Exhaust Smoke Diagnosis: Evaluating the Engine's Condition

Once the compression, cylinder leakage, vacuum, and power balance tests are performed, further testing can be done to evaluate the engine’s condition. For example, an engine with good relative compression but high cylinder leakage past the rings is typical of a high-mileage worn engine. This engine would have these symptoms: excessive blowby, lack of power, poor performance, and reduced fuel economy.

Exhaust analysis.

If comparable compression and leakage test results are found on an engine with comparatively low mileage, the problem is probably
stuck piston rings that are not expanding properly. If this is the case, try treating the engine with a combustion chamber cleaner, oil treatment, or engine flush. If this fails to correct the problem, an engine overhaul is required.

A cylinder that has poor compression but minimal leakage indicates a valve train problem. Under these circumstances, a valve might not be opening at the right time, might not be opening enough, or might not be opening at all. This condition can be confirmed on engines with a pushrod-type valve train by pulling the rocker covers and watching the valves operate while the engine is cycled. If one or more valves fail to move, either the lifters are collapsed or the cam lobes are worn. If all of the cylinders have low compression with minimal leakage, the most likely cause is incorrect valve timing. If compression and leakage are both good, but the power balance test reveals weak cylinders, the cause of the problem is outside the combustion chamber. Assuming there are no ignition or fuel problems, check for broken, bent, or worn valve train components, collapsed lifters, leaking intake manifold, or excessively leaking valve guides. If the latter is suspected, squirt some oil on the guides. If they are leaking, blue smoke will be seen in the exhaust.

Examining and interpreting the vehicle’s exhaust can give clues of potential engine problems. Basically there should be no visible smoke coming out of the tailpipe. There is an exception to this rule, on a cold day after the vehicle has been idling for awhile, it is normal for white smoke to come out of the tailpipe. This is caused by the water that has condensed in the exhaust system as it becomes steam due to the heat of the exhaust system. However, the steam should stop once the engine reaches normal operating temperature. If it does not, a problem is indicated.

The color of the exhaust is used to diagnose engine.

A. Gasoline Engine
- Visible Sign: Gray or black smoke
- Diagnosis: Incomplete combustion or excessively rich A/F mixture
- Probable Causes:
  1. Clogged air filter
  2. Faulty fuel injection system
  3. Faulty emission control system
  4. Ignition problem
  5. Restricted intake manifold

B. Diesel Engine
- Visible Sign: Gray or black smoke
- Diagnosis: Incomplete combustion
- Probable Causes:
  1. Clogged air filter
  2. Faulty fuel injection system
  3. Faulty emission control system
  4. Wrong grade of fuel
  5. Engine overheating

C. Gasoline and Diesel Engine
- Visible Sign: Blue smoke
- Diagnosis: Burning engine oil
- Probable Causes:
  1. Oil leaking into combustion chamber
  2. Worn piston rings, cylinder walls, valve guides, or valve stem seals
  3. Oil level too high

D.  Gasoline Engine
- Visible Sign: White smoke
- Diagnosis: Coolant/water is burning in the combustion chamber
- Probable Causes:
  1. Leaking head gasket
  2. Cracked cylinder head or block

E. Diesel Engine
- Visible Sign: White smoke
- Diagnosis: Fuel is not burning
- Probable Causes:
  1. Faulty injection system
  2. Engine overheating