I bought my car last year with 52k on the odometer. The 8 and I have had a few little bumps along the road but we've survived them. There is one small remaining problem. The rough Idle situation that plagues so many of this... but mine has a twist.

When I bought the car exactly one year ago today, the car had a nasty stumble and miss sometimes. Immediately i began maintenance on a lot of things. The plugs, coils, wires, oil, filters, fluids, cleaned MAF, etc. The miss promptly went away, but the rough idle hung in there, but wasn't as prevalent.

The car was then tuned via AP by the king of all ECU wizardry, the harry potter of tune-age; MazdaManiac. Of course, the product delivered everything that was promised was stated and I totally would recommend it... even if you are bone stock.

The stumble remained.

So I began fuel trials.

I tried pre-mixing with several oils.

I remained using high-test (usually speedway's garbage ass 92) but alternated companies... shell (93), Citgo, BP, etc.

I ran fuel system cleaners, seafoam, and BG 44K, and even attempted 2 de-carbs... once with zoom zoom and once with BG 44K and didn't smoke anything like the ebay videos. Still, the stumble remained. The last comp test by the local garbage mazda dealership (conducted in December) stated "all numbers are withing factory parameters"

I had essentially given up, and just assumed the BHR ignition would solve the problem when I saved up the money to purchase it. I would buy an enema bag if they sold it.

So, today when returning home from Cincinnati, I was dangerously close to running out of gas when I finally found a gas station on this souless stretch of highway.

No high test in sight.

So, I figure throw ten dollars in there of 87 and a whole bottle of octane booster in there and hope for 90-ish. Well, this mom and pop store didn't have any booster. So, I figured I'd put the 87 in and do my best and be easy on the car till I could get some better gas.

The stumble vanished. Like it never was there. I drove more and let it sit... no stumble. went about the rest of the day as normal and the stumble never came back. I was shocked. Eventually the fuel began to run very low and the stumble came back, but it's lighter than ever before.

The stumble is very inconsistent which leads me to believe it's not a motor mount.

Before it would dip from around 850-900 to around 550-630 averaging once every 6-9 seconds. Now, it with dip from 850-900 to around 700-750 once every 15-20 seconds and has always varied in intensity.

Why would lower octane potentially lead to smoother motor function?

I've searched. What other options am I not considering?

Is the "stumble" you speak of just while sitting at a light? Or does it occur even when your on the gas?

My car (an '05 w/ 51k) does the same thing while I'm at a light- but it doesn't bother me too much.
{-I do need to do my plugs.}

I run Sunoco 91 most of the time.
And I do mid-grade (89) when the price is too high on super.
I won't do 87 because i had the "marbles" problem when i (mistakenly) filled the tank w/ that once.

But the car seems just fine w/ the 89-91 octane.

some ppl can do with 87...mine doesn't like anything but 93...just doesnt' seem to run right!....

I'd say if it runs good and you're not getting MIAC then stick with the lower grade stuff....

lower octane explodes faster and is more volatile. the higher the octane, the slower and smoother the burn. im not sure why this would make a difference. maybe the fuel explodes quicker than the motor can stumble. who knows. some cars do just like a particular octane.

They say your engine is within specs..but my question is how far do specs range? High octane requires more spark as well. If your engine is low on compression...high octane wouldn't be needed. Another thought do you premix? How much do you premix? could your OMP be pumping to much oil at a idle causing the rough idle? If it's taking in to much oil...the lower octane would clear that up cause it would burn easier. So anyways I use to drive my car on 87 all the time but I'm in indiana and the car never had any problems...matter of fact I couldn't tell the difference between 87 and 93. Run the lower...and if your afraid just don't be stepping into the throttle and don't be hitting high RPM's simple as that..

octane is resistance to detonation btw!.....the higher the octane the harder it is to ignite off pure heat from chamber temps

oh BTW where is swoope to try to diagnose this problem...maybe a few too many beers last night!???

that is weird :S

Only at a dead stop.

They say your engine is within specs..but my question is how far do specs range? High octane requires more spark as well. If your engine is low on compression...high octane wouldn't be needed. Another thought do you premix? How much do you premix? could your OMP be pumping to much oil at a idle causing the rough idle? If it's taking in to much oil...the lower octane would clear that up cause it would burn easier. So anyways I use to drive my car on 87 all the time but I'm in indiana and the car never had any problems...matter of fact I couldn't tell the difference between 87 and 93. Run the lower...and if your afraid just don't be stepping into the throttle and don't be hitting high RPM's simple as that..

I'm not 100% sure. I had the paper but I can't find it at the moment. I called them and asked them to reprint it... amazingly they don't have it on "file." I would go and have another one done, but they charge $90. I could just save up and buy my own rotary specific tester for 200. I no longer premix. It didn't change much of anything at all, and I was afraid it was going to leave ash behind. I don't think its the OMP because it isn't consistent at all. I wish I had a camera to film it, but who wants to sit and watch that shit for 5 minutes? =)

im gonna say your issue is a fuel trim.

Why i say this is that the ecu will learn a certain grade of fuel and set a trim for that specific octane to function properly.

If you are saying you always put high octane and always had a stumble, then magically you put 87 in and it goes away, then your long term fuel trims are set for 87 from the original owner and you keep putting the wrong fuel in the car.

Try running your tank empty, go to a gas station, fill up with premium, THEN before starting the car, do the brake pedal reset dance after pulling the room fuse for 30 seconds, start the car, i bet you have no stumbling.

IF thats not the case, i believe you have been decarbing the motor too much. Its not good at all to constantly put seafoam or zoom zoom cleaner through your motor. Believe it or not, your motor has wear on every contact surface and actually depends on some carbon residue to fill in the gaps of uneven wear to create a smooth path. IF you keep decarbing the motor, you are getting rid of that residue and re-opening the gaps from previous wear, causing compression leaks and stumbling.

NA RX7's ran better and made more power on lower octane gasoline. When I was racing, I looked for 87 octane that didn't have alcohol in it.

im gonna say your issue is a fuel trim.

Why i say this is that the ecu will learn a certain grade of fuel and set a trim for that specific octane to function properly.

If you are saying you always put high octane and always had a stumble, then magically you put 87 in and it goes away, then your long term fuel trims are set for 87 from the original owner and you keep putting the wrong fuel in the car.

Try running your tank empty, go to a gas station, fill up with premium, THEN before starting the car, do the brake pedal reset dance after pulling the room fuse for 30 seconds, start the car, i bet you have no stumbling.

IF thats not the case, i believe you have been decarbing the motor too much. Its not good at all to constantly put seafoam or zoom zoom cleaner through your motor. Believe it or not, your motor has wear on every contact surface and actually depends on some carbon residue to fill in the gaps of uneven wear to create a smooth path. IF you keep decarbing the motor, you are getting rid of that residue and re-opening the gaps from previous wear, causing compression leaks and stumbling.


The ecu has been reset several times before. You essentially have to reset to install new maps with the AP... and to erase the gas cap CEL, I chose to reset the ecu. The motor hasn't been de-carbed too much either I believe. Only twice. Once, last september with zoom zoom and once with BG 44k in june. It had seafoam once in between september febuary, and BG 44K once in june, which was the remainder of the BG 44k can. You do indeed have a lot of good info... I just think my 8 is an outlier and chooses to be different as punishment for previously driving a ford focus.