Decided to Try a Tank of Premium...YIKES!

[The Critic]

I just did a full 1 bottle treatment of Amsoil PI in the Saturn. I used the full bottle despite the car's 13 gallon tank.

Towards the end of the treatment, the car did feel much more responsive and the idle was smoother.

For my own amusement, I decided to refill with a tank of Costco 91 octane to see what would happen. I have heard in the past that as cars get older, carbon builds up on the piston tops and thereby reduces the volume of the combustion chamber, which is a situation that could potentially benefit from higher octane.

The car runs just as smooth as it did on 87 + PI, so obviously the Amsoil PI treatment helped. However, there is one big problem.

Fuel economy.

Although this is my first full tank of fuel in hilly Santa Cruz, my gas mileage around town is absolutely awful. Yes, I do have a heavier foot, but nothing ridiculous. The weather has been between 50-70 degrees here, and each trip is at least five miles long.

As of a few days ago, I had about 100 miles so far and based on my previous experience with the gauge, I had already used around 5.5 gallons, if not more. I do not EVER recall this car getting less than 24 mpg during short-trip driving, and those "short trips" were often two miles or less.

So, could anyone explain why 91 octane caused my fuel economy to take a huge dump?

Thanks!

 

[beast3300]

The obvious reason that comes to mind is the brand fuel. Many of those places purchase fuel from different outlets. It is possible the 91 in their tanks is a different brand than the 87.

And I'm sure the 87 moves alot faster than the 91 so the turn over is faster. Another possibility may be the PCM isn't used to the octane change. To be certain of fuel economy change, maybe try the same octane/same gas station a few times for an average.

 

[Hemi426]

My owner's manual for my 2000 Dodge Dakota 4.7 warns of similar circumstances. It clearly states in the book that 87 is the only fuel recommended and that higher octane will result in poorer performance, lower mileage, etc. Re-fill with regular 87 asap to cut down that octane.

 

[ARCOgraphite]

Bad, stale gas. Not all 91 is created equal. Dont know why - everyone goes to Costco to fill up their Porsche Cayenne Turbo and Maserati

I ALWAYS get better fuel mileage with a bit 'O premium and the car "lugs" better too. It may take a few cold start cycles for the ECU to allow full advance - IF it was pulling advance in the first place. Also high octane is NOT rec for high altitude driving due to the lower cyl pressure.

 

[The Critic]

Originally Posted By: beast3300
The obvious reason that comes to mind is the brand fuel. Many of those places purchase fuel from different outlets. It is possible the 91 in their tanks is a different brand than the 87.

And I'm sure the 87 moves alot faster than the 91 so the turn over is faster. Another possibility may be the PCM isn't used to the octane change. To be certain of fuel economy change, maybe try the same octane/same gas station a few times for an average.

Without a doubt, the turnover is probably a lot quicker for 87 than 91.

However, we're talking about a 25% difference in fuel economy here, if not me. There's no reason why there should be such a huge difference between brands, or even between octanes. 10% maybe. But 25%+ doesn't seem right.

I'm starting to wonder if I got a bad tank of gas or something.

 

[javacontour]

Something else is different. More wind, different humidity or something just changed in the car, but hasn't set the check engine light yet.

It could be stale fuel. I doubt you have winter and summer blends, but if you do, is it possible the 91 is a different blend from your 87?

 

[GSDad]

The higher the octane, the slower the fuel burns. Requires different compression ratios, spark advance curves, fuel flow maps, etc. OEM's design engines and computers for a certain octane. Stick with what they tell you.
GSD

 

[Scott_Tucker]

100 miles isn't enough to accurately make any statements on fuel economy. It's also been extremely rainy and a bit chilly for the last week in Santa Cruz whereas the week before was nice and mild. I'd suggest running it for several tanks and re-do the math.

 

[PandaBear]

Costco have so high of a volume that I doubt the fuel is more than a day old, even for 91.

The formula in California is standardized, and the only thing different between brands are the detergent additives. Not something that can turn instantly into a problem.

Either they throw in the wrong fuel (diesel mix), water get into the storage tank, or it is not the fuel at all, IMO.

