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Old 07-27-2002, 08:42 PM
Keith Lucy
Posts: n/a
Rough idle after low octane gas

I recently (accidently) pumped my tank full of 89 octane. Sonn thereafter, the engine started running fairly roughly. With about 1/4 tank left, I refilled with my standard 93 octane. I've driven 80 miles (5 gallons worth) and the engine is still running fairly rough. I would have thought that the engine should have smoothed right out if it was the low octane gas, which now has me worried that either the low octane gas has tainted/damaged something, or the roughness is just a symptom of something else.

Should I wait for a few tank fulls to see if the problem goes away, or get it to a mechanic?

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Old 07-27-2002, 08:53 PM
David C Klasse's Avatar
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It seems highly doubtful that some 89 octane gas would cause the idle to run rough afterward.
I would recommend you running a bottle of Techron or two (from Chevron: fuel injector cleaner) through the fuel system, preferably close prior to an oil change.

Techron is highly recommended and has always proven to be a great product.

Good luck.
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Old 07-30-2002, 12:26 PM
Keith Lucy
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Thanks !

I wasn't able to find the brand you suggested, but I used Gumout from Penzoil instead. I used the higher concentrated option and it worked great. About 10 miles after dumping it into the tank, all is once again running smooth.

Had it up to 110mph last night and she was running as smooth as ever.

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Old 07-30-2002, 04:19 PM
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In the past couple of years, I have become a big believer in those synthetic fuel system cleaners, they really have made a difference in the smoothness of my Jaguar, and seem to help the fuel mileage on the other cars as well.

Lower octane fuel will make the engine more susceptible to knock, but FYI for others with the same problem I do not know any way it would cause a rough idle. Usually, idling is smoother with lower octane fuel.
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Old 08-01-2002, 07:43 PM
Keith Lucy
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Maybe it was crud

It couls have just been a coincidence, but it started when I pumped the low octane. Maybe I just happened to get some crud in the tank around the same time.
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Old 08-02-2002, 10:03 AM
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Join Date: Jul 2001
Location: Kingston, Ont.
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I had a similar "coincidence", where my oil level dropped suddenly after using low octane gas, and the oil was quite dirty as well, depsite being relatively clean just days before.

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Old 08-02-2002, 10:23 AM
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Join Date: Dec 2001
Posts: 565

I wonder if the lower octane fuel acted as a "cleanser" and knocked some crud loose and down into the oil pan. Techron and other such cleaners do this. Some recommend using these products maybe a gas tankful away from an oil change for this reason.
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