1. Maple or sweet syrup:
a. When: After the engine has warmed or possibly after shut down.
b. Reason: Engine coolant containing ethylene glycol [toxic] possibly leaking from somewhere
c. Inspect: Radiator body, radiator cap, hoses, failed intake manifold gasket or cylinder head gasket. If strong smell inside of rig, check heater core
2. Gym socks or musty smell:
a. When: After you turn on heater or AC
b. Reason: Possible mildew growing inside of AC evaporator or water left from your last river crossing inside the AC/heat ducting.
c. Remedy: Park rig in sun to dry out
3. Gas station:
a. When: Rig parked inside garage or when weather is warm.
b. Reason: Fuel leak or seepage. On carbureted rigs this is normal just after shut down from some fuel after-boil in the carb float bowl.
c. Inspect: Fuel injection line and nozzles or fuel tank vent hoses
4. Burnt carpet:
a. When: After hard or frequent use of brakes.
b. Reason: Overheated brake pads. If you smell this under normal driving, possible seized brake caliper piston or possible emergency brake left on.
c. Inspect: Feel brake calipers by hand for one abnormally hot, check hand-brake
5. Hot oil or acrid burning smell:
a. When: Engine is hot
b. Reason: Oil leaking onto hot exhaust manifold or pipe. If you just had a oil change, possible oil drips from filter removal.
c. Inspect: Leaking or seeping crankshaft seal, leaking valve cover gasket or lose oil filter
6. Burnt paper, smoldering newsprint:
a. When: All speeds and when shifting gears of manual transmission.
b. Reason: Clutch slippage
c. Inspect: Clutch for adjustment or replace clutch
7. Rotten eggs:
a. When: Engine is running
b. Reason: Produced by trace amounts of sulfur in gasoline.
c. Inspect: Possible failed catalytic converter, improper functioning fuel injection
8. Smoke:
Credit: March ’06, Popular Mechanics