 

[jetblacknismo]

Depending on how it is stored, gasoline can last anywhere from a few months to several years. The gas at that station would have to have sat in that underground tank for at least that long(months). Its definately possible, but usually exaggerated.

Open up your gas cap and, while being careful not to inhale a bunch, just wave your hand across the fill hole and see what it smells like. Dont huff it!

but just smell it a bit.

Does it smeel like old varnish or even nail polish? If so, then its prolly stale gas. If it doesnt, then its something else possibly. You can even check the gas itself it you can disconnect a fuel line and see if the gas is sorta sticky.

 

[MONKEYMAN]

I have run Premium in my 99 SL2 every summer for years. No mileage drop. Performance was fine. I did it since a member at Saturnfans said knock sensor would adjust timing and fuel economy would go down if I did not run Premium. Not sure that is true, but my car always ran great on Premium. Only two times did I notice fuel economy below 24 mpg. It was when I had the A/C on, used a heavy foot, and did a lot of stop and go driving. Also, my experience is it is hard to draw conclusions since some variables may not be taken into account. In your case you have at least two variable. You used a fuel injector cleaner, then a different grade of gas. You could have some issue with your car, say a bad ground on your coil with misfire, a thermostat that is not working properly, etc. The point is, unless you eliminate all variables, it may be hard to draw the proper conclusions.

 

[L_Sludger]

My Lexus is very picky about its fuel. The loss in power and smoothness is obvious with low-octane gas. But there are about three dozen reminders on the dashboard, gas cap, gauge cluster, fuel filler door, and trunk lid harshly indicating "PREMIUM UNLEADED FUEL ONLY OR ELSE WE WILL SEND NINJA SAMURAI TO YOUR HOUSE TO RELIEVE YOU OF YOUR MORTAL SHACKLES"

 

[sciphi]

On my car there was a noticeable difference running premium during the winter. The car was smoother, got better mileage, and knocked much less.

I'd spring for a second tank to get a better baseline.

 

[beast3300]

Originally Posted By: L_Sludger
My Lexus is very picky about its fuel. The loss in power and smoothness is obvious with low-octane gas. But there are about three dozen reminders on the dashboard, gas cap, gauge cluster, fuel filler door, and trunk lid harshly indicating "PREMIUM UNLEADED FUEL ONLY OR ELSE WE WILL SEND NINJA SAMURAI TO YOUR HOUSE TO RELIEVE YOU OF YOUR MORTAL SHACKLES"

LMAO, If there was room in my autosig I would use that as my quote.

 

[Nick R]

PREMIUM UNLEADED ONLY OR I WILL SET YOU ON FIRE

 

[mechtech2]

Premium fuel does NOT burn slower.
It is more resistant to detonation and pre ignition.

I noticed that this car was now seeing VERY different conditions in hilly Santa Cruz.
Add this fact to all sorts of other variables.

Losing 25% or so from going from reg to prem is too much by itself. There should be no loss.

 

[Shannow]

Originally Posted By: mechtech2
Premium fuel does NOT burn slower.
It is more resistant to detonation and pre ignition.


thankyou, and a great big +1

 

[Falcon_LS]

Originally Posted By: L_Sludger
My Lexus is very picky about its fuel. The loss in power and smoothness is obvious with low-octane gas. But there are about three dozen reminders on the dashboard, gas cap, gauge cluster, fuel filler door, and trunk lid harshly indicating "PREMIUM UNLEADED FUEL ONLY OR ELSE WE WILL SEND NINJA SAMURAI TO YOUR HOUSE TO RELIEVE YOU OF YOUR MORTAL SHACKLES"


I run 98 RON/93 AKI in mine with TC-W3. Just doesn't drive the same with 95 RON/91 AKI, and fuel economy gets just a bit worse.

 

[mrsilv04]

91 is Premium?

At the station at the end of my street, Premium is 93 octane.

It seems to me that Costco is diluting their premium with one part of 87 (to 2 parts of 93) to come up with 91 to save money.

If you're going to pay extra for premium, at least get what you're paying for.

 

[Scott_Tucker]

91 AKI is premium in our area. Actually, all of California I think.