This is in no way a joke. I have a 2002 Envoy with a rear air suspension/ load leveling system and I have a heck of a mystery on my hands. since I bought the vehicle a couple of years ago the compressor ran periodically as I was driving and I felt that this was abnormal. but a couple of months ago something started happening. the rear end would sometimes lose pressure and bottom out when i shut off the car then when I started the car it would blow up to the max height. this was very inconsistent and I could not find a rhyme or a reason for this behavior. I checked the sensors and they ohmed out properly so they didn't appear to be the problem. I still don't know the reason but about a week ago I figured out the rhyme. here'* the kicker and perhaps the biggest mystery of all time. the air only leaks out IF I SHUT OFF THE IGNITION WHEN IT IS DARK OUT!!! I have had some experiences And have done some tests to Confirm my findings. Like the following:
I shut off the car When it was light out Went on vacation for a week And when I came back it was still inflated.
I work at night so I shut off my car when its dark When I go out for lunch it is deflated So I start my car and it pumps up and in the morning it is deflated again.
When I get home in park my car in the morning it will stay inflated all day.

Well that is my huge mystery but I hope it'* not a mystery to everyone there must be at least 1 mechanic who knows what'* going on. I don't expect to get very many posts on the subject but please answer if you have any sugestions. I say good luck to me on finding an answer.

could be more of an intermittent leak i cant think of anything relating to day or night. if its pumping up a bunch of times while your driving its a leak. the trick is finding out where. take a spray bottle with some water and dish soap and spray everything down and look for bubbles. on my bonnie it was the struts themselves leaking around the rubber.

Perhaps the leak is related to hot or cold, since it is cooler at night. It really does not matter since in either case the problem is a leak in the system. Justin'* advice about spraying all the hoses and connections with soapy water is the way to go. The other option is if you have a garage or some other place that will reduce the noise level you might be able to hear the leak.

I thought about a slow leak myself but during the day when I park my car for ten hours or more (I talked about being gone for a week) the car does not drop at all. but while I'm at work at night it will fully deflate in less than three hours. This is not temperature related either because temps have ranged from zero to seventy since this has been going on. But thanks for your input I will be sure to look for some small leaks when I get this problem solved.

theres no need to complicate things, a leak is a leak and regardless how much it leaks when if you fix it you likely wont have any other issues

rkalos - I had a similiar situation with my Range Rover.

As it turned out, the rolling of the air bag during driving creates a fatigue spot in the rubber, (mine dry cracked in the center)

If it wasn't loaded, it might leak down overnite, but with a load of cargo, or while driving, it would stay inflated for days.

I replaced the bags with arnott air bags and new oring seals and didn't have any issues for the 2 yrs. I kept it.

Spraying the whole system down with soap bubles will find your leak(*)

luck,greg