Mobile Cooling System Service — we come to you

Coolant Leak: Where Is It Coming From? — Because "somewhere near the engine" isn't a diagnosis

That sweet, slightly sickly smell wafting up through your vents — or that mysterious green, pink, or orange puddle quietly forming under your car overnight — is coolant. And coolant, for the record, belongs inside the cooling system, not underneath it. Your engine runs at around 90°C by design; the cooling system is what stops it melting itself. Lose enough coolant and it will overheat. Overheat long enough and you're looking at a blown head gasket, a warped cylinder head, or worse — the sort of repair bill that makes grown adults ring their mum. SOS CarFix comes to you, finds the actual source, and sorts it before "small leak" becomes "engine transplant."

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The short version

Sweet-smelling puddle under your car? Dropping coolant level? We find the source and fix it on your driveway. No garage needed. Get a quote today.

How it actually works

Diagram of a car cooling system — radiator, thermostat, water pump, coolant reservoir, cooling fan and hoses — showing how coolant flows to keep the engine at the right temperature. — How your engine stays cool — radiator, thermostat, water pump and the coolant cycle. · tap to enlarge

Your engine produces enormous amounts of heat burning fuel — heat that, left unchecked, would destroy it inside minutes. The cooling system is a sealed loop: coolant (an antifreeze-and-water mix, typically 50/50) is pumped around water jackets inside the engine block and cylinder head by the water pump, carries heat away to the radiator at the front of the car, where airflow and the cooling fan drop its temperature, and then back round again. The thermostat controls when flow begins — keeping the engine at its ideal operating temperature (roughly 85–95°C on most UK cars) rather than letting it run cold and inefficient. Coolant also passes through the heater matrix — a small radiator behind your dashboard that your cabin heating system runs through. It sits in an expansion tank to allow for thermal expansion. Any leak anywhere in that sealed loop — a split hose, a corroded radiator, a weeping water pump seal, a cracked thermostat housing — gradually empties the system. Because coolant is pressurised when hot (typically 15 psi), even a small crack can push coolant out faster than you'd expect, and pressure testing is the proper way to find it.

“SOS CarFix comes to you, finds the actual source, and sorts it before "small leak" becomes "engine transplant.”

The warning signs

Sound familiar?

A sweet, slightly sickly smell — often described as syrup or burnt sugar — from under the bonnet or through the heater vents

A green, pink, orange or blue puddle under the car after parking, usually towards the front or centre

The coolant level in the expansion tank keeps dropping, even if you've already topped it up

The temperature gauge climbing higher than normal, sitting in the red, or spiking then dropping back down

White steam from under the bonnet — particularly alarming if you're not in a sauna

The cabin heater suddenly blowing cold air despite the engine being warm — classic sign of a heater matrix leak or very low coolant

White smoke from the exhaust on a warm engine, possibly with a sweet smell — a red flag for internal coolant loss (head gasket territory)

Common causes

So what's behind it?

1Split or perished coolant hoses — rubber degrades over time, especially where hoses clamp to the radiator or thermostat housing; the cheapest and most common source of a visible external leak

2Radiator leak — corroded aluminium radiators develop pinhole leaks; plastic end-tanks crack with age and temperature cycling; look for crusty dried coolant staining around seams

3Water pump failure — the pump seal or weep hole leaks when the bearing or seal wears; often betrayed by coolant drips directly below the water pump pulley

4Thermostat housing or coolant flange — plastic housings (common on VAG, BMW and many others from the 2000s onward) crack with age, or the gasket fails; typically a seeping rather than a dramatic leak

5Expansion tank crack — often overlooked; the plastic tank is under pressure and UV/heat eventually cracks it; coolant leaks from the tank or its cap seal

6Heater matrix leak — inside the dashboard; coolant drips onto the passenger footwell carpet (often mistaken for a roof leak), mists up the windscreen from inside, and can make the cabin smell of hot antifreeze

7Head gasket failure — an internal leak rather than an external one; coolant is drawn into the combustion chamber (white exhaust smoke, sweet smell) or oil contamination (mayo-like residue on the oil filler cap); serious, and diagnosed separately with a block-test kit

