If you’ve ever browsed Facebook Marketplace for a cheap used car, chances are you’ve seen this line: “Runs great, just needs a head gasket.” It’s become the unofficial slogan of vehicles one oil change away from the scrapyard. But why are there so many cars with blown head gaskets listed online?

A recent post on r/AskAMechanic lit up with answers. The original poster asked why these failures are so common and what causes a head gasket to “blow.” The thread quickly filled with mechanics, tow truck drivers, and gearheads sharing hard-earned experience.

So what exactly is a head gasket?

It’s a thin, crucial seal between the engine block and cylinder head. It keeps coolant, oil, and combustion gases in their proper places. When it fails, those systems can mix, leading to serious problems like white exhaust smoke, coolant loss, overheating, or even engine failure.

One Redditor correctly explained that ruined head gaskets often trace back to an overheating engine

A bad radiator hose, stuck thermostat, or leaking water pump can all cause the coolant system to fail. If the driver ignores the warning signs and keeps driving, the engine overheats, warping the cylinder head and damaging the gasket seal.

Another poster, claiming experience in towing, said the root issue is often neglect

Many drivers ignore their temperature gauge or check engine lights. Some never check their coolant at all. By the time steam is coming from under the hood, it’s already too late.

Several commenters pointed out that aluminum engines are more vulnerable than older iron-block engines

Aluminum warps quickly under heat. That makes modern cars less forgiving when cooling problems go unchecked. Once warped, the cylinder head can’t seal properly, and coolant leaks into places it doesn’t belong.

A head gasket replacement isn’t a minor repair

Even at a trusted independent shop, the job can run anywhere from $1,500 to $3,000, depending on the vehicle. It involves removing the cylinder head, checking it for warping or cracks, replacing bolts and gaskets, and flushing the coolant and oil systems. Some engines may need additional repairs if the failure caused internal damage.

That’s why so many sellers list these cars online

They’ve already spent money keeping the car on the road. When the head gasket goes, it’s often the final straw. Rather than sink more into the vehicle, they try to recoup what they can. 

One Redditor admitted doing just that: he sold a non-running truck with engine issues on Marketplace and watched it spark a bidding war because the body was clean and rust-free.

If you’re considering buying a car with a blown head gasket, proceed with caution

The repair is not quick or cheap. If coolant has mixed with engine oil, the bearings or other internal parts may already be worn. You may also need machine work or a new cylinder head altogether.

If you’re tackling the job yourself, know your limits. This is not a beginner project. You’ll need tools, time, and attention to detail. If you’re paying a shop, ask for a full quote that includes everything from head machining to oil and coolant service.

By the way, to prevent this on your own car, don’t ignore the cooling system. Check your coolant levels regularly. Flush and replace coolant at factory intervals. Watch the temperature gauge. Shut the car off if it starts to overheat. Getting towed might save you thousands in repairs.

In the end, cars with blown head gaskets are common on Marketplace because they straddle the line between being worth fixing and being better off parted out. They aren’t always bad buys…but they’re never casual ones.