ENGINE DIAGNOSTIC PROCEDURES
ENGINE PERFORMANCE
Engine performance problems include concerns of “rough idle,” “lack of power,” “excessive fuel consumption,” or “does not run.” Following the diagnostic procedure, you have verified the concern, eliminated the obvious, analyzed the possibilities, and prioritized the possibilities. Now is the time to perform the inspection, but which diagnostic tests are you going to use?
• Eliminate the possibility of fuel and ignition problems.
• Verify timing belt integrity and valve timing.
• Intake manifold vacuum test: this test is a quick and easy indicator of in-cylinder problems. Will not tell you which cylinder, but may discriminate between valves, piston rings, intake leaks or exhaust malfunctions.
• Cylinder power balance test: this test is quick and easy when using MUT II to perform. Will not point to the type of problem, but will indicate a weak cylinder. Combining results from the above two tests should tell you which cylinder has trouble and what kind of trouble it might be.
• Compression test: this test will provide direct information about cylinder condition. A wet compression test will verify piston ring problems.
• Cylinder leakage test: this test will point more specifically to the problem than all of the other tests, but can be time consuming.
ENGINE LEAKAGE
Engine leaks can occur wherever oil or coolant are located. Many times leaks are obvious and require little more than a visual inspection to find. There are procedures to follow for the “not so obvious” leaks.
External Oil Leaks
• Clean the leak area thoroughly and run the engine.
• When viewing from underneath, trace the leak if possible to point of origin. Leaks always move downward, and as the vehicle is driven, to the rear.
• Be sure to check the positive crankcase ventilation (PCV) system. If the system is not functioning correctly or if the engine has excessive blow-by, the crankcase will pressurize and “push” oil through seals and joints.
• Really tough to find leaks can be found through the addition of dye.
External Coolant Leaks
• With the engine cool, perform a coolant pressure test. Pressurize system to normal operating pressure for 2 minutes (specification can be found in the Service Manual). If gauge drops, you have a leak. Small leaks may be found through the addition of dye.
• If coolant is not found underhood, be sure to check the heater core.
Internal Coolant Leaks
• Check engine oil for contamination. If contaminated, either the head gasket or a casting has failed.
• Check automatic transmission fluid for contamination. If contaminated, the transmission cooler, located in the radiator, is leaking.
• Check for white smoke from exhaust when running. If white “sweet smelling” smoke is present, the head gasket or castings have failed.
The spark plug from the affected cylinder will look very clean relative to the others. If a head gasket leak is small, you may not notice white smoke. Use the block checker at this point to determine if combustion gases are present in the cooling system. The block checker will tell you if there is an internal leak, but will not tell you where.
ABNORMAL ENGINE NOISE
Abnormal engine noises can be caused by component failure or a lack of oil pressure, leading to a component failure. Subtle noises can be amplified using a mechanics stethoscope.
• A ticking or light rapping sound located in the valve cover area could be caused by automatic valve lash adjusters. This noise is heard at a frequency that is 1/2 crankshaft RPM. Engine load does not affect the noise. Do not confuse this noise with fuel injector operation.
• Crankshaft bearing noises caused by excessive clearance are heard as deep metallic knocks. Noise is loudest under load, acceleration, or when the engine is just started. May be accompanied by low oil pressure which can be checked with an oil pressure gauge.
• Connecting rod noise caused by excessive clearance is heard as a light metallic rapping noise under light loads. Noise and frequency increase with engine speed. Can be isolated using MUT II by cancelling out the cylinder. Noise will substantially decrease with the cylinder shorted out.
• Piston “slap” is caused by excessive piston to cylinder clearance. This is a dull metallic rattle at idle and under light loads. Noise may disappear as the engine warms up and the piston expands. Affected cylinder can be located with MUT II by shorting out the cylinder.
Keep in mind that noises might be coming from a source other than inside the engine. Belts, air conditioning compressors, timing belt idlers and alternators can all make noise.
ENGINE OIL CONSUMPTION
Oil consumption problems may be a “non-problem” on a new engine that has not been broken in. A new engine may consume some oil for the first 1000 miles until the piston rings and other components have “worn in”. Per TSB 96-11-005, oil consumption of 1 quart or less per 1000 miles is acceptable. General Procedures Follow TSB 96-11-005 procedures which may incorporate the following:
• Follow the procedure to locate and then eliminate external oil leaks.
• Inspect spark plugs for an indication of internal leakage.
• Inspect exhaust color for blue smoke.
• Perform a compression test if necessary.
• Follow procedure for cylinder leakage test if necessary.
The two prone areas for internal oil leakage to the combustion chamber are: piston rings and valve guides/seals. Excessive blue smoke during high vacuum conditions such as deceleration usually indicates excessive valve guide to valve clearance. If nothing is indicated, you should change the oil and precisely fill the crankcase. Have the customer visit every 500 miles to check consumption.
SMOKING EXHAUST
Abnormal smoke in the exhaust indicates the presence of something other than the proper ratio of air/fuel
burning in the combustion chamber. Smoke and related diagnostics can be classified by color or tint:
White Puffy Smoke
Disappears when engine warms up. This is water Cold Weather vapor, caused by condensation burning off as the exhaust system and catalytic converter reach operating temperature. This is a normal condition.
Black Smoke
This is carbon caused by an excessively rich fuel mixture. Exhaust may smell like rotten eggs if the catalytic converter is functioning. Fuel system needs to be diagnosed.
Blue/Grey Smoke
This is engine oil being burned in the combustion chamber. Follow the procedures for abnormal oil consumption.
White, Pungent Smoke
Vehicle may look like a crop duster. This is engine coolant burning. Follow procedures for internal coolant leaks.
