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Cold air flaps and gas mileage 900

posted by  RB on  Thu Feb 3 07:40 CST 2005
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last visit: Mon Jan 26 02:20 CST 2009
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Just thought I'd post some observations relating to cold air flaps. Last summer I locked the cold air flaps on both my 87 745GLE and 96 965 in the cold-air only position. The thermostats on both units had failed and they were in the hot-air only position. The general consensus is this is bad, for obvious reasons.
Last winter the 745GLE, which is my daily commuter car, averaged 25 to 26 mpg on my 60-mile commute. This winter I'm averaging 22 to 23 mpg. Since last winter I've installed new plugs, wires, and of course the normal course of filter changes, etc... In other words, I've kept up on normal maintenance.
Last winter the 965, which is my wife's car and is used mostly in town, short trips (the worst for economy), averaged 14.5 mpg. We saw lows of 13.5. This winter it's averaging 16.5 mpg. Nothing on the vehicle, save oil and filters, has been changed since last winter.
Thoughts?
Richard
--
86 740T/87 745GLE/96 960 Wagon








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    Cold air flaps and gas mileage 900

    posted by  Volvodad on  Fri Feb 4 04:24 CST 2005
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    last visit: Thu Oct 23 16:18 CST 2014
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    Single biggest factor in fuel economy is driving style and conditions, and it's tough to keep that strictly consistent from year to year. With a sophisticated fuel injection and engine management system, fuel economy won't be strongly affected by incoming air temp, but maybe more so during initial warmup when metal surfaces like the intake ports are still cold.

    My understanding is that the air preheating system is there to firstly, improve emissions performance during warmup from cold starts, and secondly prevent throttle plate icing in cool, humid condiotions, rather than give a meaningful increase in fuel economy.

    --
    Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and numerous old motorcycles)








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      Cold air flaps and gas mileage 900

      posted by  RB on  Fri Feb 4 08:52 CST 2005
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      last visit: Mon Jan 26 02:20 CST 2009
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      Bob, that's always been my understanding, as well, which is why I find my experience curious. On the older car, I'll readily accept there could some other factor influencing my decrease in mileage. Could be the O2 sensor is getting lazy, temp sensor to computer might be sending low temp signals, etc... Just thought it was curious that we've experienced at least a 12 percent increase in economy on the later car.
      Richard
      --
      86 740T/87 745GLE/96 960 Wagon








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        Cold air flaps and gas mileage 900

        posted by  Volvodad on  Fri Feb 4 09:33 CST 2005
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        last visit: Thu Oct 23 16:18 CST 2014
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        The only thing I've done on a car (our 83) that resulted in an immediate and unmistakeable improvement in fuel economy was to change out an overdue (and obviously, clogged) air filter. Fuel mileage improved by about 10% on the same 900mi round trip I was then taking every few weeks. Other than that, the results of any tune-up or other work has been hard to detect due to normal data variation. I, too keep detailed records of fuel consumption as a maintenance aid.

        Your numbers are no doubt coming from careful measurement but the real cause(s) behind them is a bit of a mystery.
        --
        Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and numerous old motorcycles)







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