Why would the A/C make my car accelerate rough?
July 30, 2011 12:44 PM   Subscribe

Why does my car's air conditioner cause it to hesitate when accelerating?

I have recently been having an issue with my car hesitating during acceleration, especially from a complete stop. I had suspected it was the fuel injector and I went to my (very trustworthy) mechanic the other day, who agreed. He suggested that I fill the tank up with high-octane gas and then pour in a fuel injector cleaner before embarking on a costly repair.

While driving to the mechanic's, I noticed that the car accelerated smoothly, which struck me as odd. After coming home from work that day, I got in the car and pulled out of a parking space...smooth acceleration. I switched on the A/C, came to a stop sign, and accelerated again. Jerky acceleration.

It suddenly occurred to me that I did not have the A/C running in the morning when driving to the mechanic's. I pulled over to a complete stop and turned the A/C off again to see what would happen - smooth acceleration. I then did the same with the A/C on - hesitation. I drove home with the A/C off, and it was smooth the whole way.

I then realized that I began experiencing this problem when I began running the air conditioner on a regular basis. It's pretty clear to me now that the A/C is somehow making the car hesitate during acceleration.

So my question is...WTF? This has never happened before. I've done a bit of googling on the issue, and it seems there are some electrical components that may be to blame, but I haven't found anything terribly specific. Also, if you can say what the problem is, about how much would it cost to fix?

BTW, car is a 1999 Camry with 135k miles. Previous owner was a relative and it's always been well-maintained. I had some major maintenance done a few months ago by said mechanic, who checked out the engine and everything in the process and said it's in good working order.
posted by breakin' the law to Travel & Transportation (9 answers total) 3 users marked this as a favorite
Your A/C compressor may be dragging against the belt and requires more energy from the engine to keep the compressor turning, thus the power during acceleration is reduced. If your usual mechanic doesn't work on air conditioner equipment (some don't, it requires equipment that can capture refrigerant and such), ask him if he knows someone who does. The compressor might just need to be oiled. However, if your compressor still runs on R12 (old "freon") and it needs to be serviced you may have to pay for a conversion to R134a (new "doesn't destroy the ozone coolant") before the equipment can be worked on.
posted by fireoyster at 1:09 PM on July 30, 2011

It is probably dragging on the belt, as has been suggested. It is possible that the belt itself is to blame rather than the AC compressor. It might need to be adjusted or replaced.
posted by twblalock at 1:14 PM on July 30, 2011

The extra drag from the belt should be compensated by an idle adjustment motor, or idle solenoid. Sometimes they get gummed up with carbon deposits. If you mention the AC symptoms to your mechanic, he or she can quickly take off the AIS motor and check for free motion. They can usually be cleaned and reinstalled.
posted by KevCed at 1:23 PM on July 30, 2011 [1 favorite]

The A/C unit requires no small portion of the entire power output of your engine at idle - we may be talking about up to 30%. If part of the A/C system isn't working right - worn out compressor, low freon charge, who knows - this power requirement may be even greater. Combine that with an older, more tired engine (or at least, one that might need plugs and a fuel system cleanup) and the switch from "idle" to "time to accelerate" is quite a change. Ergo, see if a fuel system clean out (the easiest change) works, first.
posted by notsnot at 1:27 PM on July 30, 2011

Like FO said, you're probably feeling your air conditioner compressor cycling on and off.

If you're feeling the air conditioner compressor cycling on and off at wide open throttle, which might be what you call acceleration, then you could have a malfunctioning air conditioning cutout relay, which disengages the air conditioner clutch when you are calling for more power from the engine, or when the engine is idling.

From what you describe, though, this might be normal behavior, and there might not be anything wrong with your injectors or air conditioner at all.

A 1999 Camry surely does not use R12, and if you can feel your air conditioner cycling on and off, you probably don't need to have it serviced. Do you get cold air from it?
posted by the Real Dan at 1:41 PM on July 30, 2011

Response by poster: A 1999 Camry surely does not use R12, and if you can feel your air conditioner cycling on and off, you probably don't need to have it serviced. Do you get cold air from it?

Yes, I do. I'm not sure that this is normal behavior, though, seeing as I have run the air conditioner many, many times in this car during summers past and this has never happened. By 'feel the air conditioner cycle and and off,' do you mean that it would not blow cold air periodically?
posted by breakin' the law at 1:48 PM on July 30, 2011

KevCed probably has it: idle air control. Cars aren't meant to feel boggy, and the tip-in point is changed quite a bit when the A/C compressor is running. The IAC is meant to compensate for that. If the car wasn't used with A/C much it might just be sticky. They are comparatively easy to clean, but fairly expensive.

Can you get codes pulled on the car? They may indicate something like this.

A somewhat more zebra-ish possibility is the purge control solenoid. If the bogging is worse when the car is cold, it might be worth checking next.
posted by jet_silver at 2:59 PM on July 30, 2011

AC is a huge power-sucker from vehicles. It is not your fuel injector, and PLEASE don't fill up with high-octane gas unless you literally drive some kind of cool street rod which none of the rest of us can touch.

Every vehicle I've ever owned has been sluggish off the line when the AC was on. One time my friend and I drove to Virginia Beach in my previous car. She kept fucking with me by turning the AC on and off. I'd be like "DUDE MY CAR'S SO FAST.....OH WAIT IT'S NOT." She thought that was hilarious. It really wasn't.

My current truck feels awesome all the time because I only turn on the AC to cool it off for two minutes.
posted by deep thought sunstar at 10:34 PM on July 30, 2011

I'm with the Real Dan on this, in troubleshooting spirit, if not in point of technicality. All cars made in the last 30 years have some sort of acceleration sensing AC cutout, to temporarily drop the AC compressor load from the engine when the engine is asked for maximum power for acceleration. This is usually done by temporarily dropping power to the AC compressor clutch whenever the throttle is opened to the point of hitting the transmission downshift point for a car equipped with an automatic transmission, or about 60% of full throttle opening on car with a manual transmission.

The air doesn't immediately start blowing warm, because there is some thermal mass, which is still cold, in your air conditioning system's evaporator, but if you can stay in the throttle, at full acceleration for 10 seconds or more, you may feel it warming up in the last couple of seconds, indicating that the AC acceleration compressor drop is working, or not. If not, you'll get stumbling, erratic acceleration, and constantly cold air, especially at the low end of your engine RPM until you get it fixed. Depending on your car, it could be a throttle position sensor, a MAPS sensor, an AC compressor control relay like the Real Dan suggests, or some other component in your engine management system.

A lot of auto parts stores will connect an OBD-II reader to your vehicle's diagnostic port, and give you a readout/explanation of any diagnostic codes your engine is recording for missing or poor acceleration, at no diagnostic charge. Fix/replace those items first, and your problem may be solved.
posted by paulsc at 9:42 PM on July 31, 2011

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