Defroster causes drop in mpg

ok - here's the situation: I have been keeping track of gas milage on my '96 tbird 4.6, and when I don't run the defroster, I get between 25 and 26 mpg. I ran a tank through with the defroster on and I get just
over 20 mpg. Is this signaling the A/C going out? Is this much of a drop normal? I thought when you run the defroster, the A/C comes on as well, but isn't supposed to cool anything. Am I on track or just smoking crack?
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says...

Normally, the compressor does run when in defrost. It does cool the air, but the air is then sent past the heater core to warm back up. Cooling the air squeezes out the humidity, which fogs the inside of the car. If your side windows are fogged up, use the regular A/C mode, but turn up the temp knob and aim the ducts at the windows. It'll dry them up real fast.
If you have a combined heat/defrost mode, it may not run the compressor. It may work for light defogging.
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Air Conditioning doesn't really cool air, it removes humidity, cooling is a byproduct, so yes, it's designed to run the compressor when you are in defrost. A 4-5 MPG drop seems excessive. Do you notice any drop in engine power? It's possible your A/C is over charged.
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Michael O wrote:

I suppose I have noticed some power drop, but not a whole lot. Maybe I've had it all along but just never noticed it - the car was a city driver for its first ~50k. (now at 162k)
I thought the MPG drop was a bit much- I have never had the A/C serviced. I bought the car in '96 and for the first 15-20k and at least the past 80k miles, I've just been taking it to the dealer and telling them to do the "xxx,000 mile maint". I will probably say something next time it is in (should be next month). I was worried that I might be ruining the compressor- those are expen$ive.
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Actually, it cools the air. When air is cold, it can't hold as much water as when it is warm. So drying the air is a byproduct. You got it backwards.

I also wonder if the coldness and the dampness is affecting his milage as well.
Jeff
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AC 101....air conditioning does not cool the air....it compresses a fluid which boils at a very low temperature. at 70 degrees most freon type liquids are a vapor...they've boiled now boiling removes heat...put a liquid under pressure past it's boiling point.....control the release into a small radiator type thingy, blow air over it and voila...cooler air...now cold air can't hold much water vapor so it's cool dry air...run the ac make cool air...blow that over heater core, warm it up, blow warm dry air over cold foggy windshield...voila...no foggy windshield.
with the miles on this puppy and getting 20 mpg+ with ac running I say quit sweating the small stuff and make an offering to the car gods...this thing is doing great some drop in power is normal as the air conditioner does require from 10 to 25 horsepower to run, depending on outside air temperature, blower setting, whether it's set on "normal" or max and it takes gasoline to produce that horsepower. I think you are just seeing the effect of 162 k on the motor...it's not blown up or anything but an old horse just can't do all the stuff a young horse can. just follow normal maintenance and keep on driving that puppy. might be nice to have somebody put a set of gauges on the ac to verify system charge...over charged ac system will cause high side pressure to be way high...300 psi or higher in the r12 days not sure with r134 and the harder that compressor has to work to move the 'fluid' in it the more horsepower it's gonna need...more horsepower for the ac=less horsepower for the motor...put the engine under more than normal load and it's gonna burn more gas I know that's a very long convoluted answer but I'll probably get a few comments about where I'm not exactly right but then It works for me...

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I guess I'd never thought of it that way before- the car had 205hp (according to the reviewers) when it was new, and it probably doesn't have that now. The 10 to 25 hp the A/C uses is now a more significant percentage of total engine power than it was when the car was new. Not to mention that the A/C is just as old as the rest of the car and probably isn't running at peak efficiency any more. I'll have it checked the next time I take it in. (should be next month at 165k)
Thanks!
Patrick Gilmore wrote:

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Actually he does NOT have it backwards. YOU do.
#1. There is no such thing as cooling air. You can only remove heat. #2. When you remove the heat, the moisture in the air sticks to the evaporator, hence you have dryer air. #3. See #1 and #2.
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Other way around. Expansion of gas in A/C system lowers the temperature, cooling the air. The air is cooled below the dew point so the moisture condensates, removing the humidity as a byproduct, it is then sent past heater core and warmed back up.
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