I have a 99 Mountaineer with 26,000 miles. I have developed a spark knock that happens at highway speeds (65-70) only, while accelerating up a grade (hill) on the highway. It only happens in this situation and the spark knock is so bad you have to let off the gas. Around town, it never knocks. It knocks on both 87 & 89 octane. I refuse to have to run 92 octane to correct this problem when 87 octane is approved and it used to run fine on 87 octane. I've tried every brand of gas and fuel system additives and the highway knock always comes back. I even put in a K&N air filter to help the truck breath better, but no help. I've had it to the dealer 3 times and they claim that it's normal. The manual does say that occasional spark knock is normal. But mine is not occasional, it's every time, I run on the highway.
Does anyone have any suggestions or like experiences ? My dealer says he can't help.