Lower idle when removing oil cap
 

While I checked the oil level after warming up, I noticed that the idle RPM dropped somewhat and became less stable as soon as the cap was off. When putting back on, immediately RPM reset to normal. . It's a 3.2 86 Targa. What is happening here?

Every 911 I've ever had does this. I just assumed pressure is being released with the cap off.

Completely normal, you've created a vacuum leak when cap is removed. I'd be more concerned if it didn't do that.

If I was told correctly, that lowering of idle means your vacuum system is in good order. If it didn't change, it would mean air is getting into the system from another area. I expect one of the pros will be along in a bit to confirm/correct.

here is some reading...
Ivan
https://www.justanswer.com/porsche/8f1rm-porsche-911-targa-carrera-1987-porsche-911-targa-carrera-i-m.html

great feedback from all, always good the learn that what I thought to be a problem is actually dead normal ;)

As everyone has responded, a slight drop in RPM’s is normal. However, if it is stalls you should check to make sure that the brass Venturi that is inserted in the oil hose leading to the crankcase isn’t missing or out of position as the Venturi is in place to limit the amount of air that can be drawn into the crankcase.

Regards,
David

I added about a quart of oil today, and while the drop is still there when removing the cap, it is less pronounced than before I added the oil. Is this 'normal' too?

i hope you checked the oil per the proper procedure .. fully warmed up on level ground..

all that said id stop analyzing all this minutia and just go drive the thing..

yes I did, and don't worry about my mental health: I love analysing and discovering my car(s). Next to driving, it's the way I enjoy them. ;)

At running temp keep the oil level @ HALF WAY or less on the dipstick. this will allow for oil expansion and prevent unnecessary oil consumption.
Ian

THIS!

Engine at full temp, and the front cooler feeling hot, level ground, at idle wait a full minute, then get the oil level right between the two marks.

The temp is very important. Don’t even bother to check if the engine is not fully warmed up.

sorry but you have to remember not all climates are the same.. if i waited until it was " fully warmed up" i would never be able to check the oil between the end of October and the beginning of April.

Trick is to just learn your car , and understand what the oil level will look like at 170 degrees in January in North Dakota or Canada vs 200 in Florida or California ..
its pattern recognition that is important...

just my 2c.

I agree with you this time........
 

With the engine cold, if I see oil on the end of the dip stick/s (engine not running), I know I have sufficient oil in my cars (911). That’s good enough for me to drive the car out for a drive. On my return home, a second check would show the oil level between the lower and upper markers. Then add oil as needed.

Tony