Hello,
97 F-150, 4.6 V8, 153,000 miles. I just noticed recently that when I am cruising on the highway at a steady pace of 60 mph or so, and I accelerate so the RPMs go up to about 2,000, I have a very slight knocking. Most people probably wouldn't hear it, but I know my truck so well I can hear it. Anyway, I thought I'd go up one grade of gas (currently using 87 octane) to see if that helped some. But the dealer said he did not recommend using anything higher than 87. My questions are:
1) Does anyone think this could be something more serious than just a side effect of lesser quality gas, and
2) What is the downside to using a higher octane - that is, why would the dealer not recommend anything higher than 87. I haven't tried it yet, but if I do, and the knocking goes away, what could I be hurting by using "better" gas?

Thanks for all your help, this is a great site.

 

89 grade may suppress it but I would do a service cleaning on the EGR system.
Right where you are running is the same place the EGR is in operation; at cruise.

 

If it really is slight or what some people call trace knock then it probably is nothing to worry about if its only happening during high load transient conditions. If its heavier and more sustained then I'd be concerned. That's not unusual to occur on an engine with that many miles. After time you can get carbon deposit build-up which reduces heat transfer out of the cylinder - which leads to higher cylinder temps and hence knock.

I have no idea why he say not to run a higher octane fuel. There is a misconception by some that higher octane fuel has more energy in it which isn't true - it resists detontation/knock, which allows you to run with a higher spark advance and not incur knock - so you can get more power out of it. But that's due to advancing the timing, not the energy in the fuel. Maybe he thinks you'll run hotter if you run premium.

 

When we bought my wife's 2000 Expedition in 1999, the dealer told us that it was designed to run on unleaded 87 and if we ran higher octane we would possibly burn up the sensors. I have never heard this anywhere else, so do not know if there is anything to it.

 

Try cleaning the MAF with Brakleen and install a new AIT (the little black sensor in the intake). With a K&N filter I have found I need to clean the MAF and replace the AIT about every 20,000 miles.

 

Thanks everyone for your time and thoughts - I'll try all of this. Thanks again, I really appreciate it - like I said, this is a great site!

 

Smith, where is the MAF sensor? And, what is the AIT and where is it exactly? I have an '01 5.4 w/ 124K and all I have done, besides oils, filters, is change plugs a few times. I just bought some new motorcraft plugs and some Borg Warner boots to replace tomorrow. I just noticed a slight miss this week which has been the indicator I have gone by before when plugs needed changing. I would like to do something more to improve running. I haven't changed O2 sensors yet. Another question for ya, is silicon grease the same as dielectric grease that could be used to coat boots? Thanks,

 

In simple terms, higher octane does not give your truck more power. It retards the engine's timing slightly, which make it seem there is more. If you have had a supercharger installed, all new pulleys, are using larger cylinder heads than what is suitable for your block size, and well, if you have spend grands on your system, then higher octane would be suitable. A simple FIPK or new headers or muffler system would never need higher octane, that stuff is for minor performance and efficiency alteration and/or noise.

 

John, the MAF is a round cast aluminum piece that is the first thing air goes through after the air cleaner. It has a wiring harness connected to it. I completely remove it and spray the little wires with Brakleen from the front and then turn it around and spray through the hole in the back.

The AIT is a little black plastic sensor with a small wiring harness connected to it about 6-8" behind the MAF. Very hard to remove intact from a stock setup. These are about $15 at Autozone. They can show you a picture of your intake system with the AIT included. The side that senses the air (Air Intake Temperature) looks like a very small Christmas tree bulb. In my Airaid I cut the grommet with a razor blade and then buy another 7/16X9/16 grommet from Ace.

 

Thanks for info Smith! I ordered the motorcraft brand AIT today for $20 and will try to install it this weekend. I will also use your suggestion and remove the MAF component so I can try and clean it well. I just replaced plugs again this past weekend, but I feel like engine could run just a tad smoother. Sure sounds like exhaust has a lot of backpressure but I sure don't want a loud exhaust note, nor do I want the hassles of re-programming/tuning for low restriction exhaust. My '01 5.4 is totally stock which has been fine with me, but I do like to have it running well.