Idling engine shudders when A/C on

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Over the weekend, I drove my MIL's PT Cruiser and noticed the engine was having issues with the A/C engaged. When idling, every ~10-20 seconds the engine would shudder, the tachometer would jump around, and the car felt like it was going to stall. It never actually did stall out. The car never had a particularly smooth idle but it was never this bad. The problem seems to go away with the A/C turned off. I've done the usual stuff -- new air filter, new plugs & wires, ran a bottle of Techron through the tank. I wasn't sure what the next step should be.
 
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22,698
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those cars have problems with the electric cooling fan.If the fan doesn't come on when the ac is turned on the ac aystem will build up too much pressure and cause the problem you descdribed.Go look to see if the fan is on when the ac is on.
 

kb01

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Originally Posted By: Chris142
those cars have problems with the electric cooling fan.If the fan doesn't come on when the ac is turned on the ac aystem will build up too much pressure and cause the problem you descdribed.Go look to see if the fan is on when the ac is on.
Excuse my ignorance. Check the radiator fan or is it something else? As far as I know, the fuel filter has never been changed (I think it requires dropping the tank). The IAC crossed my mind too, as did a throttle body cleaning, or possibly that it's low on refrigerant. My MIL is on a fixed income, so I'm kind of scared about throwing parts at a problem that is more of a nuisance than anything. The car is 6 years old and is only used for local driving. It only has around 35,000 miles.
 
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Indiana
Originally Posted By: KrisZ
Take a look at the throttle body, it may need some cleaning.
That is what has always caused my Fords to do this. The throttle body butterfly area gets dirty and sticks so the engine cant make the slight adjustments it needs when the added load comes on. A good dose of throttle body cleaner twice a year keeps it clean for me. You might also have another problem that needs addressed too but this area will get dirty.Good Luck!
 

kb01

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Wow, thank you for all of the advice! So, a reasonable course of action would be to: 1) Start car, run AC and see if radiator fan is turning on. If not, replace the fan. 2) Clean the throttle body. 3) Replace Idle Air Control valve 4) Fuel filter. According to all those links posted above, issues with the fan seem to be a major problem.
 
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Really stupid design with the Cooling system on these cars. The low pressure cycling switch that cycles the compressor on and off also cycles the Radiator fan on and off at the same time. This constant on and off burnes up the radiator fans lower speed. High speed still works if the engine gets hot enough.
 
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Sunny Florida
You may have that wrong if my Chrysler product has the same strategy. In my car the fans are on anytime the climate control is engaged. But the low speed is HALF voltage. That is not likely to do much at all to a motor rated for 12-14 volts! Trust me, if the fan is bad at low speed it isn't going to run high speed. Maybe a relay dropped but the motor is either good or bad, period.
 
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Originally Posted By: SteveSRT8
You may have that wrong if my Chrysler product has the same strategy. In my car the fans are on anytime the climate control is engaged. But the low speed is HALF voltage. That is not likely to do much at all to a motor rated for 12-14 volts! Trust me, if the fan is bad at low speed it isn't going to run high speed. Maybe a relay dropped but the motor is either good or bad, period.
The Pt fan motor is actully 2 motors with 2 sets of windings......Trust me when I say that one speed can stop working and the other speed works fine. I see 2-3 of those cars with the same problem every week.
 

kb01

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Originally Posted By: Chris142
The Pt fan motor is actully 2 motors with 2 sets of windings......Trust me when I say that one speed can stop working and the other speed works fine. I see 2-3 of those cars with the same problem every week.
Can you recommend an aftermarket replacement or do you stick with OEM? I just confirmed that the fan isn't engaging when idling with the AC on. The shuddering also seems to be every 6 seconds and the AC doesn't seem to be running at all, unless the car is moving. I also drove around the block several times to see if I could get the fan to kick in, with no luck. That is of course, unless the fan is super quiet to a point of not creating any air flow in the engine bay.
 
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Could be a TPS if it has one. Symptoms are up and down idle every few seconds. Replace it anyways, not expensive. A dirty IAC V symptom is idle too low when warm. When AC kicks in gets real low because of the compressor load and attempts to recover. Process repeats. Slower than the TPS symptom. An AC compressor that hasn't been charged in a long time could be low on, or have spent PAG oil = hard turning compressor, loads the engine down when the compressor clutch is engaged at idle.
 
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22,698
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Apple Valley, California
Originally Posted By: kb01
[quote=Chris142] Can you recommend an aftermarket replacement or do you stick with OEM?
The Chinese Aftermarket fans seem to out last the OE Dodge fans. I've only had 1 bad Chinese fan and the wiring was wrong and it ran backwards LOL
 
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Chicago Area
A throttle body cleaning and idle air control motor cleaning are very good maintenance, and need to be right. Most likely they will solve the problem, but if not, you did a very good thing . The fan problem may be the key, but that is very vehicle specific.
 

kb01

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I confirmed this morning that the fan isn't engaging at all, regardless of operating temperature, A/C on, or speed. I'm pretty sure it's the culprit.
 
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Texas
Odds are its indeed the fan- something of a weak spot on the PT. I disagree that the design of the way the fan works with the AC system is to blame, my daughter's 99 Jeep works that way and has its original fan at 140,000 miles. Also I think the PT actually changed the fan logic during the years it was produced- wifey's 2005 doesn't cycle the fan directly in sync with the AC clutch. Its under full computer control- the fan comes on briefly when you start the vehicle regardless of temp, then cycles occasionally as the engine comes up to temp (even with the AC off, presumably to move air through the engine compartment and also over the oil-to-air tranny cooler). With the AC on, it either stays off or runs at either high or low speed as dictated by the condensor high pressure switch.
 
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Sunny Florida
yes, modern vehicles run the fans for a variety of reasons. I was quite surprised that the same parameters trigger the PT fans as trigger my LX. On mine, the fans run at low all the time if climate control is on. I get a high speed burst when first started after a heat soak. Then I can get high speed for trans temp, AC high side pressure, or coolant temp.
 
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