might need some suggestions.
Lately I have noticed that sometimes the 4.0L pings/knocks as it shifts into OverDrive under very light throttle. Almost like the engine is running a low rating of octane. Been putting 87oct of Exxon on the last few tanks.

Last tank was shell..same result.

Could it be due a worn water pump bearing? Or failing or loose timing belt tensioner? Any help will be appreciated.

2000 XJ with 122K

 

Been tuned up lately? The 4.0L is somewhat of a noisy motor (most in-lines are) but at only 8.8:1 compression, ignition ping is not likely under normal operating/driving conditions, I wouldn't think. No timing belt/tensioner, it's a chain.

 

A way to find out is to top off the tank with Shell V Power. Even if it's not a full tank it will raise the average octane of your gas. If your ping goes away (I mean, like almost immediately, in my experience) then you know it's not a chain. I have pinging in my rig now when I use regular octane gas (so now I use 93). I think the cylinders are probably coated in carbon, making the compression too high and exploding the regular gas too soon. Sea foam through the gas tank is a way to clean the carbon out a bit. (Never used the vac line method with Seafoam but some folks on here have had good results). Cheers, and good luck!

 

Pinging (pre-detonation) can definitely be carbon related.

As mentioned, a good experiment is to fill up with premium. If the pinging goes away, the problem is carbon.

You can live with it or you can try a top end treatment (Mopar/Seafoam, etc) as that can be helpful in removing top end carbon.

 

actually I just did a seafoam ..hardly any smoke came out..I was expecting tons of it..since the previous owner was an old lady..lol

It's just weird to me that it only happens as soon as I go into over drive at very light throttle. I'm just puzzled by it.

I guess it won't hurt to put some higher oct gas in there..and see what happens.

I also just did an oil change to 10w30 Syntec, new air filer, spark plugs.

 

That is always what I try first to troubleshoot what I think may be carbon based pinging.