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Old 08-17-2008, 07:48 PM   #1 (permalink)
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AC cooling off the engine??

Today I was on the highway in hot weather (81f) and was looking at the SC2 to
see how many MPG I was losing with the AC running.
With all the hills, I couldn't really pin it down. But, on average, I was burning
more fuel than I did on the trip out this morning when it was cool..
Cool=30.5 MPG and Hot=29 MPG..

Anyways, I was displaying the radiator water temp. It was staying right at 198
just about all the time. That's the normal temp even if it's 65 out..

Then, I noticed something interesting.. Each time I would turn on the AC,
the engine temp would drop down to about 188.. A 10 degree drop!
Turn off the AC and within a few minutes, the engine was back to 198.
Just a few miles of AC on, and it was back down to 188.

My theory is, the AC coolant being pumped into the coil sitting in front of the
radiator is still cool. Cooler than outside air temp.
So, when the in-take air comes in, it cools down a little on the AC coil
before hitting the radiator and cooling it off a little..

It seems like the extra load of the compressor making the engine work harder
would make it hotter.. But, it seems the AC coolant is making it run cooler..

So, what's the real story??

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Old 08-17-2008, 08:58 PM   #2 (permalink)
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I don't know about the radiator/fan layout on a CRV, but I think the difference in temp would be mostly due to a "safety" feature. For example, a 2nd fan that kicks in automatically when the AC is turned on, or if you got only one fan, it turns faster, etc. even if coolant temp doesn't require it. Just to prevent more overheating.

Just my take on this phenomena, I know *some* cars are designed this way. Pop open your hood with the engine running and ask someone inside the car to start the A/C while you check the fans. Maybe I'm wrong though.

But the coolant gaz return is still an interesting theory... I don't know if it'd be cool enough to make a difference in coolant temperature in the radiator. Maybe.
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Old 08-17-2008, 09:18 PM   #3 (permalink)
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The dual fan theory may be closer to right. Most of these vehicles equipped with a/c will have a dual fan setup with one dedicated to the condenser. So it is likely that the second fan would and may be cooling the car a little more ...
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Old 08-17-2008, 09:25 PM   #4 (permalink)
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I 3rd the 2nd fan.
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Old 08-17-2008, 09:34 PM   #5 (permalink)
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Its the 2nd fan kicking in and the primary one switching to high.....
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Old 08-17-2008, 10:02 PM   #6 (permalink)
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Dang!

I'll bet you guys nailed it..
For some reason, I didn't think about the two fans!!

I know the radiator fan never comes on unless the engine gets up to the 200 deg range.
But, I'm unsure if the AC needs a fan running when the car is going 55 MPH..?.
I guess they would just turn it on anyways, in case you were sitting at a long traffic light..

I guess that those fans really do work, even if you are at highway speeds.

My crazy theory may have some merit.. IIRC, there was a guy around here installing insulation on his AC pipes under the hood.
He figured the pipes that were frosting up, were just wasting 'Cold' that he wanted inside the car, not under the hood..


Now that I think about it, it's nice that my AC still works okay with
80% of my grill intake blocked off.

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Old 08-18-2008, 03:15 AM   #7 (permalink)
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the refrigerant going to your A/C condenser is very hot..it goes straight from the compressor outlet to the condenser, so your theory of cool gas couldn't work.

They automatically turn the fan on when you hit the A/C button because that is the most efficient way to run the A/C. The more cooling you get in the condenser the more cooling you get in the car.

That is interesting that the guy was insulating his pipes...in any car I've seen the expansion valve in the A/C system was right in front of the evaporator in the car. But he does have an idea that you may get some heating of liquid refrigerant right before the expansion valve.
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Old 08-18-2008, 06:03 PM   #8 (permalink)
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cooling?

I can't imagine either of the condenser fans coming on at 55-mph.The ram air should take care of that even with the grille block.The compressor load might move the engine closer to the BSFC peak,and engine run cooler,however it seems like the T-stat should maintain design coolant temp regardless.I suspect paranormal activity!
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Old 08-20-2008, 10:21 AM   #9 (permalink)
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I can't say for certain this applies to every vehicle on the road, but every one i have owned turns on the condenser fan no matter what when the button is pressed. Literally, the fan is in the circuit for the A/C so there is no choice in the matter. When the button is pressed, condenser fan turns on. Most A/C diagnosis charts tell you to look at the fan when pressing the A/C button as that shows whether the circuit has power or not.

aerohead you are right at 55 mph it should get adequate airflow but there is no logic to prevent it from coming on based on speed, temp, etc.
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Old 08-20-2008, 09:43 PM   #10 (permalink)
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Likewise, all my cars' tie the fans to the A/C. A/C on = fan on. With large amount of blocking in place, ram air is probably diminished, so the fans improve air flow through radiator even at highway speed. Also, the fans are in parallel. They both turn on together.

The frosty A/C hose is probably going from the evaporator to the compressor, not the other way around.

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