Engine surges when heat or A/C is on

2000 MERCURY MARQUIS
138,000 MILES • 4.6L • V8 • 2WD • AUTOMATIC
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MGOBLUE78
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When in gear at a stop with heat or A/C on the car will lunge/engine surge. Does not do this when heat or A/C is off. Any idea why it does this?
Apr 27, 2019 at 10:19 AM
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BMDOUBLE
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There are literally dozens of possible scenarios that can cause this. First start with the basic visual inspection of the intake and hoses for vacuum leaks. Also remove the belt and turn the compressor by hand, it should turn relatively easy when rotating the center part of the pulley, if it is as hard as opening a Gatorade bottle, then the compressor is bad. Also verify that the voltage does not drop excessively when the compressor kicks on, lower than 10 volts will indicate a voltage issue which could be anything from a weak battery to a weak ground.
Apr 27, 2019 at 10:48 AM
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MGOBLUE78
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Thank you for the help. I will check this out.
Apr 27, 2019 at 11:02 AM
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STRAILER
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Hello,

It sounds like the AC is low on charge which cycles the compressor, but to be sure here is a guide to help you test the system charge:

https://youtu.be/uZrQCGwXfek

Please let us know what you find. We are interested to see what it is.

Apr 29, 2019 at 1:07 PM