HELP! knocking qustion

I hear some knocking on the highway sometimes. Not too loud, but then again, if I can hear it above the wind noise and tire noise, it might be louder than I think. Louder under higher load. What are the causes of knocking? I put in better gas (88 oct) and the knocking reduced somewhat. But the conpression of th 4.0 shouldnt require high octane.

You may need higher octane. The engine control used on the 91 and later XJs do not have a knock sensor. As the engine gets miles and carbon builds up the compression ratio will rise.

I have seen posts over the years from guys saying they run 89 or 91 in their XJ to prevent knock.

I think their are cleaners you can pour in the throttle body to clean out the carbon but someone on here should know what product that is to use.

Does that 96 have a knock sensor? I don't know. I r&r'd mine and engine runs better.

Nope

No knock sensors after Renix. Not exactly sure why. Almost everything else does. The main problem i have seen is carbon build-up. After a hard run out west here cruizing up these mtns at 85mph with pedal to the floor i tend to see less detonation when i get back here to Ogden. After a while of putting around town, back to the same tendency. Chevron mid-grade seems to help the most as compared to other's top grade. Techron is really about the only fuel additive that does help guard against intake valve misc accumulation and injector clogging too.

Well, I started running 91 and the knocking stopped, gas milage was up, and the motor felt and sounded a lot better. So, I guess thats what I have to use now. Sucks having to pay close to $2 a gallon when you only get 15 mpg. I need to get a little honda beater for around town.

<rant>

Hondas are nice...
until some jim-bob hillbilly with a lifted XJ ends up blinding you with his headlights...

Oh wait that's me any time a punk cuts me off with his "thinks a wing and big bore mufller that makes it sound like a weed-eater makes a race car" POS Honda. Then, when the turns green they learn that torque is far better in a drag race than 120 whining horse power.

really... Get a diesel 5sp xj and enjoy upper to mid 20's for mileage and good power (not quite as good as 4 litre)
</rant>

lol... easy there buddy. I drove a Type R (Acura Integra) before my xj. Not exactly a 'big bore muffler and wing the size of texas rice burner'. I hate those guys. I just miss having a car that can easily smoke V8's and still get 25+ mpg in the city, and 35+ on the highway. But I'm happy with my cherokee. Except the milage. But thats the way things go. When you make a vehicle perform ouside its stock form you're going to run into 'side-effects'. My integra was unbearably loud. My XJ milage sucks. It did make me feel better when I saw a sticker on the new G500 Mercedes saying that they get 12 city/15 hw.