QuestionMy 1999 Jetta runs very well until the A/C is on. At idle it seems to surge from 550 -850 rpm. While driving and coming to stop it will sometimes stall. When the AC is off it smoothens out. What do you think?
AnswerHi there Bill;
Remember that turning on the air conditioning in any car adds a load. So, when the air is turned on in your car, the engine control module sees this as a load, like going up a hill. So, if the rpm range fluctuates, either the ECM sees the load come, and go, like going up a hill, and then going down on the other side, OR one of the "input devices"....(like the engine coolant temperature sensor, or the mass air flow sensor, or the throttle position sensor....etc.) is fluctuating in it's behavior as the air conditioning is turned on, and off.
In this particular case, I am assuming that you have "Climatic," and not "Climatronic" air conditioning. In other words, you have a manually operated system. You select the fan speed, and you select the temperature range, and you push the button to turn on the air conditioning. Not the "Climatronic," where you select a temperature, and the system does the rest.
So, I'll cut to the chase. It sounds to me like the problem is in the air conditioning system itself. The compressor is the "load." When the refrigerant is able to flow without restriction through the thermal expansion valve, the engine runs just fine. However, when there is significant increase in resistance to flow of the refrigerant inside the system, the load increases, and the engine slows, or stalls. In the shop we would watch the pressures on a "manifold, and gauge set." But since I am doing this without the benefit of even seeing the car, I would have to say that it sounds like there is a small amount of moisture inside the closed air conditioning system...ie, a little bit of water. When the water gets to the thermal expansion valve...and this is where the system gets cold.....it freezes into little hard crystals of ice, and blocks the flow of refrigerant for a short period of time, and the engine rpm goes down, and the temperature of the thermal expansion valve goes up, because the flow of refrigerant is blocked, so it can't stay cold, the water rushes through the "TXV" and the system runs OK, until that little packet of water droplets gets back to the same spot, and blocks the TXV again, and the load increases, and the engine slows down. This happens again, and again. The system is 8, almost 9 years old. I would say the thing to do is recover the R134a that is in the A/C, remove, and replace the receiver/dryer, or accumulator/dryer(one goes on an R-12 system the other goes on a 134a system...I can't remember off hand which one goes with which one, but anyway....)whichever one you have, and then make sure all of the fittings, and unions are tight. Then evacuate the system, down to 28 inches of mercury for 30 to 45 minutes, then close the system to see if the vacuum holds, and if it does, recharge the system with 134a in the amount that is listed on the tag, or sticker under the hood...it's like almost 1 kg., or 2.2 lbs....a little less though. If you want to, and it might be a good idea, add an ounce or so of florescent dye so that any leak detection, in the future would be made easier.
If you don't have all of the equipment, or question the process, take the car to an air conditioning specialist, and let them do it. The whole process, including parts, as long as the compressor is still OK will be less than $300.00 I figure. If the compressor gives out due to the increased pressure in the system, the cost will go to $800.00...just like that.
Diagnosing this stuff over the Internet, without the opportunity to even look at the car isn't as easy as it looks.
The only possibility I can think of, if it were an engine problem, and not an A/C problem would be the mass air flow sensor. But I don't think it's that here, because you say the engine smoothes out when the A/C is turned off.
Hope this is of some use, Bill. Good luck.
