Gearbox oil leak: breather, overfill and seals

What happened

An owner had a new gearbox fitted, drove it for a few minutes and noticed a plume of grey smoke behind him. Looking under the car, he found the nearside suspension and driveshaft dripping in yellowy oil. He did not know whether the oil in his old gearbox had been red.

What owners suggested

1. The breather. several owners both said to check it first. One owner's idea: the box has been overfilled and the excess is coming out of the breather near the gear links. The breather is the small nipple just above the gear change shaft, and it usually has a plastic top pushed into the hole. Another owner's turned out to have no bung, which he thought was the cause.
2. Overfill. One owner said the only way to see whether it has been overfilled is to take the undertray off, remove the filler bung while catching the excess, and when the oil stops coming out that is the right level. He said it typically takes 1.9 litres but most people put 2 in. Others said to drain the box and refill to about 2 litres. Another owner said that overfilling isn't harmless: it just means the oil is churned up and runs hotter sooner, on the track if not on the road. A third owner thought a bit too much is better than not enough, provided the car isn't parked facing uphill. See Gearbox oil replacement, which has a case where an underfilled box was damaged.
3. Hydraulics. One owner asked about the hydraulic fluid reservoir at the front, shared by the clutch and the brakes. If a new slave cylinder wasn't fitted with the gearbox it may have failed. He also said the hydraulic fluid inlet valve block on top of the gearbox is prone to leaking if it isn't connected properly. The owner had a new slave fitted and the fluid was full.
4. Clean it and look. One owner: clean it off and see where it comes from. Another owner: spray foot powder or talc around and take it for a spin.
5. The seals. When the leak returned (below), one owner said it would be the gearbox oil seals where the shafts go into the box. See Driveshaft removal and gearbox seals.

The breather mod

• Several owners said to push a short piece of pipe on to the breather stub with a jubilee clip and run it up towards the header tank, and CHILL added a small filter on the end. He said he refilled his own gearbox through that hole after fitting the pipe. One owner's advice was to do a search for the gearbox breather mod.
• In 2009 one owner's one-line answer to an owner with oil on the engine and diffuser was "gearbox oil... search for gearbox breather", and in 2010 another owner said oil on the inside of the near-side rear wheel could be from the gearbox breather.
• The pipe route, the filter, and where exactly to end it aren't specified in the thread.
• One owner put the cap from his old box on the stub as a temporary measure, and said he would do the mod when he could be bothered.

Where the plugs are (disputed)

The thread muddled these for a while, and the owner was told different things.

• One owner (with a photo, below): the filler bung is on the top, near the shift mechanism. The level plug is half way down the face of the gearbox that faces the rear of the car. The drain is at the bottom, under where the driveshaft goes (also confirmed by several owners's own look).
• Earlier, one owner had said the filler bung is an 8 mm hex "half way up the transmission, facing the front of the car", and CHILL showed a photo of the bottom plug in answer to another owner's question. A third owner told them that photo was the drain. Another owner ended up taking out the level plug to let the excess run off, which a fourth owner agreed with, and a fifth owner pointed out that it is the level plug, not the filler (which is on top).
• You need to raise the rear and take the rear undertray off to get to the level plug. Park on level ground and lower the car on to it after taking the filler out, if you have jacked it up.
• One owner later removed the level plug and no oil came out. He topped up about 100 ml before oil started to dribble. So by that check the box was a little low rather than overfull, but the thread doesn't say how much oil had already been lost.

Which oil

• One owner said anything 70W-80 or 75W-80 should work, to API GL4 specification. He wrote "APL", which we take to be a typo. He linked a retailer's gearbox oil page. This is one owner's view, not a TIS grade. See the gearbox oil guide for the filling.
• The older (2009) thread has an owner asking whether the colour and smell of oil from the breather differ. Answers were that gearbox oil smells horrible and is red, but his oil was brown-orange, which he took to be engine oil.

What happened next

One owner topped up and went for another drive. As soon as he got above 30 mph on a slip road the smoke started again, exactly as before, and this time the breather cap was on. Another owner said it is probably the gearbox oil seals. One owner agreed, wanted both sides done, and asked which seals to buy. A third owner said the blue ones are the standard Vauxhall seals and the red ones are probably the same as a pattern part. One owner found Vauxhall wanted £26 for one. CHILL's tip for the fit was to freeze them first so they can be pressed in. One owner said it was going to a mechanic, so the thread ends without a result.