Engine knock?

With the recent upgrade to 31" tires; I now have knocking engine during moderate acceleration. Anyone know why an engine knocks under load and what can be done to stop it. Other than turning up the radio. Thanks!

It all depends on the year of your XJ, wish you had posted that or included it in your profile.

If you have a 87-90 XJ with the 4.0 then you have a knock sensor. If you are getting knocking then the sensor is bad or the wiring has come off it.

If you have a 91 or later XJ with the 4.0 you are out of luck other than buying a higher grade gasoline. Chrysler decided a knock sensor was not required.

Add a bottle of octane booster....if it cures the problem, you may want to consider using it regularly...or change brands of gas to see if that helps.

It's too much ignition lead, poor fuel, hot spots in the combustion chamber, poor fuel distribution or maybe just a bad need for a good tuneup.

Either way...knock (detonation) will damage the engine in the long run.

Ooops! It is a 99' w/68,000 miles.