years ago before I had my Explorer there was someone on Ebay in Australia who specialised in the doing the timing chains on Explorers. They did the entire job for about $750. Maybe someone will do it again, they would have plenty of buisiness.
To finish, I will post a link to some helpful threads written by the master of the 4.0L SOHC V6 timing chains... 2000StreetRod...
http://www.explorerforum.com/forums/showthread.php?p=2349038
