So long story short my Dakota is falling apart piece by piece or it feels that way. Right now i am trying to track down a vacuum leak that sounds close to the EGR valve. and i believe an exhaust leak and on top of that i have a misfiring plug. As you can imagine it's a rough idle right now. So today i decided for the heck of it to put 89 octane in. I know the manual says 87. I drove 40 minutes and averaged 19.2mpg according to the truck. Usually i start at 17mpg and 20 minutes later it's down to 13-14 and never comes back up. On the way home after letting it idle for 5 minutes it started at 17mpg and was back up to 19.0. My idle also seems smoother. Is this all in my head?

 

I can get 19 on the freeway at 70mph with 87 octane. I've never tried the higher octane stuff in this truck. Might try a tank while prices are cheap and see how it does.

 

Running higher octane gas in an engine not designed to run on it is a waste of money. Probably a placebo effect.

 

Yeah i know it's considered a waste. My motorcycle manual had a whole section about not using higher octane and that's where i learned not to use higher octane. I never put anything other than 87 in my truck. That's why i only went up to 89 and not 93. But it is hard to ignore why for some reason the engine at idle is noticeably quieter. When i made the 80 min round trip it was at 60mph with cruise set. The whole trip was a constant up and down hill. My normal driving is stop and go through town and a 30 minute round trip to and from work on relatively flat highway at 70. I figured 60mph up and down hill constantly would be close to my fuel consumption of my normal driving. Maybe not. I guess i will find out when I go back to work.

 

So: octane is a rating for the resistance to pre-detonation. Can octane have an effect on horsepower, torque, efficiency? Yes. However, its situational. If the ecu interprets O2 sensor voltage, air flow (from the map sensor) and it determines that based on the knock values, the existing fuel does not have a high enough octane rating (based on ambient air temp/throttle position/engine rpm/coolant temp, etc) it will retard the ignition timing and increase fuel delivery. This would mean fuel mileage would decrease. Now, if you do have a vacuum leak, and unmetered air is entering the combustion chamber, a higher octane fuel would help the air/fuel ratios. Meaning you may get better fuel economy. Again, this is situational based on variables: air temp/density, possible vacuum leak, etc.

 

Used to have a motorcycle that required 93 octane. Just for the hell of it I ran a tank of 105 (I think, been a while) from the marina in it. My top speed went from 125 to 135, plus the engine was smoother all around.

 

There is one consideration. Cars are tuned to run on 87 octane due to the federal regs.

OTOH, a few cars out there are actually tuned for 89 or higher, and let the knock sensors degrade performance to where they're safe at 87 octane.

My old Malibu (with knock sensors) really liked 89+ octane due to that.

And Murphy Gas (the stuff WalMart sells) would kick my gas mileage down about 3-5 MPG(!!) due to the pulled timing and enriched fuel mix, caused by Murphy's knocking on regular on the Malibu.

It's worth trying the next step up to see if the motor has been pulled due to knocking, and really needs a higher octane to run properly.

And if it makes no difference, well, drop back to the cheaper gas - if it's not knocking, then you don't need the higher octane (which will technically slightly reduce gas mileage and performance - it's harder to light up, so there's less energy released, by a small fraction.)

RwP

 

Thanks everyone for the replies. I do trouble shooting and repairs for living, but not for engines. When it comes to the in depths on how the engine works I can't tell ya a lot of things, but do know the basics and can do the labor as long as its not deep into the engine or transmission.

Donkeypunch I think your last statement may hold true for this situation. I know i have a vacuum leak, but i was wrong about it being at the EGR valve. I took a medical stethoscope and went around the vacuum lines and intake areas. I heard the hissing sound on the passenger side of the throttle body at the mounting gasket. I may have more than one because it's odd i heard it plainly by ear on the driver side close to the EGR. I really couldn't get the scope down there to tell. amazing how those things almost cancel out the engine noise and that loud cooling fan. That thing moves some air. I don't remember my old Dakota's fan blowing that hard.

 

higher mileage vehicles run better on higher octane gas, also now that the winter blend gas is here, running a higher octane will perform better.