Symptom library
Leaks, smells and smoke
A puddle and a smell are the two symptoms most likely to be either completely harmless or genuinely urgent, with very little in between. Color, location and timing tell you which one you have.
These pages help you identify what you're looking at before you decide whether to drive the car to a shop or stop where you are.
Start with what your car is doing
- A puddle under the car you can't identify
- Burning, sweet, or rotten-egg smells
- A smell that only shows up after driving
- Visible smoke or steam from under the hood
Strange smells
Your nose is a decent diagnostic tool. Each smell has a short list of causes and a very different urgency.
Read itFluid leaks
Color and position tell you most of it, and one of the puddles on this list is completely normal.
Read itBurning smell after driving
A burning smell that appears right after you park, especially after a long or hilly drive, has a shorter and more specific list of causes than one that happens all the time.
Read itGas smell
Fuel vapor is the one smell we never tell anyone to monitor for a few days. Some causes are trivial, but the difference between trivial and dangerous is not something you can determine from the driver's seat.
Read itWhat usually causes these
Several symptoms in this group share the same underlying causes, which is why describing the pattern matters more than naming a part.
- Fluid leak onto a hot surface
- Oil, coolant or transmission fluid dripping onto exhaust is the most common source of a burning smell.
- Cooling system failure
- A thermostat, water pump, fan or radiator that can no longer shed heat fast enough.
- Low or degraded transmission fluid
- Burnt or low fluid is the cheapest cause of slipping — and the one that stops being cheap if you keep driving on it.
- Oil burning past seals or rings
- Hardened valve stem seals or worn rings let oil into the combustion chamber, which shows up as blue smoke and a dropping dipstick.
- Brake fluid, air or master cylinder
- The hydraulic side of the brakes: old fluid that boils on grades, air in the lines, or a master cylinder bypassing internally.
- Low coolant or stuck thermostat
- The heater runs on engine coolant, so it goes cold first when the level drops or the engine never warms up.
- Worn brake pads or rotors
- Normal wear that squeals as a warning and grinds once the friction material is gone.
- Sticking caliper or dragging brake
- One brake stays partly applied, which heats the wheel, pulls the car, and can smell hot after a drive.
- Fuel delivery or fuel pump
- A weak pump, clogged filter or failing injector that cannot keep up under load — and often leaks vapor as it ages.
- Evaporative emissions leak
- A hardened gas cap seal, purge valve or heat-cracked vapor hose letting fuel vapor escape where you can smell it.
Questions to ask before you approve work
- What did you find, and what test confirmed it?
- Is this a safety item, or can it wait?
- What happens if I wait a month?
- Which part actually needs replacing, and why that one?
- Can you show me the failed component or the measurement?
- Was anything else damaged by this?
- What is the diagnostic fee, and does it come off the repair?
- Is any of this quote optional or recommended rather than required?
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