“Testing” compression through oil cap.

If you remove the oil cap, with the engine at idle, and a good amount of crankcase gas pumps out of the hole, is that a rough indicator of a bad compression engine?
 

I've noticed that some vehicles pump quite a bit of gas and oil droplets out, while some have no blow-by.  
 

Thank you. 

Typically, yes. That is called blow-by. Usually indicative of worn piston rings.

Yeah with a working PCV system and a normal-ish amount of blowby gas, if you open the oil cap at idle that should cause a vacuum leak.

Beware that some engines will spew a mighty oil fountain out of the oil cap if you try this though.

I'll agree with Cousin_Eddie, that it can indicate worn rings, but it's subjective and there can be other causes. 

What are you trying to determine based on blowby?

If you are looking at a used car, then excessive blow is potentially a red flag or at least a negotiating point. 

If you are trying to diagnose an issue, blow-by is just an observation that may steer you in a particular direction for further testing. Also check that it's air only moving out, and not just air pumping in and out as the pistons move up and down in the cylinders.

Remember, each piston is moving just as much air below the piston as in the cylinder. That creates noise, air movement, and oil mist when you open the cap for the oil fill.

Depending on the engine, there may be mechanical component that sling oil out the fill (1990's Corolla cam gears are an example). Firing order, crankshaft design, and crankcase volume can affect how the air moves when the engine is running. Crankcases can be pressurized by other systems connected to the crankcase such as fuel pump, air/vaccuum pumps, turbos, air compressors (vehicles with air brakes), bad head gaskets, and poorly operating PCV systems, just to name a few  

GameboyRMH said:

Yeah with a working PCV system and a normal-ish amount of blowby gas, if you open the oil cap at idle that should cause a vacuum leak.

Beware that some engines will spew a mighty oil fountain out of the oil cap if you try this though.

Tangentially related....

Assume a tbi 454.  Hypothetically, if you lost the oil cap and just shoved a rag in there, would that cause things like the vacuum-operated EGR system to not be quite right?

Hypothetically, of course.

TBI 454 should run with woodchips in place of the oil.  

Diesel bros check for blowby by removing the oil fill cap on a warm idling engine.  A little smoke/oil vapor should come out.  Danger is if it looks like a freight train (puff puff puff) or if you see one intermittent puff along with hearing an audible stumble/miss at idle.

That all being said, an engine can have a ton of blowby and still run just fine.  

The way to check this is by running a pressure gauge on the dipstick tube, with and without the PCV blocked off.

The problem with "cap off causes wind" is that all engines do that.  If you have a 2 liter engine, there's two liters of air getting pumped around in the crankcase per revolution.  Pistons move air on both sides. Because the airflow in a crankcase is generally poor (making room for airflow means some combination of insufficient crank counterweighting, increased engine dimensions, or reduced strength due to windows in the mains webbing)  this makes for some pressure pulses moving up and down the cylinder head oil drain backs.  In some engines like Honda D series and Nissan RBs, this is even enough to hinder oil return to the pan.  You see the air pumping out because it carries oil mist, but you don't see it getting sucked back in.

And if you are thinking, hmm, can this crankcase pumping be used to supercharge an engine... you are imagining a crankcase scavenged 2 stroke engine and they work rather well 

(You know what doesn't have all sorts of air moving back and forth in the crankcase all the time?  WANKELS.  )