Why is my car leaking oil?

A dark patch on the driveway is one of those faults that manages to feel serious and vague at the same time. The car drives normally, nothing is flashing on the dashboard, and the only evidence is a stain that was not there last week. The useful news is that an oil leak is one of the very few car problems you can narrow down a long way yourself, for free, with a piece of cardboard and ten minutes.

The less comfortable news is that oil leaks do not settle down on their own and they get more expensive the longer they are left. Oil is the only thing keeping metal from touching metal inside the engine. A slow drip that costs a couple of hundred pounds to cure this month can, if it is ignored long enough to run the engine low, turn into the kind of bill that writes off an older car. Past a certain size it also stops being your problem alone and becomes an MOT failure, because the test has a measurable limit for how much fluid a car is allowed to leave on the floor.

First, work out what is actually leaking

Plenty of drivers arrive at a garage certain they have an oil leak and drive away having been told it is condensation from the air conditioning. Before anything else, find out what the fluid is, because the answer changes the urgency completely.

Park on a clean, dry, level surface. Slide a sheet of plain cardboard or a large piece of paper under the middle of the engine and leave it overnight. In the morning you will have a sample in a known position, which is worth far more than a smear on tarmac.

Now match it. Engine oil is light golden brown when it is fresh and turns dark brown to black as it ages, it feels genuinely slippery between finger and thumb, and it will not dry up. Automatic gearbox fluid and power steering fluid are usually red or pink and noticeably thinner. Coolant is dyed a bright colour on purpose, green, orange, pink or blue, feels slightly sticky rather than slippery and smells faintly sweet. Brake fluid is clear to pale yellow, feels oily, and tends to appear near a wheel or under the pedal end of the bulkhead. Clear, odourless water in warm weather is almost always the air conditioning doing exactly what it is designed to do, and needs nothing at all.

If the answer is coolant rather than oil, read our guide to why cars overheat next, because losing coolant slowly is how head gaskets fail. If it turns out to be brake fluid, stop reading and stop driving: our guide to checking your brake fluid explains why, and the brake warning signs guide covers what else to look for.

Where the drip lands tells you which seal has gone

Once you know it is engine oil, position is the next clue, and it is a good one. Engines leak from a small number of places, and each one sits in a different spot along the length of the engine.

A drip toward the front of the engine, often accompanied by an oily film on the front of the block, points at the rocker cover gasket at the top or the front crankshaft seal at the bottom. A drip directly under the middle of the engine, in a neat spot, usually means the sump plug washer or the sump gasket. A drip at the back of the engine, where it meets the gearbox, with oil visible around the bellhousing joint, is the rear main seal, and that is the one worth being careful about because reaching it means removing the gearbox.

Oil found around the side of the engine block on a modern diesel is very often the oil filter housing gasket or the oil cooler seal, which is one of the most common leaks on cars built in the last fifteen years. Oil in the turbo area, usually with blue smoke to go with it, points at the oil feed or return pipe, and our guide to what exhaust smoke colour means will help you confirm it.

Position is not proof. Oil runs downhill and along the underside, so what drips at the front can perfectly well have started higher up and further back. That is exactly why a garage traces a leak rather than guessing at it.

The most common causes, cheapest first

Ranked roughly by how often they turn out to be the answer:

The sump plug and its washer. The plug at the bottom of the sump is removed at every oil change and its crush washer is meant to be replaced each time. Reuse it a few times, or overtighten the plug, and it weeps. This is the cheapest possible outcome and it is more common than people expect.

The rocker cover, or cam cover, gasket. A rubber or cork seal sitting on top of the engine that hardens with heat and age. Because it is at the top, oil from here runs down the back or front of the block and can drip a long way from where it started. It often also drips onto the exhaust, which is where the smell comes from. Our guide to what a burning smell from your car means covers that specific case.

The oil filter housing gasket or oil cooler seal. Very common on modern engines, particularly diesels. It is not a difficult part in itself, but on some engines a lot has to come off to reach it, which is what drives the price.

The sump gasket. The seal between the sump and the bottom of the engine. Replacing it means dropping the sump, which usually means draining the oil, so the job normally comes with a fresh oil and filter anyway.

Crankshaft seals, front and rear. The front one sits behind the crank pulley and is frequently done at the same time as the cambelt, because the belt has to come off to reach it. The rear one sits between the engine and the gearbox. Same part, wildly different labour bill.

The oil pressure switch or sensor. A cheap electrical part that can weep from its seal or its body. Worth mentioning because it sometimes gets diagnosed as a much bigger leak.

A failing head gasket. Less common as a pure external oil leak, but it happens, and it usually comes with other symptoms. Our head gasket repair cost guide sets out what to look for.

What an oil leak repair costs in the UK

These are typical independent garage ranges including parts and labour. Where a range is wide it is because access varies hugely between engines, so the same named job can be a ninety minute task on one car and most of a day on another.

Two of those are worth planning around rather than simply paying. If the front crankshaft seal is weeping and the cambelt is anywhere near due, doing both together saves most of the labour twice over, so it is worth checking when your cambelt is due before you agree to anything. And if a rear main seal is quoted on a car that also has a tired clutch, the gearbox is coming out either way, so ask what a combined price looks like against the clutch replacement cost on its own.

Is it safe to drive with an oil leak?

For most leaks, and most short journeys, yes, provided you actually keep an eye on the level. The leak is not what damages the engine. Running low on oil is, and a car can lose a surprising amount between one weekly glance at the dipstick and the next.

While you are waiting for the repair, check the level before every journey. Our guide to checking your engine oil covers doing it properly, on level ground with a cold engine, and topping up with the right grade rather than whatever is in the boot.

