# Gas Octane Rating and Carbon Build Up **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** volkswagen, passat\ **Created:** [October 16, 2011, 2:41am UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676 "2011-10-16T02:41:09Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Vanhower](https://avatars.discourse-cdn.com/v4/letter/v/8e7dd6/32.png) [@Vanhower](https://community.cartalk.com/u/Vanhower)\ **Post date:** [October 16, 2011, 2:41am UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676/1 "2011-10-16T02:41:09Z")
I recently took my 2007 VW Passat 2.0T in b/c the check engine light was on. They initally replaced a leaking valve and a bad ignition coil, but the car was still misfiring on cylinder 3. They took off the intake manifold and found that there was significant carbon build up. The service writer said it was from “low-quality” gas. The car requires 91 octane. I’ve used 93 octane since 2007 but started using 89 octance around 6 months ago when the gas prices spiked again. Could 6 months of 89 octane cause significant carbon build up to the point it would trip the check engine light? Or could something else have caused it that I need to watch out for? ---
**Author:** ![Mechaniker](https://avatars.discourse-cdn.com/v4/letter/m/13edae/32.png) [@Mechaniker](https://community.cartalk.com/u/Mechaniker)\ **Post date:** [October 16, 2011, 2:45am UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676/2 "2011-10-16T02:45:24Z")
You’re burning oil, probably from bad valve seals. But in a 2007? What’s the mileage? ---
**Author:** ![Caddyman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/caddyman/32/23_2.png) [@Caddyman](https://community.cartalk.com/u/Caddyman)\ **Post date:** [October 16, 2011, 4:12am UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676/3 "2011-10-16T04:12:34Z")
It’s all guesswork until you post the codes that were set when the CEL came on…They might be recorded on your repair invoice… Low quality gas?? As far as I know there IS no “low quality gas”…It’s all the same gas… ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [October 16, 2011, 6:57am UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676/4 "2011-10-16T06:57:06Z")
The octane of the gas had nothing to do with this problem. Lack of maintenance and/or a problem elsewhere could be behind it though. Per the usual, I’ll add that you should never, ever put much faith into what you’re told by a service writer or a service manager. Very, very few of these people have spent time as mechanics and most have little or no mechanical aptitude at all. They get through each day reciting BS, old wives tales, innuendo, and so on. ---
**Author:** ![B.L.E](https://avatars.discourse-cdn.com/v4/letter/b/848f3c/32.png) [@B.L.E](https://community.cartalk.com/u/B.L.E)\ **Post date:** [October 16, 2011, 12:32pm UTC](https://community.cartalk.com/t/gas-octane-rating-and-carbon-build-up/68676/5 "2011-10-16T12:32:57Z")
Carbon builds up in intake manifolds because the vacuum in the intake manifold actually sucks exhaust gasses back into the intake manifold while both the intake and exhaust valves are open at the same time. On racing engines, the intake/exhaust overlap is so large that the exhaust dilution of the fresh intake charge causes random misfiring resulting in that loping dragster idle. It has nothing to do with the fuel’s octane rating.