Changed the Gearbox and Diff Oil today on my 2.0 Sport 2006. When I removed the fill plug from the gearbox, air escaped as if it was pressurised. Is this normal? If not, should I be concerned/how should I go about fixing?
Many thanks.
There is a breather on the gear box and diff, I always give the caps a twizzle during service to keep them clear. The diff one is visible on the top of the diff when viewed from the rear, you’ll have to have a look for the gearbox one as I can’t remember its exact location.
Thanks!
I assume this is the vent, drivers side just back from bell housing?
Probably just going to be a case of reaching up the side from below.
It depends on what else you were doing at the same time, or just had done and you forgot to tell us the full story.
There is no need to touch the breather.
Hi, only changed the gearbox oil and diff oil, nothing else.
When I opened the fill plug, there was a hiss of pressure being released which I thought was odd, that is the full story 
Was the oil draining out as you undid the fill plug?
Yes it was!
There is the missing info, and here is the answer:
Gearboxes are not under pressure, what you caused there by draining the oil off is the opposite - a vacuum. When you released the filler plug air was rushing in to fill the void left by the escaping oil.
Strangely, I’m sure the air noise was first, oil only came out once the plug was fully removed. Perhaps the air was filling the vacuum through the small gap created towards the top of the plug and the oil half way up the plug could only pass once the plug was out. Slight vacuum when the oil is cool could be normal?
