The clutch on manual transmission cars is protected by a bell housing that seals the clutch plate and flywheel inside. Sometimes, it’s still possible for the clutch plate to become contaminated with oil, and this can cause the clutch plate to fail.
As a general rule, the symptoms of oil on a clutch plate are a slipping clutch, a burnt clutch plate, a foul smell from the clutch and difficulty changing gears. It’s usually not possible to remove oil from the clutch plate, and most of the time a replacement clutch will need to be fitted.
The friction material on a clutch plate is usually very hard wearing and can tolerate high temperatures and lots of friction. However, the material can also soak up oil and other fluids, causing it to slip and burn if it comes into contact with a high-speed flywheel.
What are the signs of oil on a clutch plate?
- Slipping clutch when changing gears
- Burnt clutch plate and foul smells
- Rough changing gears
- Engine surging
- Noises from the clutch
If you find that your car is using more engine oil than usual, or you notice oil or transmission fluid under your car after it has been parked, then these are the signs of a fluid leak that will need fixing. If you are also having problems with the clutch, then there may be oil or fluid getting onto the clutch plate.
If enough oil gets into the clutch and onto the clutch plate friction material, then it’s almost certainly going to affect how the clutch performs. The severity of the symptoms depends on how much oil or fluid has been soaked up by the clutch plate.
symptom 1: slipping clutch when changing gears
One of the most obvious signs of oil on the clutch plate is a slipping clutch, especially if the clutch is relatively new and you can rule out a worn clutch as the cause.
The reason the clutch may slip is because oil on the surface of the clutch plate will make the clutch friction material less capable of gripping and holding the flywheel.
This slippage will be most noticeable when changing gears, as this is when the most stress is put on the clutch plate as it attaches to the flywheel when you take your foot off the clutch pedal. You may also notice some slippage when changing gears while going uphill or when the engine is under increased load. This is because under these conditions the flywheel will be under more strain and this will increase the amount of friction needed by the clutch plate. If the surface of the clutch plate is contaminated then it’s more likely to slip if the flywheel is turning faster or with more force.
symptom 2: burnt clutch plate and bad smell
Another very common symptom of a clutch plate that has become contaminated is a burning smell. As outlined above, a clutch plate that has oil or hydraulic fluid on its surface is more likely to slip, especially when under pressure.
A slipping clutch occurs when the clutch plate doesn’t fully engage with or fully grip the flywheel. This causes it to burn as it rubs against the spinning flywheel, scorching the clutch plate surface and creating a burning smell. The burning smell can also be caused by the oil on the surface burning.
The smell associated with a burning clutch can only described as awful. It can be described as an acrid, rotten egg smell that comes from underneath the outside of the vehicle.
If you continue driving the vehicle with oil on the clutch, the surface of the clutch plate will eventually become burnt and scorched and will need to be replaced.
symptom 3: Rough changing of gears
If the clutch plate has oil or grease on it, then this can make changing gears more difficult, and not as smooth as it should be. This is because the clutch plate will not be able to engage with the flywheel as smoothly because of the damage to the friction material.
Gear changes will also be more difficult because of clutch slippage caused by the oil that has contaminated the surface of the clutch plate. The clutch slippage will cause the car to jump and surge when you take your foot off the clutch. It may also be difficult to select a gear if the clutch plate is not disengaging from the flywheel smoothly.
symptom 4: engine surging when under load
Another symptoms of oil on a clutch is engine surging, especially when the engine is under load or going up a steep hill. The engine surging is caused by the clutch plate slipping against the flywheel, instead of gripping it. As described earlier, this slippage is caused by the damage done to the clutch plate surface by the contaminating oil or fluid.
Engine surging when the vehicle is pulling a heavy load or is accelerating in a high gear is a common symptom of a damaged clutch. It occurs when the clutch plate is worn down or has become damaged and can no longer grip the flywheel. The result is a slipping clutch plate that grips under normal circumstances, but slips when put under increased stress. The reason the situation is made worse when the engine is under increased load is because of the the increase in rotational force applied to the clutch plate via the flywheel.
This symptom is most noticeable when the car is driving up a steep hill, pulling a heavy trailer and when accelerating in a high gear.
symptom 5: noises from the clutch
If the clutch has become noisy when changing gears and when the clutch pedal is pressed, this could be a symptom of oil or some other contaminant on the clutch plate. The noises that you are hearing are caused by wear to the clutch assembly that is usually caused initially by damage to the clutch plate.
Damage to the clutch plate caused by oil or hydraulic clutch fluid can cause damage to the flywheel, the clutch fork, the pressure plate, the diaphragm spring and the master cylinder. If the leaking fluid is hydraulic fluid from the clutch system, then this is going to affect the operation of the clutch mechanism.
A damaged clutch plate can cause cracks in the flywheel due to excessive heat. It can also cause vibrations in the pressure plate and damage to the clutch fork and throw out bearing. If any of these parts are not operating smoothly, then the clutch is going to be noisy when you try to change gears.
what happens when oil gets on a clutch plate?
