Posted by: @carlosg

: 40, 50, and 20 psi on cyl 6, 4, and 2

those cylinders are all on one bank.

it looks a lot like failed head gasket.

failure on 3 cylinders is too severe for additive to fix. It's likely to just make matters worse.

Posted by: @carlosg

I had already replaced transmission 3 years ago. If I pull the head myself it will be cheaper than buying another SUV.

If you can do the work yourself (most people can't) that's a horse of a different color.

Although head gasket seems most likely of course you may find other problems. Might want to use a borescope to check cylinder condition before doing all that work, and check the other 3 cylinders for compression as well. The GM 3.6 is notorious for timing chain failure and it has 3 timing chains. A chain could have stretched and skipped a few teeth, throwing off valve timing.

Once you're satisfied the head gasket is the problem you'll want to be sure to check the head and block for warpage when the head is removed.

have you tried boosting it from another running vehicle?

Posted by: @carlosg

I just checked compression on 3 cylinders and see very low compression: 40, 50, and 20 psi on cyl 6, 4, and 2

Your engine unfortunately is shot, probably a blown head gasket as indicated by @imperator. Cost to repair is very likely to exceed the value of a 16-year-old Acadia. Even if you did repair the engine, those vehicles are notorious for transmission failure and chances are the trans would blow out soon afterwards. Realistically it's time to shop for a replacement vehicle.

https://www.carcomplaints.com/GMC/Acadia/2008/

I did a cylinder leak down test with homemade tester. See 50 psi out of 60 psi.
Used balloon to see it inflate some to identify compression stroke, then put long screwdriver and little further turn of crank showed it reach top.
Loud hiss/leak is heard out of other cylinder.
Cyl #   Leaks out of
4            2
6            4
1            5
3            1
I did not test cyl 2 since I could not identify the compression stroke (balloon did not inflate any) .
I also did not check cyl 5.
I am puzzled why cylinder 3 leaks out of 1 but 1 leaks out of 5?
If it was a head gasket issue, would not 3 leaking out of 1 mean 1 would leak out of 3?
What does it all mean? Needs a head gasket, cracked head, valve job, or timing chains?

I learned all that leaking from one cyl to another is likely because valves are not seating.. because of timing failure leaving valves open. I'll check with borescope. I now understand it is an interference engine and so valves could be bent.  Would it be advisable to replace chains and set timing and then check if valves seat before considering removing heads? Replacing valves myself looks easy with a compression tool but want to avoid unnecessary work.

Posted by: @carlosg

I learned all that leaking from one cyl to another is likely because valves are not seating.. because of timing failure leaving valves open. I'll check with borescope.

See what you can with a borescope but you're getting to the point where the heads and timing chain cover really need to be removed to see what's going on inside that engine. If you're lucky the valve timing is off just enough to cause compression problems without damaging valves, bores, or pistons.

The timing chain setup on the GM 3.6 is complex. If valve timing is off it could be due to any of the 3 timing chains or all of them.

You'll want to get all your ducks in a row. This is not a job you want to do more than once.

Posted by: @carlosg

Could something be wrong with the crankcase? I will try pumping air into a cylinder and see if it comes out of another cylinder again as before.

I'd think if something bad was going on in the bottom end you'd heard some knocking down there when it does start. A leakdown test is probably a good idea.

Did you send the cylinder heads to a machine shop to check them for problems? Did the valve seats look OK? New valves lapped into the seats for a good seal?