How to track down an oil leak

QI’ve been trying to trace an oil leak since buying my Scirocco MKII. After a 15-mile drive there is oil around the sump gasket, on the bottom timing belt cover and on the suspension wishbone. My first thought was that it was a leaky sump gasket, but there is no obvious trail of oil. I can’t see any oil when looking from the top down. Any suggestions? Phil Norton, Chester

ATracing leaks can be tricky because the escaped oil tends to migrate around the engine, disguising its source. The starting point in any investigation, therefore, is to clean the engine. Brake or carburettor cleaner is effective for localised areas – or a grease solvent sprayed on, worked in with a brush where necessary and washed off with water.

We can’t really advise which particular part of your engine is soiling itself without seeing for ourselves, but here’s a useful leak-tracing checklist starting with the easiest items.

The rocker cover is the first thing to check. Cork gaskets are often a tad iffy in their sealing and they tend to be reused many times. Some people like to glue them to the cover or cylinderhead with sealant or adhesive, which stops them popping out of place when (over)tightened. Rubber gaskets are generally better, but they can harden, shrink and split.

Look also for any holes in the rocker cover caused by corrosion or the flexing of spotwelded brackets supporting the air cleaner, throttle linkage, etc. Lastly, make sure the gasket face of the cover is flat: many are distorted by overtightening.

Next, have a look at the crankcase breather hoses. These take oily fumes from the crankcase and feed them to the air cleaner to be burned off. If a pipe splits or falls off, oilyness will creep onto anything in the surrounding area. If the breather system is blocked, choked-up or messed around with, the crankcase will become pressurised. This will push large amounts of oil out of all sorts of gaskets, seals and orifices that wouldn’t usually leak, especially around the crankshaft. If the engine actually drips while running, check this first.

Fuel pumps and other replaceable items attached by a flange are prime suspects. Old gaskets get reused and flanges may be buckled by overtightening. The sump suffers the same maladies as the rocker cover, but it’s surprising how many are not very tightly attached. This is due to the cork or rubber gasket shrinking over time. It’s worth going round and seeing if the nuts or bolts seem slack.

When tightening sumps, rocker covers, timing chests and similar, use a ¼in drive ratchet and nip up the fixing with your hand cupped around the head of the tool. Don’t swing on the end of the handle, as this will probably be too tight. If screws are installed instead of bolts, this is a hint that not much force is necessary.

The timing cover at the front of the engine can leak – problems are as for the rocker cover. The crankshaft will have oil seals front and rear. These can develop large leaks as there’s a lot of oil moving around behind them. Replacement of the rear seal is a pain, so many owners live with it until the engine needs some other form of major repair.

Finally, check for loose or missing bolts in the engine block. The holes may be drilled through the crankcase wall and could provide a point of leakage. In this case, the installation of a springor shakeproof washer will sometimes cause oil loss: use a plain steel washer instead.