1500-2000rpm idle once warm - I know I know

[littlered]

So I have a 93 318i M42
Since the day I bought it it has had a irratic idle .
Meaning - When you start the car from cold , it does its normal high idle until its fully warmed up then idle sits nice around 900rpms .
Once you shut it off , like to go into a store and then come back and fire it up , the idle sits at 1500-2000 rpms and stays there at idle until the car cools off fully .
Things I have replaced .....
Vac hoses
Intake boot
ICV
spark plugs
wires
fuel filter
Coolant temp sensor
Throttle position sensor
cleaned throttle body
intake manifold gasket
valve cover gasket
throttle body gasket
air filter
fuel injector O rings
Camshaft position sensor
Timing chain tensioner (for a different reason)
All to no avail

I have a peake scanner and I get nothing and no CEL light on either.

Can a bad O2 sensor cause this ?
Thats about the only thing I have not replaced.

Any ideas would be greatly appreciated

[littlered]

Also , I notice during the cold start and its 1st run cycle where the idle is smooth as butter my timing chain is smooth as butter as far as audible noise.
Once you shut the motor off , after its warm and idling properly , when you restart it , the idle is high between 1500-2000 rpm and the timing chain is noisy .
I have already replaced the chain tensioner .

[Darky]

Hi

How's your oil pressure and type of oil?

Cheers Rohan

[littlered]

Fresh oil Rotella 5w40 synthetic and filter .
Suspect the oil pressure is good since I just completely rebuilt the head .
Too thin of oil maybe ?
strange though as my chain is quite as a mouse when the idle is proper and quite thru all the power band when the idle is working properly.
noisy at about the 1500-2000 range once the idle goes poo poo

[littlered]

The motor in question ......

[Darky]

Yes I'm thinking it might be to thin but it depends on your ambient temperature?

[littlered]

Its been low to upper 80's here in Bend Oregon

[littlered]

So a short update ......
Changed the oil from Rotella Syn 5w40 to Rotella T regular oil 15w40 and that seemed to quiet down the chain chatter .
Spent some time going back thru my vac lines to address the idle and I though , well .... lets pull the ICV (which is brand new) and reinstall my old cleaned one .
So far .... its idling warm at 900-1000rpms so thats a definite improvement .
Since I reset the DME hopefully some run time and it will settle in at a constant 800 ... lets see.