Oil Breather

Symptoms

A common problem with the G40 is oil coming out of the top of the dipstick tube. This is an indication that pressure is building up in the engine crankcase and is caused by a blockage in the crankcase breather system. It is not always obvious where the oil is coming from as it tends to only be thrown out when the engine is running at high rpm. If you suspect an oil leak from the dipstick tube you can check by taping some tissue around the top of the tube (with the dipstick still in place) and then checking for signs of oil loss after a test drive. Don't get paranoid about minute traces of oil, but if there are a few drops this indicates the start of a problem. A badly blocked breather will throw enough oil out of the dipstick tube to turn on the oil warning lamp after a few hundred miles.

Another symptom of breather problems can be when oil seals on the camshaft or crankshaft start to leak. If a seal has failed it is wise to check the breather or the replacement seal may soon fail as well.

It has been known for some garages to suggest that the excessive internal engine pressure is due to badly worn pistons and a complete engine rebuild is needed. If this is actually the case it is likely that the car would be incapable of passing the MOT emissions tests. If the emissions are OK and there are no obvious signs of burning oil (blue smoke from the exhaust) then the problem is almost certain to just be the breather on an otherwise healthy engine.

Breather Operation

Pressure builds up in the engine crankcase due to vapour from the hot engine oil and also from exhaust gases that escape past the piston rings (this will be more apparent in a high mileage engine where the piston rings are starting to wear). A crankcase breather system is used to release this pressure. To avoid the polluting effects of allowing the oil vapour to escape directly into the atmosphere, the breather feeds the crankcase gases into the engine where they are burnt and so converted to less harmful compounds.

In the G40 engine the crankcase breather starts with an oil separator (also called 'breather' or 'breather pod'). This is a dome shaped object with a short metal outlet pipe and is fitted on the back of the block below the intake manifold. It contains wire mesh and its purpose is to filter out any liquid drops of oil contained within the crankcase gases and prevent them escaping via the breather system. Fitted into the oil separator is a plastic one way valve. This is a push fit (using an O ring seal) into the outlet pipe of the oil separator with a plastic securing clip. Fitted to the other side of the valve is a rubber pipe which joins the large diameter supercharger air return pipe between the throttle bypass valve and the cam cover. From here the oil vapour and other gases are sucked in by the supercharger and eventually end up in the combustion chambers to be burnt. The one way valve prevents air being forced back into the crankcase under conditions where the crankcase pressure is lower than that in the supercharger return pipe.

If the breather system becomes blocked the crankcase pressure will build up. The only way out for the gases is via the dipstick tube which tends to throw out lots of liquid oil as well.

Maintenance and Repair

Occasional maintenance will help to prevent breather related problems before they happen (including oil seal failures) and is probably worthwhile on higher mileage cars. This involves dismantling the breather system and cleaning/replacing the faulty parts.

To locate the breather system first find the smaller diameter pipe that joins the large supercharger pipe between the throttle and the crankcase cover. Follow this back to find the one way valve and oil separator. (The thin rubber pipe that joins part way down is concerned with the fuel tank breather system.) The breather system is partly located under the inlet manifold and can be difficult to get to, so it may be useful to release the valve from the oil separator and pull the pipe up with the valve attached (replace in the same way).

The oil separator is a push fit into the block (?) and can be replaced with a new one or cleaned out with a suitable solvent.

Part number: V030 103 129 Approx cost from VAG: £23

The one way valve is probably the main cause of problems and can block up badly with oil and other deposits. The valve has a small bypass hole so it will give some air flow in both directions; the flow from the direction of the crankcase should be very much greater. Replace if you have any doubts that it is working properly after cleaning.

Part number: V030 115 327 Approx cost from VAG: £25

The rubber breather pipe itself can become blocked. It also tends to rot due to the exposure to oil. This is an integral part of the larger supercharger pipe and the VW replacement is extremely expensive (over £100). A cheaper alternative is to cut out and replace just the damaged sections with suitable rubber tube. Note that holes in a rotted breather pipe could lead to unfiltered air getting to the supercharger.

Additional Problems

The increased pressure due to a blocked breather can force oil past the crankshaft and camshaft oil seals. If failure of an oil seal is suspected, first correct the breather problems, clean the engine, and then confirm the leak is still present. Another problem is contamination of the cambelt and supercharger belts with oil. The oil from the dipstick tube tends to run down behind the cambelt cover and gets onto these belts. Unclip the plastic cambelt cover to check for problems. It is wise to replace a badly oiled cambelt as the oil will attack the rubber which could eventually cause belt failure. This will result in seriously expensive damage to the engine! The charger belts are less of a concern although the oil contamination may cause slipping. Wash off the oil with detergent in less serious cases.

Nick Dilley