Stop and get help rather than carrying on if any of these apply. The oil pressure warning light comes on, which means stop immediately and switch off rather than press on to the next junction. Oil is running rather than dripping. You can smell hot oil while driving, which usually means it is landing on the exhaust and is a real fire risk. Or the level has dropped below minimum and you have nothing to top it up with. If the engine management light has also appeared, treat that as a reason to have it looked at now rather than at the weekend.

The MOT rule: 75mm in five minutes

This is where the guesswork stops, because the MOT has an actual measurement. Section 8.4.1 of the inspection manual covers fluid leaks. The tester checks with the engine idling, and the rule is that a car fails if a fluid leak creates a pool on the floor more than 75mm in diameter within five minutes, or if several smaller leaks together leak at the same rate.

The defect is recorded as Major where fluid is leaking excessively and is likely to harm the environment or pose a safety risk to other road users, and as Dangerous where fluid is leaking continuously and is likely to pose a serious risk to road safety. Seventy five millimetres is about the size of a drinks coaster, which is a lot less than most people picture.

Three details are worth knowing. First, coolant, screen wash and the fluid used for selective catalytic reduction, which is AdBlue on most cars, are specifically excluded and are not reasons for failure, so our AdBlue guide applies rather than this one. Second, brake fluid is dealt with elsewhere and far more harshly: a leaking brake pipe, hose, connection or master cylinder on a hydraulic system is graded Dangerous in section 1 of the manual, full stop. Third, a tester is allowed to refuse to carry out the test at all if a fluid leak is excessive, and must refund the fee, so a badly leaking car can waste the appointment as well as fail it.

If your MOT is close, this is a good reason to have a pre-MOT check done first rather than gambling on the day, and our guide on how to prepare for your MOT covers the other cheap failures worth clearing at the same time. A leak that is not yet bad enough to fail will often be recorded as an advisory, and our MOT advisory guide explains why that is the test starting a clock rather than giving you a pass. If you are buying a used car, past advisories for fluid leaks show up in the free MOT history check, and a leak advisory repeated across three years tells you a great deal about how the car has been looked after. Our guide to why cars fail their MOT puts it in context alongside the more common failures.

Why the puddle matters beyond your driveway

Oil stains block paving and concrete more or less permanently, which is the part everyone notices. The part most people do not think about is where it goes after the rain. GOV.UK pollution guidance is explicit that surface water drains discharge directly to the environment rather than into a treatment works, so oil washed off a driveway or a road ends up in a watercourse. That is the thinking behind the MOT wording about a leak likely to harm the environment. Oil on the road surface is also a genuine hazard for motorcyclists, which is the other half of the same defect wording.

What a garage actually does to find it

A proper leak trace is not a look under the bonnet. The car goes on a ramp, the underside and the suspect area get degreased and cleaned, and a fluorescent dye is added to the oil. You then drive the car normally for a few days and bring it back, and the technician goes over it with a UV lamp. The dye glows at the exact point the oil is escaping, which is often several inches above where it was dripping.

Expect somewhere around £40 to £90 for that, sometimes across two short visits. It is the single best money you can spend on a leak, because the alternative is paying for a gasket on a hunch and still having the drip afterwards. Our guide to what a car diagnostic check involves explains why a plug in code read on its own will not find a mechanical leak, and the questions to ask a garage before booking covers getting the trace and the repair quoted separately.

What you can check yourself in ten minutes

None of this needs tools beyond a torch and a rag.

Check the oil level on the dipstick and write down where it sits, then check it again in a week. The rate of loss matters more than the stain. Look at the top of the engine for wet, oily grime around the edge of the rocker cover. Look underneath, if you can do so safely, for a wet trail rather than a clean drip, since the trail points uphill to the source. Look at the sump plug at the lowest point of the engine for a bead of oil hanging off it. And finally, check whether the oil is where it should be at all: a car that is losing oil with no external leak at all is burning it, which is a different conversation and usually comes with smoke.

Do not steam clean the engine bay before the garage sees it. It is tempting, and it destroys the evidence the technician needs.

Stop leak additives, and when they are a bad idea

Bottled stop leak products work by swelling rubber seals. On an older car with a slightly hardened seal, they occasionally slow a weep for a while. They will do nothing at all for a leaking gasket face, a cracked housing, a stripped sump plug thread or a split pipe, and those account for most real leaks.

The catch is that a product that swells seals swells all of them, not just the leaking one, and it does not make the MOT defect go away if the leak is already pooling. Treat an additive as a way to buy a few weeks before a planned repair, never as the repair itself, and never on a car you are about to sell to someone else.

Getting a fair price for the repair

Oil leaks are one of the areas where quotes vary most, because the diagnosis is genuinely open to interpretation and the labour times differ enormously between engines. That makes it worth comparing.

Ask for the leak to be traced and reported before any parts are ordered, and get the trace quoted separately from the repair. Ask which specific seal has been identified and how it was confirmed, because a confident answer with a UV trace behind it is very different from a shrug and a rocker cover gasket. Ask whether the oil and filter are included, since many of these jobs involve draining the oil anyway. And if a rear main seal or a crankshaft seal is on the table, ask what else is worth doing while the access is open.

Our guides to choosing a good garage and the signs a garage is overcharging you cover the rest, and if you would rather have the work absorbed into scheduled maintenance, check what is already covered in a full service and how often your car should be serviced. Keeping the paperwork matters too, since a documented repair is part of a full service history when you come to sell.

When you are ready, compare local garages and ask two or three of them to quote for the trace. The price of finding out is small, and it is the only part of this job worth paying for twice.