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Steve W's Avatar
Join Date: Nov 2001
Location: PV Estates, CA
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I'm far from an expert in gasoline, but oxygenated fuels do have a significant effect on fuel consumption and it's effect on air fuel ratios. Two common oxyenates are ethanol and MTBE(derived partly from methanol), which can be mixed with gasoline at a 10 and 15 percent rate respectively. Ethanol with an octane of 129 and MTBEs with a rating of 118 are mixed in with low octane gasoline to bring up the octane level of the fuel to the required level. However alchohos have one drawback in that they only have half of the energy output per cc compared to gasoline. This means for alcohol to produce the same amount of energy and maintain the same air fuel ratio of gasoline, twice the volume of alchohol must be injected. If 10% ethanol has displaced gasoline at your pump, the gasoline you are now pumping is only 95% of straight gas, which means your car will now run leaner at full and part throttle and deliver less power, or if your fuel system is adaptive or in the case with closed loop fuel systems, will get poorer fuel efficiency to make up for the reduced density.

When driving under part throttle load conditions, the O2 sensor constantly tries to maintain the stoichiometric ratio of 14.7:1, and sensing the leaner fuel ratio of the oxygenated fuel, will try to constantly richen the fuel mixture, resulting in increased fuel consumption. At full throttle though, the O2 sensor is ignored and a preset/preprogrammed fuel curve is used. But now the fuel system has no idea it is using an oxygenated fuel and not being adaptive here, will actually cause the engine to run leaner at full throttle than it should. For example if your full throttle AFR is tuned/programmed to with an afr of 13.0:1, using an oxygenated fuel with 10% ethanol, it will now lean out to 13.67:1, resulting in reduced power and a higher propensity to predetonate.

Old 01-02-2004, 10:21 AM
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I think a bad/stuck CSV would have shown up on the spark plugs as soot ...

Look for a dynamic fuel leak elsewhere ... cracked fuel line fitting at pump, leaking line into accumulator, etc.

Downwind or behind car 15 - 20 feet with engine idling after warmed up ... do the fumes seem particularly noxious, or eye-stinging??? I think they would be ... if the mixture was the problem!

On the other hand, winter-mix gasolines from all of the major brands seem to cause a loss of power and fuel economy ~ 20%, perhaps, on '60s high-compression V-8 engines! This is a recent phenomenon of the past 20 years with the 'improved' fuels to meet CA & Fed stds!
Warren Hall, Jr.

1973 911S Targa ... 'Annie'
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Last edited by Early_S_Man; 01-02-2004 at 10:34 AM..
Old 01-02-2004, 10:30 AM
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Location: Lacey, WA. USA
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Warren, your thinking is the same as mine. If the mixture were rich, as it would be if the CSV were always open, there would be sooty spark plugs, black smoke and other telltale indications of a rich mixture. But that's not what I'm finding. When JW richened my mixture, the fragrance of my exhaust took on that wonderful, slightly noxious aroma but not an over-rich smell. A/F was perfect on Dyno Day, the first of November. That's about when my mileage went downhill. Hmmmm.

And if there were a leak, then I would be able to find a fuel spot under the car after extensive idling, or I might smell fuel. I have never smelled fuel.

But again, I feel a little better now that I've heard of others whose fuel mileage is low. Cooler air, etc. When mileage was great, I could still get as low as 16-18 with plenty of spirited city driving. So, it's probably nothing to worry about, but it's likely to always be a question in my mind.

Oh, one more thing. Sometimes with an overly rich mixture, I find that lifting the throttle a touch at times can cause subtle acceleration. I'm not seeing that either.
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Old 01-02-2004, 11:47 AM
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I once achieved 25.1 MPG on a long road trip on I-95 (850 miles totol). Actually, it wasn't me -- my brother was driving while I slept. When we stopped for gas, I did the usual gallons/miles and came up with 25.1 MPG. I don't know how he managed to do this, my I guess he was tired and driving below my 75-90 MPH average. At those speeds, I normally get 20-22 MPG.
In the city, 15 MPG is not unusual.

Just an idea: I installed an AIR/FUEL meter and now I Know exactly what my mixture is doing.
TODD (the past):
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Old 01-02-2004, 11:48 AM
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Superman...I didnt read all the posts...but I believe that some gas here in the NW is oxygenated in the winter months.....for example I get much worse mileage with ARCO gas than my usual Chevron or Texaco..... My 85 gets 22-25 on the highway if I am not going too much over the limit...and around 18 in town with spirited driving....this is with a Steve W chip also. I burn 92 octane....but once when I used ARCO gas out of necessity the mileage was much worse....I'm pretty sure it was oxygenated.
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Old 01-02-2004, 10:07 PM
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ARCO in Oregon is most definitely gasahol...year 'round. I avoid that brand like the plague. A local wrench I trust seems to believe that all brands contain some alcohol now, especially since MTBE has been banned in California. Super, it would be an expen$ive experiment, but you might see if you could find a station there that sells unleaded racing gasoline...still legal for the street. Try a tank of that, see what your milage does...
"Now, to put a water-cooled engine in the rear and to have a radiator in the front, that's not very intelligent."
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Old 01-02-2004, 10:21 PM
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Jim: I tried the racing fuel route. It made no real difference in gas mileage or performance. I'd save my money on that option.
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Old 01-02-2004, 10:40 PM
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I get crappy mileage too, but my CIS hasn't been tuned for altitude yet.

sub 15 mpg, I think.
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Old 01-03-2004, 03:20 PM
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Here's a really oddball suggestion, but from experience. Go check your odo against a known distance. Mine shed a couple teeth (over time, about 1/2) and my first indication was that I thought my mileage took a crap. Just a thought.

Greg Lepore
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Old 01-03-2004, 03:48 PM
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