I have recently seen a few threads on here about a knocking sound at idle and my 1991 xj was doing the same..and when the engine was cold i heard no knocking at all just when the engine began to warm i heard the knocking sound at idle speed only..I tried marvel in the oil for about two weeks and no change..so i decided to put some in the fuel tank and poured a whole quart in about 10 gallons of fuel..After about a week and 280 miles later the darn knocking sound is gone away all together..Just thought i would share this..I do not know what it affected to cure the knock but i swear it's the quietest i have heard that engine run since i bought it...Heck it's only 2.50 a bottle try it!!

 

Can't imagine anything poured into the gas tank making an engine quieter. Coincidental, IMHO.

 

Not sure myself but when i put it in the oil after two weeks no change..then added it to the fuel and after a week it's gone..Maybe carbon buildup?I dunno but it's gone..Thats for sure..

 

buy a lottery ticket! sounds like you got lucky!

 

i put a quart in my oil and gas.. no such luck