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Mileage

The mileage I'm getting has fallen considerably. Prior to the rebuild, I was getting as much as 22-24 mpg, on occasion. Never less than 18-20. After the rebuild, it seemed that I lost a mile or two, but still reliably 18-21. Around early November, mileage changed. The last couple of fill-ups would suggest my mileage is not much greater than 10 mpg! I'd say I'm getting 14 mpg average.

The engine runs just as well as it ever has. Air filter and ignition cap and rotor have been changed, no affect. I changed spark plugs yesterday, so we'll see. Ignition wires seem to be working fine. I'm suspicious of a fuel leak, but I see and smell no fuel.

I don't imagine you guys can troubleshoot this from your computers at work , but I'm open to suggestions of where to look. I suppose a compression and leakdown could reveal something. But again, this engine seems to still be running like meth freak with his hair on fire.

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Old 01-02-2004, 04:54 AM
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Super I'm just firing in the dark, but could it be a leaking cold start injector? I have heard if this happens it will cause mileage to drop.
Old 01-02-2004, 06:59 AM
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I had that thought on the way to work this morning. I have not seen clouds of black smoke but the CSV may be worth looking at. Seems to me that if my engine were running righ enough to get only 14 mpg, then there'd be other indications of a rich mixture, like black smoke. Also, the outgoing spark plugs were white, not black.

I sure appreciate the advice, Matt.
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Old 01-02-2004, 07:07 AM
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My mileage changes a fair amount when the weather turns cold. I go from averaging 18-20 (to and from work mostly) to 15-17 for the same driving.
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Old 01-02-2004, 07:07 AM
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Quote:
Originally posted by masraum
My mileage changes a fair amount when the weather turns cold. I go from averaging 18-20 (to and from work mostly) to 15-17 for the same driving.
Same results here. My milage all summer long was right around 20 mpg. This first tank of gas during the cold weather was 15 mpg! I was pretty shocked at first, but though it must be to running rich longer to get the engine up to operating temp. Either that or my lead foot.
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Old 01-02-2004, 07:22 AM
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In CA they switch over to the oxygenated gas for the winter, and mileage drops on all our cars by 2-3 MPG. Do they do that in WA?

Your drop is pretty large, though.
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Old 01-02-2004, 07:32 AM
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Wet weather does it for me too. I suspect my mixture may be a little rich, but lately I've been getting around 12-15mpg! I think my plug wires could be on the way out- shorting in the wet a weak spark maybe.
Old 01-02-2004, 07:45 AM
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In Minnesota, they used to change over to 10% ethanol from October to April, which resulted in a coincidental 10% average drop in MPG!
I never understood the value to the environment if you burn 10% more fuel as a result. Oh, I forgot, FOLLOW THE MONEY!
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Old 01-02-2004, 08:11 AM
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I don't know where you guys get this 20+ mpg in an SC. If I'm *only* doing freeway miles at cruising speed I can get about 20mpg. But for typical around town/freeway mix, I sit at about 13-15mpg. Then again, I like to keep the revs up...
Old 01-02-2004, 08:22 AM
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I've seen 23mpg on the I-5 in summer. Thats definitely a best case scenario.
Old 01-02-2004, 08:33 AM
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I must need a tune-up! I'm getting about 15mpg in the city and if I'm lucky get 20-21mpg on the highway, but I'm also cruising along at 85+
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Old 01-02-2004, 08:49 AM
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I once saw 24.5 mpg going east on I090 across Washington with a tail wind. Interestingly, my velocity was around 85-90 mph, which is where the sweet spot was in my power band.

I like to chat, and I use that sickness to get information. For example, I learn stuff when I chat with the tanker drivers that fill up gas station tanks. The Chevron guy (I only use Chevron gas) said no, Western Washington fuel is not oxygenated. He also said, by the way, that Chevron, unlike other brands, uses different gas for premium versus regular. Most other brands use regular and just add octane booster.

Okay, you guys are helping me feel a little better. But I'm still curious about my mileage change. The car likes cool weather, makes more power. So, I'm not sure why cold weather driving should be LESS efficient. But if my mileage is still in line with other 911's, and if it seems to still run strong and smooth, I'll quit worrying. But I'll still scratch my head over this.
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Old 01-02-2004, 08:52 AM
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SUPE! i think the book says poor fuel economy is leaking CSV or injectors, or your air fuel mixture is not right.

one time when i was young my camaro suddenly developed crappy fuel mileage. turns out some dude was stealing gas from me on a regular basis. my car was the easiest to siphon from with the gas cap behind the licsence plate.
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Old 01-02-2004, 08:53 AM
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Who tracks their mileage? Is this done purely for a "just curious" reason? Economics? These cars get crappy mileage....pass the nuts. Superman, don't get too woried, it is probably just the weather.

Jeff
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Old 01-02-2004, 09:08 AM
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Well, economics is not my motivation. I just like to ensure my engines are running as well as possible. Is there dip for these chips?
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Old 01-02-2004, 09:13 AM
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Just one more point. Superman, if your car likes cool weather and makes more power then it is because the cool air is more dense. If it is more dense won't the car compensate with more fuel to keep the A/F correct? Result, worse mileage. I agree that my car feels stronger in the cold wather and I drive it a liitle harder I think because of it. This also would decrease mileage. Just some thoughts.

Jeff
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Old 01-02-2004, 09:14 AM
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It's curiousity and striving for efficiency
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Old 01-02-2004, 09:14 AM
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Harlan, yes. Jeff, thanks...I think you're right. I'm glad you guys are always here for me. A couple of hours ago I was a little worried, and now I'm proud to be burning so much fuel.
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Old 01-02-2004, 09:23 AM
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Super,
Concur with Jeff. Denser air would have a profound impact on the A/F ratio. Maybe I am WAY off base, but I would think that the colder, denser air would be able to move the sensor plate higher for given amount of throttle input ergo the need for more fuel...opposite of the way warm, less dense air causes a balloon to rise...?
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Old 01-02-2004, 09:55 AM
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My '82 has always gotten ~20+mpg on freeway, (4th-5th gear, even @ 80-100mph), and ~13mpg in town, no matter what. City driving habits are fairly mild, everyone drives fast here and I keep up w/ traffic.

My car has something wrong where it runs rich until fully warmed up, idle is "lopey", then once hot it's perfect. I am going to look into that CSV when I get the time and motivation. It also has something going on where it gets richer as the revs climb, this makes for great power, but still isn't really right. I have a new WUR, injectors and seals are perfect or new, I've been told that the fuel distributor is suspect, not much else it could be(?)

At any rate, it runs like an open sore, everyone who's driven it gives it a huge thumbs-up, so no big hurry to take things apart.

It does bother me that the city gas milage is so poor, though, it baffles me how a 2600 lb. car w/ a 3 liter, 6 cylinder, fuel injected motor and manual trans could be so inefficient.

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Old 01-02-2004, 10:14 AM
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