I’m driving a 2002 Subaru Forester S. It was running rough and my mechanic said that was because it needed premium gas (he also drives a 2002 Forester). OK, so I’m running premium (and I know about knock sensors) and it runs a lot smoother. But there’s the ethanol question, so I switched from 93 octane with 10% ethanol to 91 octane with no ethanol. I’m getting slightly better MPG but now the car seems to be running rougher again. Thoughts?
Check your owner’s manual. Look carefully for the words “recommended” or “required.” I believe the Forester S requires the use of a premium gasoline. I would give 91 octane a little longer and see if the engine smooths out. If not then you will have to opt out for the 93 octane since you already know it makes your engine run better.
Where you live, is 91octane mid-grade? If so, you need to switch to 93.
I sympathize with you about the ethanol - I think it has no business being in gasoline, and am annoyed that it’s almost impossible to find non-ethanol-contaminated gas where I live.
Actually, where I live (upstate NY) 89 is mid-grade; the 91 I’m using is advertised as high-test but, of course, they can advertise anything they like. Sounds like I should give this 91 a chance (a few tanks) before switching back to 93–but that ethanol bothers me probably more than it should.
Thanks!
Any gasoline with 10% ethanol will return less mpg than conventional gasoline, regardless of octane rating. Ethanol is just not as good a fuel as gasoline.
I would also make sure all maintenance is up to date and that there is no underlying issue causing the rough running. It’s hard for me to explain how a difference of 2 octane points makes a noticeable difference in how the car runs.
@jbcarter Gas stations can CALL it what they like but the number has to be at least 91 minimum if that is what’s on the pump label or the state will come down on them. Your owners manual should say specifically what octane number your car requires. Usually the knock sensor can compensate for lesser octane fuel but your car, for some reason, seems to need 93 (not 89, not 91 but 93) octane gas.
My guess is the problem is not related to gasoline octane rating at all.
There are a number of things that need to checked before blaming the octane rating and your mechanic should know this.
Carbon buildup in the cylinders or an EGR problem can make an engine more sensitive to octane.
A Subaru that requires premium fuel is loves labor lost…
I’m not a fan of Ethanol as I think it’s a total scam but a local guy around here has a Mustang that hit over 250 MPH on corn squeezings…
The old timers told me when I was in high school that they increased the octane of gasoline by adding mothballs to the gas tank. These old geezers claimed much improved performance on their Model T Fords and that they could set the spark advance(it was a lever on the steering column) way up to take advantage of this souped up gasoline. I don’t know if this is the answer for your Subaru, however.
What I don’t understand is how the miles per gallon would increase going from 93 octane with 10% ethanol back to 91 octane with no ethanol and yet the engine would not run as smoothly. Tom McCahill (the only authority I ever understood) claimed that whatever fuel and engine settings gave the best performance also gave the best mpg. Now if your engine doesn’t run as smoothly on 91 octane, it isn’t performing as well and I wouldn’t think that mpg would be better. One time after replacing the spark plugs and distributor points in my 1971 Ford Maverick, I set the timing a little too high. I didn’t realize what I had done until I started on a road trip. The engine had a pretty bad spark knock on regular, so when I stopped for gas, I filled up the highest octane premium I could get at the Sunoco blending pump. The spark knock disappeared, the acceleration was really improved, and my mileage increased by 2 or 3 miles per gallon.
jbcarter–Is your Forester turbocharged?
If not, then your mechanic is…wrong…about it needing premium-grade gas.
The only Subarus that need premium gas are those that are turbocharged and the early 6-cylinder Outback & Legacy models (2001 to 2009).
One thing that does aid in the ability to use lower octane gasoline and/or prevent pinging is the centering of the spark plugs in the combustion chamber so my gut feeilng is the problem is not octane as much as it is an ignition miss, etc; or hopefully not a valve lash problem.
@asemaster, I don’t think the guy had fuel mileage at the top of his bucket list on the salt flat runs but it was probably gallons per mile at speed… 
Another Mustang of his hit over 260 MPH on corn juice and went airborne for a 1/4 mile on the return run.
He’s also got a record holder Ford King Cab pickup that ran 182+ on bio-diesel and 169 on regular diesel. Remove the stickers and someone at a traffic light would think it’s just another stock, normal sounding truck.
Owner’s manual simply says “87 or above is recommended.”
Has there been any actual diagnosis of this car at all or is the octane statement just being tossed out there as a guess?
Your car should run fine on 87.
So it’s not a turbo?
Ethanol is a great fuel IF your car is set up for it. Running straight ethanol (or E-85) makes a great race gas. It is high octane (113-staight ethanol) and the need to use more of it with a given volume of air means the engine will run cooler. So a 250 mph Mustang should be quite happy with corn squeezin’s. Indy cars now run on Ethanol and used to run on Methanol (or wood alcohol). NASCAR racers run on 15% ethanol/ 85% gasoline (E15)
It is, however a scam, since it uses more energy to produce than you get from a gallon of the stuff.
Since the OP has told us that the car’s Owners Manual specifies 87 octane gas, it is fairly clear that his mechanic has not diagnosed the engine’s problems properly.
Using higher octane gas may mask symptoms that actually result from…ignition problems…or carbon build-up in the cylinders…or problems with the EGR…or an engine that is running too hot…or possibly other problems. However, masking a problem and resolving it are two different concepts entirely.
I suggest that the OP take his car to an indy mechanic who specializes in Subarus, and hopefully that person can give better advice than to simply use higher-octane gas that would not be needed if the engine was running properly.