band@hao.UUCP (Peter Bandurian) (12/02/83)
Here is a wierd problem. I keep track of the fuel mileage any car I have as a matter of course. I do this not so much because it is interesting in itself but because it is an indicator of when something may need maintence or repair. Last winter I noticed that the Rabbit's fuel mileage started dropping. Actually it varied wildly, from the accustomed 30mpg to a low of 21 mpg. I have a 1976 Rabbit with a carburated engine and a Heathkit capacitive discharge system. I looked for a possible cause. I played with the timing. I played with the carburation. I tried different brands of gasoline (supposedly, the majors make better gasoline than the minors), super as well as regular (unleaded). I tried BG Supercharge (the theory was that with 99,200 miles on it a little cleaning up would not hurt; the VW dealer's idea; it didn't help). I tried octane booster. The choke was okay. I replaced all the rotted little rubber vacuum hoses. I checked the temperature controlled vacuum valves of which there are many in this vintage. The anti-backfire valve, the by-pass valve, the by-pass by-pass(!) valve. I replaced the thermostat (the old one turned out to be marginal but certainly not bad). I even tried ignoring it. Nothing helped. By Spring I noticed a correlation between fuel mileage and average ambient outdoor temperature. A normal winter in Boulder, Colo. has a tremendous temperature variation: from warm, spring-like to, well, winter. Sometimes all in one day. The colder it got, the worse the fuel mileage. If it warmed up, up went the mileage. Like a yo-yo. Like Boulder's weather. Well it's doing it again this very snowy Fall. The only thing I learned this summer is the mileage stayed consistently around 27 (compared to the former 30mpg). I realize a small mileage loss in winter is normal: the first two winters with this car saw a 10% mileage drop (ie 30 -> 28mpg); but nothing like this. Anyone have any ideas? Peter Bandurian ucbvax!hplabs!hao!band decvax!brl-bmd!hao!band seismo!hao!band
knutson@ut-ngp.UUCP (Jim Knutson) (12/02/83)
I am by no means an expert, but temperature and humidty can effect the
fuel/air ratio. Perhaps some of the racing team members can give a
better idea of just how much of a variance there might be. Also, if the
air is not heated sufficiently before entering the carbuerator, icing
can occur inside the throat. This will reduce air intake causing a much
richer mixture.
--
Jim Knutson
ARPA: knutson@ut-ngp
UUCP: {ihnp4,seismo,kpno,ctvax}!ut-sally!ut-ngp!knutsondmmartindale@watcgl.UUCP (Dave Martindale) (12/04/83)
1) You could have something wrong with the cold-enrichment system (choke or whatever else) that is making it run richer than it should be, either just during warmup or all the time 2) Whenever you drive around in snow that is not very well packed down, the rolling resistance of the tires will be much higher than on dry pavement, producing poor mileage.
dmmartindale@watcgl.UUCP (Dave Martindale) (12/04/83)
If the carburetor contains no mechanism to compensate for outside temperature, the mixture will actually be leaner in cold weather, due to the presence of more oxygen in a given volume of air. In most recent cars, the systems which preheat intake air try to maintain a reasonably constant air temperature going to the carb, minimizing the effect of this (if the system is working).
wookie@alice.UUCP (12/05/83)
Temperature and humidity certainly affect the air/fuel ratio and in our racing effort the jetting is set accordingly. I find just a ten degree change will greatly affect the engines performance and it is not unusual to have to change jets from morning to afternoon or even in the same morning at a race. Another thing that greatly affects the mileage in the winter is the amount of time the choke is closed causing a necessarily rivher mixture. If you are doing mostly short hop driving of just a few miles I have seen drastic reductions in fuel mileage between summer and winter. Driving six miles to work I have seen 30% drops in the winter in my old gas guzzler! Keith Bauer White Tiger Racing
porges@inmet.UUCP (12/10/83)
#R:hao:-75000:inmet:2700034:000:599 inmet!porges Dec 5 11:58:00 1983 Not only do I get only about 15 mpg in my 1979 Rabbit in the winter, down from 30 during the summer, but the Chilton book warns you that you could get as little as 1/2 your normal mileage when the car is cold. Since I mostly just drive 15 minutes back and forth to work, this explains it for me. I also notice than when I take long highway drives so that the car is warmed up I get my 30 mpg back. If your driving is under similar conditions, "your mileage may vary." -- Don Porges ...harpo!inmet!porges ...hplabs!sri-unix!cca!ima!inmet!porges ...yale-comix!ima!inmet!porges