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Old 04-15-2009, 05:24 PM   #5 (permalink)
Master EcoModder
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Location: South Dakota
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WonderWagon - '94 Ford Escort LX
Last 3: 51.52 mpg (US)

DaBluOne - '99 Ford Escort SE
90 day: 48.97 mpg (US)

DaRedOne - '99 Ford Escort ZX2 Hot
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Originally Posted by hummingbird View Post
1. With increase in temp I saw a decrease in FE. Should it not be the other way round?

2. The ECU mistook my ~88 octane fuel to be 93, and gave me waay higher FE. What did it do differently with that assumption that it adjusted for the later tanks? (If I know, I can mimic it, so I get back the FE)
Everything else being equal, on normally expects better FE in warmer summer temperatures. Since you switched from 93 octane to 87 octane (the first tank being a mix of ~88 octane??? the second tank being almost exactly 87 octane), everything else isn't equal. Looking at you fuel logs, I also see that you had it in for an oil change on the last tank. Any chance the guys servicing the vehicle bumped the tire pressure down from 50 PSI??? (This the very first thing to look at.) I also see that you've used the AC more often.

It's a misunderstanding to think the ECU mistook ~88 octane for 93 octane. Octane is basically a measure of how subject the air/fuel mixture is to pre-ingnition. As the piston compresses the air/fuel mix, it's temperature rises. If the temperature and pressure are high enough, the mixture spontaneously ignites instead of being ignited by the spark plug. The lower the octane number the lower the temperature and pressure at which the mixture pre-ignites. When the combustion of the mixture is induced by a carefully time spark, a wave of flame travels though the mixture from the spark plug tip to the top of the piston and cylinder walls. When the mixture pre-ignites, it happens more or less everywhere in the mixture at once. The burn is a lot quicker and resembles an explosion. Pre-ignition can quickly damage an engine. If the ECU detects that pre-ignition is (occasionally) occurring, it adjusts the spark timing to prevent pre-ignition occurring.

Anyway, if the tire pressures are still up and you really didn't use AC enough to account for the FE hit, then octane may be the culprit. To test this theory, at your next fill start with ~3 gallons of 93 octane then top off the tank with 87 octane.
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