Hi All, When I had my 97 5.4l I had use 93 grade gas or the engine woud rattle. Now I have the 2000 5.4 and the manual says 87grade .Should I use 93 anyway? It does not rattle with 87 like the 97 model did.Any suggestions. and I also noticed the Navigator reqiures 93 as well.

 

use 87 if it works, why pay more? The Nav uses 93 b/c its a DOHC and IIRC it has a timing advance also that requires 93. if you have an Expy, use 87.

 

So it's not gonna change gas mileage anyway right? and why did the 97 rattle with 87 and not 93?

 

Gas mileage? Without a tune to advance your timing its a waste (in most cases, if you get pinging then you need the higher octane) to run 93. In fact you should get a better, more complete burn with 87 than you will with 93 and no tune. In other words, you are probably getting the best gas mileage you can running 87 than you will with 93, as more than likely with 93 you will be dumping excess fuel into your exhaust.<O</O

Not sure about the 97. There can be a number of things that could cause it to pre-ignite with 87.

 

My personal experience with a 2001 XLT 5.4 is that 89 and 93 octane gas DOES get me increased mileage. It is about a total offset on cost vs savings though.

I mainly use the good stuff when towing, I get a little more range out of a tank that way. from 8 to 11 MPG is the difference I see at 87 vs 92-93 octane. That is almost 90 miles on a tank. Again, do the math, you pay more to get more, so it is totally offset, I just like the better range.

 

Why pay more if you don't need to and your getting the same performance with the lower octane gas?

 

Some vehicles do better with different octanes than those recommended by the mfg and used on other vehicles of the same type.

Not to doubt your numbers, but that seems like a huge difference between the two. Most times a vehicle not designed to run the higher octane fuels do not burn them completely resulting in some fuel loss. Now on some of the more modern vehicle computers (Not sure when or if Ford does this) but the computer can sense the fule being used and advance or retard timing to allow the engine to perform its best. Even with some loss, its usually never noticed in the overall MPG.

 

Stick to the low octane gas unless you want to gunk up your valves with carbon deposits or have excessive spending money you like burning...

 

87, 93 is a waste for the expy

 

There is a good explanation to this. What's happening in yor case is the knock sensor retarding timing due to spark knock when using 87 octane fuel. This can be caused by many things,and it's not always audible to the human ear,so you may be experiencing knock and don't even know it. The MAF could be dirty,you could have a vacuum leak alowing unmetered air to lean the mixture,there could be a large amount of carbon buildup causing hot spots in the combustion chamber,or it could simply be low-quality fuel.
That's why you're gettign more economy out of the higher octane fuel-the knock sensors aren't seeing any spark knock,so the PCM is actually advancing ignition timing a slight amount,and the enging is running more efficiently. It doesn't have anything to do with a particular fuel octane being beter for fuel mileage-it's just a side effect of the knock sensor's input and the PCM's resulting actions.
JL

 

Fuel octane has absolutely nothing to do with "gunking up the valves" with carbon deposits.
JL

 

Well maybe not on these trucks, but try it in a low compression air cooled motorcycle engine and you WILL get carbon build up on the valves.... (just trying to keep the guy from wasting money on gas)

 

It depends. Straight pump gas... true. But some aftermarket octane boosters will cause problems with build-up. For example, its one of the reasons spark plug fouling isn't covered by Roush's warranty if you use octane booster.