I have a noise that in noticeable under heavy acceleration (sometimes) that sounds like marble rattling around in a tin can. Anyone experience this?

 

What did you think an xj was ran off of?

 

I've had that issue. It happened going up gradual hills too. I eliminated the knock by running higher octane gas. Basically (and someone please correct me if I'm wrong) the compression in your cylinder gets too high due to carbon buildup, so the regular octane gas explodes a half a second too soon. This isn't good for the engine because the force is pushing down on your piston when it should be going up. The higher octane gas is more stable so it explodes at the right time (when sparked) and, depending on the brand, it also might have cleaners in it that slowly dissolve away carbon gunk. I use Shell V-power for this reason. Now, some folks will say it's against their religion to pay for premium gas in an engine that was made for 87 octane. Sure, but I guess the alternative is to take the head off and clean it out. Or Seafoam it, but I haven't tried that except to run it through the gas tank. I only got reduced knock/ping that way. The higher octane gas eliminated the noise entirely and my Jeep is much happier.

 

yes, do the seafoam thing, 1/3 in the gas, 1/3 in the oil and 1/3 into the vacuum system.

 

I agree and I find it hard to believe that my 2001 engine was built to perform its best on 87 only. I had this same clatter a week ago and I dont care what anyone says. It was bad gas. I added some octane booster and then switched back to Shell 89. The clatter is all gone.

 

Thanks for the suggestions, but if it was pre-detonation, wouldn't the knock sensor pick that up and then the ECU pull the timing back to compensate?

Thinking of that, it might be a good idea for me to pick one up anyways. The PO put a used engine in before i bought it, and i know there a easily damaged item.

Well, im off to seafoam it, and switch grades of fuel. Ill let you know how it goes.

 

its called spark knock and 9 times out of 10 the censer doesn't pick it up not sure why it is caused by either to low octane or fuel with water in it

 

The censor didnt pick mine up. Then again, I didnt drive around with it long enough to find out either.

 

Sometimes, the gas is crappy. The engine runs perfect on 87 octane but not all gas you buy is "good gas".

Stick to name brands and buy from busy stations that have good turnover. Don't add anything to the fuel except Stabil and only do that if absolutely necessary (like deployment overseas for a while). Even then...drain and discard Stabilized gas that's older than a year.