Engine

A car's engine oil should stay clean looking for 1,000 miles or so after it's changed.  The engine should start quickly and run smooth without any kinds of knocks, bangs, or rattles. Dash gauges should stay within range, and dash warning lights should go off immediately after the engine is started.

If any of the things on the chart below are occurring, it's a sure sign of trouble, and a sure sign that repairs are in order:

Symptom Problem Diagnosis Engine Oil appears creamy. Brown paste on dipstick and oil filler cap: Engine oil is contaminated with water. Head gasket failure. Cylinder head Cracked. Engine block cracked. Oil is visible in the radiator coolant: Coolant is contaminated with oil. Head gasket failure. Cylinder head Cracked. Engine block cracked. Erratic temperature gauge: Coolant system failure. Thermostat faulty. Radiator blocked. Radiator fan/sensor fault. Head gasket failure. Cylinder head Cracked. Engine block cracked. Hard starts: Engine takes more than 4 seconds to turn over and start Weak battery. Weak starter motor. Spark plugs need replacing (gas engines). Glow plugs need replacing (diesel engines). Fuel or ignition related fault. Worn or damaged piston rings or valves. Engine rattles, ticks or knocks: Worn engine component  Worn or damaged camshaft, rockers, lifters or pushrods (where applicable), crankshaft bearings, connecting rods. Engine misfires: Fuel or Ignition system failure. Dirty fuel. Plugged fuel filte. Spark plugs need replacing. Ignition fault (distributor cap, rotor, leads on older engines). Faulty engine sensor, wiring, ECU (newer engines). Engine moves more than 1" when revved up: Wear due to age or damage. Worn engine or transmission mounts. Excessive pressure from the oil filler hole while engine is running: Blow by*: Worn piston rings. Worn cylinder bores.

*How to check your engine for blow by: With the engine running, carefully loosen the oil cap. Slowly loosen your grip on the cap so it’s resting on top of the oil filler hole. The cap may bounce around a little, which is normal, but should not be blown off the oil filler hole by the force of the gases coming out. Excessive “blow by” is a good indication that the engine is excessively worn and needs to be rebuilt or replaced.

Exhaust Color

There are five types of smoke which will come from the tail pipe:
Clear or very Light Gray, Frosty White or Gray, White, Blue, and Black.
Normal exhaust smoke will be clear or a very light gray color. On a cold day this smoke may appear a lot, but this a normal result of hot gases from the combustion chamber of the engine coming into contact with the colder ambient air, just like our breath when we breathe on a frosty morning.
However, smoke other than Grey / Frosty type smoke could spell trouble. See the table below:

Color Problem Diagnosis Clear or very Light Gray: None Engine is running normal. Frosty Gray: None in cold weather Normal on a cold day. Gray or White smoke in warm weather: Water is being burned with the fuel mixture. Head gasket failure. Cylinder head Cracked. Engine block cracked. Blue smoke during first minute of start up: Oil is being burned with the fuel mixture Leaking valve stem oil seals. Blue smoke continuously: Oil is being burned with the fuel mixture. Severe damage to valve stem oil seals. Damaged pistons, piston rings, engine bore wear, Possible turbo charger seal failure (if engine is turbocharged). Black smoke (gas engines): Overly rich air fuel mixture. Dirty air filter. Blocked air cleaner passage. Faulty air sensor. Light black smoke (diesel engines): None Engine is running normal. Dense black smoke (diesel engines): Fuel system failure. Dirty or faulty injectors. Injection pump incorrectly timed or faulty.

Transmission

Automatic transmissions don't require any regular service beyond changing your transmission fluid and filter (if equipped). Other than failure to change the fluid on a regular basis, heat is main cause of automatic transmission failure. Installing a heavy duty transmission cooler can significantly increase your transmission's life. While there are some transmissions that were poorly designed, regular fluid changes and the addition of a transmission cooler can make your automatic transmission outlast the life of your engine. While modifications, such as shift improver kits, are normally intended for high performance, these can also increase the life of your automatic transmission, but it should be noted that the shifts between gears will be harder.
This is because automatic transmissions use a series of bands and clutches to shift gears. The longer the bands and clutches are engaged, the smoother the shift. Unfortunately, the longer the bands and clutches are engaged, the faster they wear out as well.
Transmission repair requires more knowledge than this article can cover. If your automatic transmission is slipping (taking longer than 1 second to shift gears), or vibrates and/or jerks when shifting gears, you need to take the car to a repair shop.

Manual transmissions require regular clutch adjustment and replacement. Again, this requires more knowledge than we can cover here, as different makes and models of cars require different adjustments and adjustment procedures.
Common problems with a worn manual transmission include:

Symptom Diagnosis Whirring, whining or rattling noises when the clutch pedal is depressed: Worn clutch release bearing. Worn flywheel. Hard to shift gears: Worn transmission gears and/or synchromesh. Worn clutch. Whining noise while driving: Worn transmission gears and/or synchromesh. Car vibrates when accelerating: Worn flywheel. Clutch slipping: Worn clutch plate. Worn pressure plate. Loud clunking noise when pressing quickly on and off the accelerator: Worn transmission, transmission mounts, prop shaft, differential mounting bushings.

Suspension

A cars suspension contains a number of parts such as bearings, bushings, ball joints, shock absorbers or McPherson struts, springs and Constant Velocity (CV) joints on front wheel drive models.
Other than worn CV joint or shock absorbers, identifying a specific component that is worn or damaged requires having the vehicle inspected by a mechanic.

Symptom Diagnosis Clicking noise when turning left or right: CV joints need to be replaced. Car rocks from side to side and/or front to back. especially in corners or when the brakes are applied: Worn shock absorbers. Clunking or banging sound when driving over rough surfaces: Worn suspension components. Car wanders when you hit a bump: Worn suspension components. One corner of the car sits lower than others: Worn or damaged springs.