Unscrewing Coolant Tank & Oil Filler Cap on Running Engine: Check Bubbles, Drops of Oil, & Effects

The old mechanic said buying a car with the engine running
you can unscrew the coolant tank cap and check
whether it beats "does not put bubbles" and whether there are no drops of oil

What does it mean when it beats these bubbles?

And how is it with unscrewing the oil filler cap when the engine is running?
Can you do something like this?

Apparently, when the car goes out after unscrewing the filler cap, it is better to give it a rest, why?

Can someone explain to me?

As for the "bubbles", "beating" in the coolant - if such a thing occurs, it means damage to the gasket under the head. This also applies to the presence of oil in the coolant. As for the oil filler cap - I'm not sure what the old sages mean by this, but maybe it's about a lack of pressure. The old Polonaises even had such a clock on the dashboard showing the oil pressure. If the pressure is too low, it may be caused by, for example, lack of compression by the pistons

cordoba_2004 wrote:

Apparently, when the car goes out after unscrewing the filler cap, it is better to give it a rest, why?

Can someone explain to me?

It meant very large "blow-bys", that is very low compression pressure on at least one cylinder. But when it goes out, it's already hardcore. There should be no visible exhaust fumes or oil spray (single droplets will always be there).
But this is deceptive and we are not talking about all engines. I remember a Mazda, in which the engine always went out after unscrewing the filler cap - such a design and it was probably a V6 engine. So it is different.

But such methods of checking a car when you buy it, do not allow it, because if you do not even know what it is, you will not detect anything - they are good if you know what you are doing and what you are looking for.

Please visit the cool forum www.bezwypadkowe.net, there you will find out more.

Karol966 wrote:

The old Polonaises even had such a clock on the dashboard showing the oil pressure. If the pressure is too low, it may be caused by, for example, lack of compression by the pistons

But you're talking nonsense! What does compression in cylinders have to do with oil pressure in the lubrication system?

When it comes to "bubble" in the expansion tank (or radiator), this means that the exhaust gases get partially into the cooling system. This is caused either by a damaged (burning) gasket under the head, or microcracks in the head itself. a well-warmed engine and with "sharp pogazówkach".

When it comes to unscrewing the oil filler plugs on a working engine and observing "blowouts and oil splashes", the situation is different. One engines, for example: have a plastic or metal cover just above the valves, which effectively prevents direct splashing of oil to the sides by working valves of other covers They do not have. Some blow more, others less, some reduce the revs (and sometimes increase), other engines can even stop at low revs. Remember that the "top of the engine" (conventionally: over the pistons, i.e. compression chambers, intake manifolds, air filters, etc.) and the "bottom of the engine" (i.e. conventionally: "under the pistons", i.e. generally the oil pan) are connected with each other by the so-called: pneumothorax. When unscrewing the oil plug, "pressure imbalance between the top and bottom of the engine" occurs, these changes are captured by pressure sensors / vacuum, flow meters, etc., introducing "chaos in calculations", with carburetor engines there was a temporary change in fuel atomization, causing waviness of engine rotations.
I will say this: if we take the first better engine and put on it, let's say 6 (better or worse) mechanics, and with the engine running, we unscrew the oil filler cap, the 3 will say that the engine is in perfect condition, the other three will say that the engine requires a general overhaul This is a very subtle observation and everyone has an opinion on the "strength and quality of blows from under the plug".

PS: By the way, I am curious if there is any instrument, a pressure gauge that can be screwed in the place of the oil filler cap and can "quantify the technical condition of the engine on the basis of the excess pressure blown out by the cap" ???.