2005 rough idle have tried everything!!!

[beautyfiend]

hey guys
so i have a 2005 vz auto 3.6l ecotech wagon.
it was idling rough when i bought it but being mechanically minded i thought it would be easy to diagnose. boy was i wrong.....
these are the things i have tried to fix the problem.
i currently always run the ac when driving cause having it on it doesn't idle as bad
my fuel consumption is pretty high at around 15L/100km



Replaced
oil/oil filter,
fuel filter (still had factory one from gm when car was assembled and its done 288,000kms)
air filter
spark plugs

Troubleshooting Ive tried
tested for vacuum leak by spraying carb cleaner near hoses etc
cleaned the throttle body
cleaned the purge valve
cleaned the maf sensor
checked fuses
checked hoses
checked coolant
checked transmission fluid levels and color

 

[grey_hawk]

Lower Intake Manifold gaskets?

 

[lout]

do a compression test

 

[beautyfiend]

how do i do that

 

[lout]

how do i do that

there are many ways
cheapo https://www.amazon.com.au/YUAN-Prof...8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
leakdown https://www.amazon.com.au/Cylinder-...8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
scan tool balance test depends on scan tool
current draw during cranking requires oscilloscope

 

[Fu Manchu]

I'd be doing a smoke test to find vacuum leaks. Using solvemts to do it will not be comprehnsive. Smoke testing can find the smallest leaks and you'll be able to visually detect them. Isolate the interior vacuum line off the back of the intake. Then isolate the brake booster line by plugging it with a bolt and a hose clamp.

Where did you source the spark plugs? The previous owner could have used some cheap eBay counterfeit plugs without knowing they were. Then you could have done the same without realising. No one would be able to spot the cause being faulty plugs. This would be diagnosed using osciloscope, looking at the spark wave form. Or you would source plugs from an actual shop you walk into like Ripco or Supercrap.

Do the PCV mod. Theres a thread I did in the How To section.
Clean out the PCV hoses or replace them with new ones.

Make sure you've used the metal intake plenum gaskets when you had the plenum off.

Some intake plenums have plastic barbs for the PCV lines. One type has an o-ring that perish. They pull out. Others screw in. Then all going well, you have the ones that have a brass barbs that are pressed in and don't fail.