As we have already seen, if oil, or any fluid gets onto a clutch plate, it’s going to cause damage to the plate.
The friction material on a clutch plate is usually made from an organic-metallic compound that is generally hard wearing. Unfortunately, just like a brake pad, it is also porous and can absorb liquids fairly easily. If the clutch plate comes into contact with a large enough amount of liquid such as engine oil, transmission fluid or hydraulic clutch fluid, the liquid will be absorbed deep into the friction pad.
If it is left long enough, the oil or liquid can appear to dry out but will reappear when the clutch plate heats up. This will cause the clutch plate to get ‘wet’ again and can cause clutch slippage and clutch plate burn if it’s not fixed. This can happen over and over again until causing the clutch to fail completely. Once this happens the entire clutch assembly will need to be replaced.
What causes oil on the clutch plate?
Oil on a clutch plate can sometimes come from a crankshaft oil leak or some other engine leak that manages to get into the clutch bell housing. However, it’s much more common for oil to come from the transmission or from the clutch hydraulic system because of a leaking slave cylinder. Here are a few of the most common causes:
Cause 1: hydraulic clutch fluid leaking from slave cylinder
Fluid leaking from a bad slave cylinder is probably the most common source of oil or fluid on a clutch plate. The slave cylinder, along with the master cylinder, clutch fork and throw out bearing are responsible for the operation of the clutch. To move the clutch pressure plate, fluid (usually brake fluid shared with the brake system) is pumped under pressure within the slave cylinder. This moves the slave cylinder piston, that in turn moves the clutch fork, pushes the throw out bearing and disengages the clutch pressure plate.
The slave cylinder uses fluid to move the clutch fork and disengage the pressure plate. If the slave cylinder is leaking, then fluid can often find its way to the pressure plate and from there to one side of the clutch plate. Because the fluid is operating under pressure, any damage or wear to the slave cylinder seals can cause a fairly substantial leak.
If enough fluid makes it’s way onto the surface of the clutch plate it will need to be changed. Brake fluid will cause irreversible damage to the friction material on the clutch plate even if you manage to clean it off.
Cause 2: leaking sump gasket
A leaking sump gasket is another reason way that oil can get onto the clutch. It’s not very common in newer vehicles, but it can still happen on older cars where the oil sump is not made from a cast alloy or is positioned directly beside the clutch housing.
In general, the oil sump on a vehicle stores the oil when the engine is turned off. When the engine is running the oil is pumped around the engine under pressure and then allowed to return to the sump to be pumped around again. Some sumps are made from cast alloy that is part of the engine block. Other vehicles use a sheet metal design that is bolted to the underside of the engine block, with a gasket or sealant used to stop leaks.
The oil pan is a very common place for an oil leak to occur. Oil spray can find its way from a leaking sump to the underside of the engine. A lot of older vehicles, especially those with a transversely mounted engine will have the clutch and gearbox in close proximity to the oil pan. If there is a significant oil pan leak, it can lead to oil getting into the clutch assembly.
Cause 3: gearbox oil leaking or overfilled
Sometimes gearbox oil (or transmission fluid) can contaminate the clutch plate. This can be caused by failure of one or more gearbox seals.
The clutch disc is connected to the transmission via the input shaft (or clutch shaft). It transmits power from the engine to the gearbox via the clutch. The point at which it enters the transmission is sealed by the input shaft seal. This seal prevents gearbox oil (transmission fluid) from leaking from the gearbox out into the clutch.
The input shaft seal can fail due to age or if too much oil, or the wrong oil has been added to the gearbox.
Cause 4: Rear main seal failure
Probably the most common way that oil can find its way onto the clutch plate is via a leaking rear main seal.
The rear main seal of an engine stops engine oil from escaping from the base of the engine. It is positioned at the point where the crankshaft leaves the engine before it joins the rear of flywheel. A leaking main seal is a very common problem in older vehicles, and can be expensive to fix.
A leaking rear main seal will allow oil to escape from the engine and collect in the clutch bell housing. Depending on the size of the leak, the clutch may continue to perform properly for a while, until enough builds up that it starts to soak into the clutch plate and cause slippage.
Replacement of a rear main seal is a big job that involves removing the clutch assembly and replacing all of the parts, including the rear seal and gasket between the engine block and housing.
how do you get oil off a clutch plate?
Most of the time, if you get oil on a clutch plate then the clutch plate will need to be replaced. It’s pretty much impossible to remove the oil completely as it will have soaked into the clutch plate friction material.
In some cases, it the oil leak has been very small, then it may be possible to remove the oil using a solvent such as brake cleaner. Most of the time this won’t work and for any chance of success the clutch plate will need to be removed from the vehicle. If you are going to go to the trouble of removing the clutch plate then it usually makes more sense to just replace it.
Sometimes, at the early stages of a leak the oil may ‘burn’ off by itself when you drive the car and the smell of burning oil can be an early sign that something is leaking onto the clutch.