Mileage intermittently drops by over 20% with A/C on lately

[mfa-prius]

My '05 Prius has always had a split personality -- under seemingly identical conditions, mileage would either be very good or very bad. A couple of days ago, things got ridiculous -- mileage dropped to 41mpg while driving at 40mph on a flat road I drive every day, usually seeing 50-55 mpg.

It was hot - 90+, and I had the A/C on, of course. As an experiment, I turned it off, and the mileage immediately jumped up to "normal" -- 50ish. I repeated the experiment for the 10 minutes it took to get home, and it was consistent -- A/C on, 40ish mpg; A/C off, 50ish. The A/C, when on, does work.

Suspecting an A/C problem, I called the dealer and they said to bring it in. When I got there this morning, however, they said the only thing they do in response to low-mileage complaints is to check the tire pressure. My description of the problem fell on deaf ears -- they said there was nothing they could do. They couldn't tell me if the A/C was belt-driven or mechanically driven, or electrically-driven or what, or what an A/C failure might do to the ICE.

So I'm appealing to the forum -- any ideas? I have done the A/C on/off experiment in the past, and generally found a very small, usually unnoticeable, difference in mileage, at all speeds and temperatures. Now, however, something in the A/C operation seems to be sucking a lot of power out of the ICE. For now, I'm assuming something is broken a little bit (maybe a bearing going bad?), and when it finally breaks big-time, I'll take it in to another dealer for repair. But I'm not happy with this strategy.

[morpheusx]

When you run the A/C try to keep it at a higher temperature like 77 and on one of the 2 lowest fan speeds.

As far your other question the A/C is not belt driven in the 2004+ and is all electric. This is good as it does not require the engine to run while stopped at a light in order to stay on.

[EZW1]

I notice the same phenomenon here in Phoenix. Today its 114 and I set my AC to 78 in Auto mode. The system cools the car down and the fan speed drops to an almost imperceptable speed. I've noticed when the fan is running very high, likely the compressor is also and the drain on the battery is substantial. One time at a drive through, the battery gauge dropped from 5 blue bars to 2 pink bars over the 10-mins I was working my way through the line. So, this is evidence of the high drain on the battry to run the compressor at high speed.

Conversely, when the fan has dropped to a very slow speed, likely the compressor has also and I barely notice a drop at all on the battery.

So, my answer to you, mfa-prius, is what you're experiencing is normal. Under high demand from the AC there's a large load on the battery and the system will run the engine a bunch to keep the battry charged and hence, lower gas mileage. Last winter I was getting 60+ mpg but this summer its dropped to 50+. That's about a 15% drop. But, I'm still doing much better than the EPA numbers.

[mfa-prius]

Originally Posted by morpheusx

When you run the A/C try to keep it at a higher temperature like 77 and on one of the 2 lowest fan speeds.

As far your other question the A/C is not belt driven in the 2004+ and is all electric. This is good as it does not require the engine to run while stopped at a light in order to stay on.

I did all that (normally set to 77); as long as the A/C is running at all the mileage drops severely. Recirculate or not makes no difference.

Thanks for the info re electric drive -- I should have thought of that.

[mfa-prius]

Originally Posted by EZW1

So, my answer to you, mfa-prius, is what you're experiencing is normal.

I can accept some additional drain, but this is something new. Never noticed significant mileage drop with A/C until last week, had the car for 4 1/2 years.

[morpheusx]

it could be that you are driving shorter routes then you used to as well. The battery usage to propel the car is much greater while the the vehicle is in its warming up cycles (ie the first 5 minutes) so you add the warming up + the A/C use and you are using alot of battery

[mfa-prius]

Originally Posted by morpheusx

it could be that you are driving shorter routes then you used to as well. The battery usage to propel the car is much greater while the the vehicle is in its warming up cycles (ie the first 5 minutes) so you add the warming up + the A/C use and you are using alot of battery

Don't think so. Wednesday's test was done on the last 5 miles of a 40-mile trip. It's a relatively flat stretch of road, 40mph limit, constant speed. I set the cruise control, toggled the A/C on and off about once a minute, and watched the instantaneous economy bars jump up (A/C off) and down (A/C on) about 15 mpg.

[bedrock8x]

With high temperature 90+ and high humidity, the AC compressor will draw 4KW/hour. Just doing some simple calculation, typically the Prius will need 250Wh/miles driven, so for 4KWh is equivalent to 16 miles driven in an hour.

The power will come from the ICE and this power is robbed from the FE,
this is like losing 16 mpg with the AC on. This is an approximation to explain why the drop in mpg.

Originally Posted by mfa-prius

Don't think so. Wednesday's test was done on the last 5 miles of a 40-mile trip. It's a relatively flat stretch of road, 40mph limit, constant speed. I set the cruise control, toggled the A/C on and off about once a minute, and watched the instantaneous economy bars jump up (A/C off) and down (A/C on) about 15 mpg.

[ETP]

Originally Posted by mfa-prius

I can accept some additional drain, but this is something new. Never noticed significant mileage drop with A/C until last week, had the car for 4 1/2 years.

You might need to check everything over just to be sure. 0W30 oil, air filter, tire pressure, change transmission oil, fan on low and recirculate, coolant drain and fill, and most of all have your wife push at stop and go traffic.
Your AC needs a full complete check up. Did they actually do a freon pressure check and an AC compressor cycle test or just shove you out the door? Sounds like a compressor/electric motor bogging down to me.
Get a second opinion.

[mfa-prius]

Originally Posted by ETP

You might need to check everything over just to be sure. 0W30 oil, air filter, tire pressure, change transmission oil, fan on low and recirculate, coolant drain and fill, and most of all have your wife push at stop and go traffic.
Your AC needs a full complete check up. Did they actually do a freon pressure check and an AC compressor cycle test or just shove you out the door? Sounds like a compressor/electric motor bogging down to me.
Get a second opinion.

That's better advice than I got from dealer #1, who just shoved me out the door, and is in agreement with what I feel. Fortunately, dealer #2 is about the same distance in the other direction, so I'll be seeing him next week.

Thanks also to bedrock8x for the engineering data. If correct, that still doesn't explain why I haven't seen this in the last 4 years, but makes me feel better about seeing it now. Where did you get the 4KW/hour figure?