Hello,

I had recently failed my emissions test, so I had my neighbor, who is an experienced mechanic take a look at it. He told me I needed to replace the cata converter. I told him that I had replaced the cata converter before and asked why I had to replace it again. He told me to stop using cheap gas (arco, safeway, costco, etc.) and to fill 89 octane instead of 87. He said that it would be cheaper in the long run because I would get better mileage.

Does this sound right? I just wanted some opinions on this. Please let me know what you guys think. Thanks.

 

1) Yes, the octane of the fuel can affect emissions, but not much. This is due to the slower burn of higher octane fuel.
2) Higher octane doesn't do anything for economy. Either the car needs the increased octane to not knock, or it doesn't. The (only very recent) exception to this is the few new cars that try and run on the verge of knock and pull timing as needed. You'll see the manufacturer claim the power figure with (on 93 octane) in the advertising for these cars. And even with these cars, the mileage rating stays the same regardless of fuel used.

Post up your emissions sheet. What reading did you fail on? There are several different issues that could kill a cat quickly, but the most common one is buying a cheap cat and expecting it to last.

 

Thanks for the reply.

I have already gotten my cata converter replaced and passed the emissions test. The error code I got was Catalyst System Efficiency Below Threshold (Bank 1).

The old cata I ordered online from RockAuto 4 years ago. Two years later, I failed the next emissions test. I brought it to the shop and they told me all I needed was a tune up. They reset the CEL and I passed it. However, after driving for a bit, the CEL came back on.

My mechanic ordered the cata and did the work on the same day. I'm not sure where my mechanic got my cata, but he told me it was new and it is illegal to sell used catas.

 

Your neighbor should have told you to use the gas recommended in your owner's manual, to get an ignition system tune up, and to fix any CEL codes.

 

Straight from Sunoco race fuel home page on fuel burning speed.
cliffs: Read the last sentence first.

Naturally aspirated race motors with large combustion chambers spinning at high RPMs really like high-octane, fast burning fuels. They need the octane to prevent uncontrolled combustion, and they need a fast-burning fuel so that the flame front can span the large bore of the combustion chamber quickly. If you’re not sure which fuel burns faster than others, one indicator is specific gravity. “Lighter” fuels – fuels with a lower specific gravity – tend to burn faster because fast burning hydrocarbons are themselves light. Look for a specific gravity close to 0.70 and you’ll likely find a fast burning fuel. Of course, consult with the fuel producer to verify your assumptions. You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!

 

what car/engine is OP's car?if you failed due to catalytic convertor, get a new catalytic convertor. the cheap rockauto ones are not long lasting.

 

Uh, I've heard that before. But, I've experimented with it over the last year. I get better MPG when on 91 compared to 85. Without a doubt. The 1.5L in my civic is a smaller difference, 35.7 compared to 34.5.

With my GSR. I would get 22mpg on 85, and 26mpg on 91.

 

I have a 1999 Civic LX. My neighbor mechanic told me 89 octane gives better mileage and that he verified it by testing it.

So, should I stop filling cheap gas and should I change my gas octane?

 

The brand of gas plays a bigger role. Using cheap gas will cost you more in the long run. You should probably have the fuel system cleaned, then continue to use high quality gas. You can use 87.