Air pressure under oil filler cap. - Ben {P}
Just had a guy come and look at my mondeo estate. All was going well untill i started the engine. There was no tappet noise, despite the cold weather up here. No smoke etc. All sounded spot on to me. Then his expert friend removed the oil filer cap.

The bloke then said, "nope sorry, don't want it, too much pressure there."

Now i thought plenty of pressure here was a sign of a healthy oil pump. Am i wrong?

Air pressure under oil filler cap. - Sooty Tailpipes
Yes, nothing to do with the oil pump/pressure, as the oil is in a closed circuit.
It will be blow-by gases, how bad is it, were they cloudy vapours?
Air pressure under oil filler cap. - Ben {P}
The vapours were not cloudy at all. Its a 99v with 66k and full history. Are you saying this is a sign of bore wear then? I have had two mondeo's previously with this sort of milleage, and the pressure has been about the same on all of them.
Air pressure under oil filler cap. - Robin the Technician
Hi Ben,
Clearly this person didn't understand the reason behind the 'pressure'. Pressure inside the engine is quite normal due to it being a fully enclosed system. The usual way this disposed of is via a breather pipe usually from the crankcase or from the rocker cover area depending on the design of the engine. If there were a problem with excessive pressure, your engine would be plagued with oil leaks (which I assume it is'nt).

Hope this helps.

These are the views of Robin the Technician with 35 years in the trade. I fix, therefore I am...
Air pressure under oil filler cap. - Ben {P}
Thanks for the responses above.

I can't find any oil leaks. I have not added any oil for the time i have had it. In this time i have done about 3.5k miles. There have been no perceivable sign of oil consumption going by the dipstick. This engine performs far better than the other mondeo i have had in the past.
Air pressure under oil filler cap. - Sooty Tailpipes
Sounds like theses people who came to look at your car had already decided they didn't want it when they took the cap off. I wouldn't worry, I doubt anyone else will complain.
Air pressure under oil filler cap. - RogerL
Every engine has blow-by gases, whether it's brand new, just run-in, nicely bedded-in, high mileage or knackered. This is because the piston rings aren't a perfect seal. The blow-by will vary during the warm-up cycle and will gradually increase as the engine covers more miles.

This case sounds to me like a nerd who thinks he/she knows it all but knows very little in truth. Many of us have sold cars to nerds like this who "knowledgeably" point out "the perfectly obvious" while missing the clear signs of major faults which are the reason we're selling the car! Put this down to a time-waster.
Air pressure under oil filler cap. - DysonC
This pressure is quite normal and is due to the crankcase gas recirculation system.
Air pressure under oil filler cap. - Cliff Pope
I think the nerd is probably right, but for the wrong reason.
Yes, all engines have blow-by gases which if unrelieved would cause crankcase pressure build-up. But in all engines I have encountered there is a ventilation/circulation system based on suction from the inlet manifold which if working properly more than compensates for the pressure. What often happens is that the small orifice in the connector to the inlet manifold blocks up over time, so the circulation system gets sluggish, pressure builds up, and this can cause oil leaks. Cleaning out the orifice is often neglected at services.
In my experience there should never be any pressure under the filler cap, rather a suction showing the recirculation system is working.
Unless there are special features of this engine, I would have thought bore wear is extremely unlikely, but it might be worth checking the engine ventilation system for a minor constriction.
Air pressure under oil filler cap. - M.M
Every post makes a fair point but Cliff gets the nearest to it.

Frankly few folks can judge crankcase pressure from whipping off the cap and looking/listening.

The only way you will ever really know is to fit a balloon over the filler! And then the way systems work due to the emission control design will often confuse.

Example my old Land Rover diesel....

As purchased the engine was about finished. The underside of the oil filler was covered with oily crud and taking it off while the engine was running would cause a plume of vapour and very definate positive pressure...due to bore wear.

Now the engine has been rebuilt and run-in the results are interesting. The underside of the filler stays oil free. With a balloon over it, and the engine at idle, there is a mild pulsing of the balloon but it doesn't even start to inflate. Rev the engine and the crancase pressure goes negative so the balloon actually sucks down a little into the filler. This is the emissions control design at work whereby the hose from the valve cover to the inlet manifold draws the fumes away to be consumed in combustion.

So I too expect this guy didn't know what he was looking at or wanted an excuse. Also unlikely at 66K with history you have much wrong.

But also note it is not so clear cut that you can say there should be/not be any feeling of pressure.

Air pressure under oil filler cap. - Ben {P}
Thanks for all the repsonses.

I only noted any outward pressure at idle. I would of described it more as turbulance. There may well be an issue with the inlet side of things, the car sounds very slightly more rorty than others i have had, but this could be exhaust related. But as it runs so well, and the noise is only VERY slight, i have never bothered to investigate. I'm interested how this ventilation system from the inlet manifold might work now.

At the time of viewing i did suggest the air coming from the rocker cover was unlikely to indicate possible big end problems as their expert suggested. I have received a call this afternoon from the couple saying they now deffinately want to buy the car!... I haven't put a date on the receipt just yet.
Air pressure under oil filler cap. - jc
It is a legal requirement on new cars that there is a depression in the crankcase at idle and at two conditions under load;this has to be demonstrated to the certifying authority by the manufacturer-if your car cannot do this there is either a problem with the crankcase ventilation system or your engine is knackered.
Air pressure under oil filler cap. - DL
Don't forget the air the is displaced in the crankcase by the rapidly moving pistons - this gives rise to the pulsing effect when you take the cap off.

-- - Pictures say a thousand words.....
Air pressure under oil filler cap. - jc
Turbulence in the crankcase-yes.Pulsation-no.The air displaced by the pistons going down must equal that of those going up or it wouldn't work.

Value my car