My 4.6L Expedition knocks really bad when I use 87 Octane the manual calls for. It sounds like it's going to blow through the hood at times. If I use 93 it is much better, but still slightly there.

Anyone else have this problem so severe?

 

Mine knocks really softly when its first started up. I have read many postings regarding engine knock. Many have advised that it was "piston slap". What year is your truck? Still under warranty? I wouldn't wait bring it in running 87octane.

 

It's a '98 w/ 94k miles, but I first started noticing the problem when it had about 50k on it. I always ran the good stuff in it until the gas prices sky rocketed so that's why I noticed so late. I rean 87 for about 4 tanks trying different brands, then I went back to 93. I put 87 in it this week just to see if it would still do it, and it still does.

 

Tried resetting your computer? Disconnect the negative battery terminal and turn on the headlights to drain residual current. Leave it for 10 mins, then reconnect?

I'm not sure if it was an "old wives tale" but I've heard if you run premium from day 1 the motor gets used to it and will definitely ping when you step down in octane.

 

That is true. There are several reasons, one of which is unburned deposits. Using a gasoline with a higher octane than you need is detrimental to the engine. The line of thinking that suggests occasionally giving your vehicle high octane fuel as a treat is backwards. It's really more like a spanking.

Take care,
-Chris

 

I had the same problem with detonation. Brought it to the dealer a few times but they could never find the real problem. They tried pulling out the fuel shortening bar to change the timing but I caught it before I left...They said they forgot to put it back. Anyway I had my family loaded and pulling a pop-up camper, Hit the highway... first hill ... my check engine light comes on and I could tell I lost a cylinder.....To make a long story short My #8 cylinder exhaust valve looked like an inverted mushroom. They said it was a defective valve. They replaced the head with a new one. I've also been running 98 oct. (Low price) doesn't knock anymore (it used to knock a little with 98oct. before the repair)
Ford gave me an "AfterWarranty Adjustment" so I'm covered till 100,000 mi. or 2004. Ford will not tear into an engine unless it gives a code or something breaks. Good Luck! I still like my truck!