What we do — at your door

We come to you — driveway, work car park, wherever the car is sitting being quietly dramatic — and run a proper cooling system diagnosis on the spot. That starts with a visual inspection for obvious external drips, staining and crusty coolant deposits, then a pressure test: we fit a pump to the expansion tank cap and pressurise the system to the cap's rated pressure (usually around 15 psi). A leak will show itself immediately; a slow one we give time to develop while we watch. We also check the coolant condition (colour, smell, contamination), test the cap's pressure-relief valve, and run a combustion gas sniff test at the expansion tank if there's any suspicion of a head gasket issue — because that needs a completely different conversation and a clear-eyed repair estimate before you commit to anything. Once we've found the source, we give you an itemised quote and, in the majority of cases, fix it the same visit.

What affects the price

Cost depends entirely on where the leak is coming from, which is why finding the source first is non-negotiable. A split coolant hose is at the cheap end — hose and coolant, straightforward labour. A thermostat housing or expansion tank replacement is a step up, though still usually reasonable. A radiator replacement varies significantly by car — some are accessible in an hour, others require half the front of the car coming off. A water pump often involves timing belt access and is priced accordingly. A heater matrix is labour-intensive (dashboard removal on most cars) and the bill reflects that. Head gasket failure is in a different category entirely — materials and labour for a full head gasket job represent a significant investment, and whether it's worth it depends on the car's value and overall condition, which we'll be honest with you about. We don't invent prices; we quote after we've actually found the problem.

Random knowledge you didn't ask for

Coolant (antifreeze) is sweet-tasting and genuinely toxic — it contains ethylene glycol. Cats and dogs are particularly attracted to the smell and the taste, which is one reason a leaking car on a driveway is a genuine hazard.

Modern coolants are formulated to last five years or 100,000 miles before the corrosion inhibitors degrade — but most people never change it. Old coolant turns acidic and corrodes aluminium components from the inside, which is partly why cooling systems develop so many leaks on older cars.

The pressurised cooling system raises the boiling point of the coolant significantly — at 15 psi, the boiling point rises from around 108°C (for a 50/50 mix) to roughly 125°C, giving a useful safety margin on a hot engine. A failed pressure cap or a leak drops that margin instantly.

Questions you're probably asking

Can I keep driving with a coolant leak?

It depends how fast it's leaking. A very slow seep with the level only dropping slightly over weeks is different from a hose that's weeping steadily. The rule is: never let the temperature gauge climb into the red. If it does, pull over and stop — continuing to drive an overheating engine risks warping the cylinder head or blowing the head gasket, which turns a £150 hose job into a £1,500 head gasket job. Top up, monitor, and get it looked at promptly.

What colour is coolant, and does the colour mean anything?

UK cars use various coolant colours — green, pink, orange, blue and yellow are all common, depending on the type (OAT, HOAT, silicate-based). The colour itself just tells you the formulation; what matters is that coolants shouldn't be mixed (they can react and form a gel that blocks passages). If your puddle is clear and odourless, it's more likely condensation from the air-con drain. Sweet-smelling and coloured means coolant.

My coolant level drops but I can't see any puddle — where is it going?

Three main possibilities: a very slow external leak that evaporates before it drips (pressure testing will find it), a heater matrix leak that drips inside the cabin onto the carpet rather than the ground, or — the one nobody wants — it's being burnt internally through a head gasket leak (watch for white exhaust smoke and check the oil filler cap for a brown, mayonnaise-like residue). We test for all three.

Is a coolant leak an MOT failure?

Yes. A significant coolant leak is an MOT failure — it falls under fluid leaks likely to cause environmental harm or be a safety hazard. Even a minor leak will get flagged as an advisory. Worth sorting before the test rather than having to rebook.

How long does a coolant leak repair take?

A hose replacement or thermostat housing is typically under an hour. A radiator replacement is usually one to two hours depending on the car. A water pump can be longer if it shares access with the timing belt. A heater matrix — honestly, most of a day, because it often involves removing significant dashboard. We give you a realistic time estimate when we quote.

Coolant Leak — sorted at your door

Stop procrastinating. Get a transparent quote and we'll come to you.

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