tylerspyler

Unfortunately I have an 01 with NLEV 50 state Emissions. I was wondering if the system will function better if I use a higher octane fuel? I am looking for a quieter engine, smoother running, and so on. I am going to try 89 octane tomorrow, but any input on the subject is appreciated.

This is not an off road vehicle. I am getting 20 mpg on the highway and 12 going to school and back (1.2 mile trip).

djb383

Try Googling octane and u'll learn fact from fiction. 87 is acceptable in the XJ.

outlawcherokee

your engine is designed around 87 octane, running higher octane won't benefit you and could actually do harm. Your best bet is to stick to manufacturers recommended octane.

djb383

I believe the book says 87 minimum octane so higher octane wouldn't harm anything.....wouldn't accomplish anything either. Only time a higher octane, than factory minimum, might benefit is trailer towing in high ambient temps.

SilverHaze

In my 2001 with 50 state emissions I ran 93 octane for a while and I noticed no difference in MPG's or performance. Save your money.

5-90

Higher AKI (Anti-Knock Index) fuel won't cause any trouble, but could actually drive your HC and CO emissions up.

"Octane" is a measure of the resistance of the fuel to combustion - higher-AKI fuels are used to prevent detonation/preignition in high-compression engines. Our beloved AMC sixes are certainly not high-compression engines, even when built as strokers, unless you redesign them as such.

If you're finding yourself needing fuel with a higher AKI, you probably have buildup of carbon deposits in the chambers - which can be corrected in various ways, search up around here to find them (BG44K, Seafoam, and just plain water can be used for this purpose.)

Short form - using a higher-AKI fuel won't reduce emissions, else 87AKI fuel wouldn't be available in CA in the first place.

tjwalker

Agree with the above. 87 octane is all you need for the factory stock 4.0 engine.

There is still a bit of a "myth" floating around out there that the higher the octane, the better it is for the engine.

If your engine "pings" on acceleration and using a higher octane fuel resolves the pinging, you have carbon deposits as 5-90 mentions. For vehicles that ping, the first thing I do is to fill it up with a tank of higher octane fuel to see if that resolves that symptom as an initial troubleshooting strategy.

djb383

........and why is it your quoting me?.......I'm not finding myself needing fuel with a higher AKI.......didn't even ask the question.

4.3L XJ

Another issue that is coming up around the country with the infatuation we seem to have with alcohol in fuel is that emissions go up from blending it in gas. The greenies haven't figured this out yet. Here in CA, I run the high octane stuff in everything because they are selling e85 in place of e90. When they do this, we take a 25% hit on milage. It is also a known fact that when you smog, put the good stuff in and if you are anywhere near marginal for HC emissions, you will probably pass just fine. Alcohol makes the gas molecules sticky and they don't evaporate well and don't burn as completely. The extra HCs are burned in the cat, which only makes a hot cat.

Gee oh Dee

Maybe he was just expanding on your post?

RTorrez1

After working in a refinery for over 15 years here is a little info that not many people know. When they formulate the fuel say for instance 87 octane. That is the minimum that is allowed to be formulated into the gas. But, 50% of the time it is over the minimum amount. So when you are buying 87 octane you may be getting anywhere from 87 to 89 octane. Yes the do try to keep it at the required octane. But they never want to go below or it will cost them big in fines.
Just to let you know.

Slick761

My hc number was just under the limit and the guy told me to throw a can of octane boost in. I figure it was due to 3 month old gas do I wasn't too worried. Anyhow I have to go back cause I failed due to a missing pre heat tube (a holes passed me without it last time) I've also heard of people using heet... That stuff is alcohol. Bad info?

djb383

Maybe quote/expand post #1 & #3.......

tylerspyler

OK, Firstly I do know that the jeeps require 87 octane, secondly I could care less about emissions, I have no testing or inspections. thirdly I know that higher octane means it has a higher resistance to combustion and i also know that fuel burns easier in heat. My intake at idle is over 200F according to the scan tool. I also found my thermostat was bad and made the engine run at 220-230F.

What I am getting at is that MY jeep is running hot, rough, and had a bad case of heat soak. I am running 6 gal. of 87, and 14 gal 89. I DO see an improvement in throttle response, idle, and power.

djb383

Hum, must not have Googled octane fact and fiction.