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Twice in the last two years upon returning to Texas from Colorado I have experienced severe engine missing when I get back to a lower altitude. The problem generally shows up around Amarillo which has an altitude of about 4000 feet. The problem seems to get worse as the fuel tank gets closer to 1/4 full. I generally drive from Colorado to Amarillo in one day, stopping once for fuel. In my part of the country (altitude @ 1000 feet) I use 87 octane unleaded fuel and in Colorado I use 85 octane unleaded fuel. It was my understanding the engine fuel adjustment is made automatically at the higher altitudes so the use of 85 octane was OK. After I return home the problem clears up so when I take my vehicle in to the dealer they don't have any idea what the problem may be. I don't have any engine missing in Colorado at an altitude of @ 10,000 feet. Ideas? Maybe I should only burn 87 octane in Colorado?
 

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jimmarf said:
Twice in the last two years upon returning to Texas from Colorado I have experienced severe engine missing when I get back to a lower altitude. The problem generally shows up around Amarillo which has an altitude of about 4000 feet. The problem seems to get worse as the fuel tank gets closer to 1/4 full. I generally drive from Colorado to Amarillo in one day, stopping once for fuel. In my part of the country (altitude @ 1000 feet) I use 87 octane unleaded fuel and in Colorado I use 85 octane unleaded fuel. It was my understanding the engine fuel adjustment is made automatically at the higher altitudes so the use of 85 octane was OK. After I return home the problem clears up so when I take my vehicle in to the dealer they don't have any idea what the problem may be. I don't have any engine missing in Colorado at an altitude of @ 10,000 feet. Ideas? Maybe I should only burn 87 octane in Colorado?
I would stick to 87 octane. Why?

1. Your ECU has to learn to adjust to the A/F changes-just like when you install a new CAI.
2. Pre-ignition can damage your engine.

I would also suggest that you put in a can of dry gas in case you've got moisture in your tank...
 

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My suggestion is use 87 octane in Colorado so when your headed back home, it's an easier transition for ecu adjustment to altitude. I've driven to sea level many times (using 85 octane in CO) and have never had a problem before so not sure why your having them.
 

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As already stated here and as it reads in the manual - 87 is the min for our engines. That said - burn the rest of that 85 out of there and on your next fill put in some ISO Heat and a bottle of octane boost with a full tank of 87. Don't worry about using the boost as it says on the label. Then reset your ECU after you get 1/4 into the new gas. That should put you back on track.
 

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Doing it the 'fast way' works - you may have to do it a bunch of times until you get the sequence right. It took me like six tries to get the first pedal clicks fast enough but it works!
 

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i agree no less than 87, but if that doesnt help, try turning the key to on for a few seconds before starting it, changes in altitude mean changes in baro pressure, which can affect the reading the computer reads. turning the key to on allows the ecu to read the baro pressure, it also reads it at WOT while driving... this used to be true, not sure if the latest fuel injected vehicles still operate this way but i would have to assume it does.
 

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The 87 Octane requirement is for sea-level. 85 is fine for Colorado, you just need to throw the 87 back in the tank for the drive home so when you descend and the air thickens up, you won't knock. This is very simple. The ECU can only retard the timing enough to keep the motor from knocking on 87 at sea-level. Using the 85 on your way home, the computer will attempt to retard the timing when it begins to knock, but it can only do so much. As you go up in elevation your octane requirement decreases and vice versa as you go down.
 

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I'm no physicist but yeah, what he said.
 
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