My owners manual for my '03 GT recommends gasoline octane no higher than 87 points, which of course I ignored, resulting in fouling my spark plugs. After a stern warning from my dealer, he replaced them under warranty. My question is this: 1) Has anyone else experienced this? and 2) What is the remedy so that I can use higher octane gasoline (assuming that higher octane = better performance)?

 

What a GREAT question! Wonderful lesson for so many to learn.

Plugs foul when the temperature in the combustion chamber does not get hot enough. This can be caused by using too high of an octane OR by using the wrong plugs for the application.

Spark plugs serve two purposes:

1 - They make spark and thus ignition
2 - They cool the combustion chamber.

Colder plugs, take more heat away then hotter plugs. Using colder plugs prevent "pre-ignition" but lead to fouling. As a rule, you should always use the coldest possible plugs w/o having fouling problems.

Cars with turbos, s/c, and nitrous require colder plugs due to the more fuel and higher compression thus more heat which tends to cause pre-ignition. Pre-ignition is when you get combustion BEFORE spark. Factors that lead to this are load on motor, temp of motor, ambient temp, using wrong octane fuel (too low).

ALWAYS USE THE FUEL WITH THE RECOMENDED OCTANE RATING FOR YOUR CAR!!!!!

Lower octane fuel burns faster and resists knock (pre-ignition) not as well as higher octane. The higher the octane fuel that you use, the slower it burns and the more it resists knock. To take advantage of higher octane fuel, you have to either increase compresion (which makes the fuel burn faster) and/or increase timing advance. Increasing TA makes the spark happen sooner which gives the slower burning fuel more time to burn.

In your case, by using 93 octane, you are not burning all the fuel and are ending up with fouled plugs and thus loosing performance. Your car will have more hp and fuel economy using 87 octane!

By having a chip or a programmer, you can have your timing advanced so that you can use higher octane fuel. I have an 87 octane program and a 93 octane program for my car. The difference is very noticable in power when driving with the 93 octane program and 93 octane fuel.

NEVER! use fuel that is too low of a grade! You merely fouled your plugs by using too high a grade, but using too low a grade can destroy an engine quickly.

Ask you service manager if he recomends using Sea Foam or similar product to clean your heads. You need to find out if you simply had fouled plugs or if the combustion changers are all fouled.

Using a chip/programmer will void your warrenty BTW. Though, if you use a programmer, you can have the stock tune re-installed before service work and they will never know it was ever there. If warrenty is a big concern for you, you may want to consider staying away from one.

 

i called a ford dealership regarding this as i have always filled up 89 thinking this was a good way to keep a car from knocking. she told me she has never heard of a problem with the spark plugs like this im sure at 93 it is an issue as was explained thouroughly by Mt. She doesnt recommend anything higher as a normal fillup than 89. once a month at 91 wont hurt though.

 

Keep in mind 87 is the recomended octane rating for a GT. As you move up in octane, knocking is not an issue at all. Knock happens when you go down in octane rating from what you should be using.

If you use higher than the recomended rating, you will not burn the fuel properly, loose performance, and foul your plugs.

 

will there be a performance decrease at all using 87 as opposed to 89? id be very happy to pay 12 cents less a gallon haha

 

No and as a matter of fact, ive heard that your car can sometimes be slower by using the 93 instead of the 87. Some people have issues like bucking and a lack of performance and then they switch back to the 87 octane and the problem goes away.

 

wow crazy thanks for the info gonna downgrade asap on next fillup i guess if knocking starts go back up to 89.
thanks for all the help

 

Before my Diablo chip, I could still tell a noticable difference between running 87 na drunning 93. The 93 had more punch, but I'd have some dirty plugs.

 

The two golden rules of fuel and plugs:

1 - Always run the lowest octane of fuel that you can get away without having knock.

2 - Always run the coldest plugs possible w/o getting fouling.

Problem is, in todays age of high-tech autos, there are things called knock sensors. When they detect knock, they make adjustments to the ignition. They can only react so much, and then the car will knock noticably.

My theory is that when high octane fuel is used over a period of time, the cumbustion chambers build up with deposits. This actually reduces the volume inside the compression chamber a small amount, but small enough to increase the pressure and thus the temperature. This results in either knock or the knock sensors kicking in reducing the amount of available power. So, when high-octane fuel is used, there is the appearance that the car has more power. So then, there is more power when high octane is used in this situation, but for the wrong reasons.

 

roger that

 

great info thanks alot