Mist coming from AC on decelleration ?

[smokum]

I've noticed when I have my AC on (normal, fan selector on #1) and make a hard (foot to floor) pass on the highway that when I come off the pedal and decellerate to speed that a mist/vapour comes out my center dash vent.

Its happened a few times (but not always), and when it does happen, if I switch to either MAX AC or to just vent with the fan on #1 position it goes away immediately, however if I switch back to REG AC it will mist/vapour again for a while and then stabilize.

This has happened on both blistering hot as well as just warm days. My AC is ICY cold and with no apparent issues, noises, odours, or window film deposits, etc.

Any idea's ??

Thanks,
Greg

[shelbyx1]

is it a mist or tiny white flakes?

[smokum]

Sorry, was early when I posted.......

A better discription would have been "fog" with no slippery residue.

-Greg

[b4000]

This is just sort of a guess, I'm not an expert.

It sounds like what is happening is that I think the AC system is designed to run optimally with the AC compressor running at "normal" engine RPM.

But, when you gun the engine, you also momentarily increase the RPM of the AC compressor, because it's driven off the crankshaft pulley via a belt. As a result, the differential pressure across the evaporator expansion valve increases, making the AC system increase it's output.

The expansion valve is usually thermostatically controlled by a capillary sensor the reads the temperature of the evaporator coil and regulates the amount of refrigerant passing through the coil. Like so:

http://image.rodandcustommagazine.com/f/9817792/0805rc_03_z+air_conditioning_system+.jpg

It takes a certain amount of time for the expansion valve to respond to the temperature change in the evaporator coil. Usually that's not a problem at fairly constant RPM. Theoretically, I think if you kept up the high RPM for an extended period, it would give enough time for the expansion valve to compensate.

In other words, the increase in RPM on the AC compressor is almost instantaneous, but the AC system itself has a latency period before it can adjust to the new conditions.

As you observed, it takes a short time for the system to stabilize after you take your foot off the gas. Until it does, the evaporator coil is still colder than normal, until the expansion valve catches up and puts things back to normal. The fog you see is caused by the super-cooled air condensing the ambient water vapour, and then goes away, once the evaporator coil warms up a bit.

If the AC is working fine, like you say, then my guess is it's acting in a way you'd expect and there's no real problem.

Does that make sense, or am I on crack? :icon_surprised:

[smokum]

That make a lot of "logical" sense I'm sure...... so much and so far out of my realm of mechanical understanding that I'll just have to take your word for it, lol.

Other than what I've mentioned, the system on the whole works very well. I've also had comments that when the truck is off and sitting after driving with the AC on mild, that the amount of condensate that accululates drips under the truck on the ground is rediculasin comparison to what anyone has seen, as it actually makes quite a large puddle. What that means........ I'm really not sure.

All I "am" sure about it that I don't want to remove the coil/heaterbox/or anything else within the entire system anytime soon, lol.....

Thank-You for taking the time to explain the system works and providing a diagram.

-Greg

[b4000]

All I "am" sure about it that I don't want to remove the coil/heaterbox/or anything else within the entire system anytime soon, lol...

I don't blame you! Sounds like a nightmare job. And if it ain't broke ...