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Differential Oil Leak

Thick, dark oil with a strong sulphur smell under the middle of the rear axle (or the front on four-wheel drives) usually comes from a differential seal or cover gasket.

Normal risk

Have it checked soon. A differential that runs low on oil can whine and eventually fail.

Short answer · How do I know it is differential oil?

Gear oil is thicker than engine oil and has a strong, sulphurous smell. Leaks usually come from the pinion seal where the propshaft enters, the axle shaft seals or the cover gasket. Over time the level drops and the gears whine or wear.

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Safe checks you can do

  1. Smell the oilGear oil smells strongly of sulphur.
  2. Look at the differential housingWet areas around seals and covers show the leak.
  3. Listen for whiningA whine from the axle can mean the level is already low.

Questions that narrow it down

  1. Rear- or four-wheel drive?Front-wheel-drive cars often have the differential inside the gearbox.
  2. Is there a whine?The oil level may be low.
  3. Was the car towed or driven through deep water?Seals can be damaged or water can enter.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Pinion sealLeak where the propshaft enters.
  2. Axle shaft sealOil on the inner side of a wheel.
  3. Cover gasketLeak around the cover edge.
  4. Blocked breatherPressure pushes oil past seals.

Questions about Differential Oil Leak

Can oil on the wheel come from the differential?

Yes, a leaking axle seal can put oil on the brakes, which is dangerous.

Is a seal replacement a big job?

It depends on the seal; a workshop can advise.