Testing an engine / buying a car

[Vultch]

Testing an engine - Surf or otherwise

I have heard / read that one test is to start the engine and then remove the oil filler cap, if the engine starts to run rough or makes a different noise there may be a problem.
Is this true, I have used my google-fu skills but I keep getting alternate answers.

[J i m s t e r]

What problem is that supposed to reveal? Oil pressure? There's a light for that. Unless seller has taken the bulb out / put black tape over the light...

I was told to inspect the oil filler cap for "Mayonnaise", if it's present, then it could be condensation from lack of use or worse, the head is on the way out.

[red barron]

you could take it for a test drive? Judge the performance and eat ginger?

[Vultch]

From what I can see, excessive pressure in the rocker cover sump etc is an indicator or excessive wear in the piston rings and your pressurising the engine. This can cause all sorts of problems least of which are oil leaks etc as seals, gaskets give up and poor performance reduced power / excessive oil consumption. Oh and if the PCV valve is stuck open it ruins the fuel mixture causing your engine to run lean...higher temps.

There always will be a little pressure in the engine but there is a system PCV valve? (positive crankcase ventilation) thats uses the engine vacuum to remove it as you rise above tickover.

The test

let engine warm up and carefully remove oil filler cap, place your hand over the 'hole' and it should be slightly pressurised at tickover you should feel the pressure pulsing in time with the engine pistons moving, as you increase the throttle this positive pressure should turn negative as the PCV valve does its thing so you should feel a gentle suction, if not the PVC gear is faulty or blocked with gunk.

If the engines changes note when you remove the oil filler cap it indicates the engine vent system was working and you have disturbed it.

Of course the above may be may be complete Bo**ocks but its as much as I can gleam at the moment.