Modern Automotive Technology PowerPoint for by Russell Krick Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois

Chapter 48 Engine Mechanical Problems

Contents Why is diagnosis important? Symptoms of engine mechanical problems Decide what type of engine repair is needed Evaluating engine mechanical problems Service manual troubleshooting charts

Why Is Diagnosis Important? If a technician does not know how to properly diagnose engine problems, time, effort, and money will be wasted An untrained technician may rebuild an engine when a minor repair would have corrected the fault

Symptoms of Engine Mechanical Problems Excessive oil consumption Excessive crankcase blowby Noises Exhaust smoke Poor performance Coolant in the oil Engine seizure

Engine Mechanical Problems

Pre-Teardown Inspection Run the engine Look for external problems oil leaks vacuum leaks part damage contaminated oil Listen and watch for problems

Check Oil Condition Check for coolant in the oil shows up as white or milky oil Check for gasoline in the oil smell the dipstick for gasoline

Oil-Fouled Spark Plugs Indicate internal oil leakage into the combustion chambers—caused by worn rings, cylinder walls, or valve seals

Oil in Coolant Causes: a leak in the radiator oil cooler head gasket leakage a cracked block or head

Engine Oil Leaks Causes: To isolate leaks: gaskets harden and crack seals wear fasteners work loose parts become warped or cracked To isolate leaks: clean the affected area trace the leak upward to its source

External Coolant Leaks Causes: hose problems rusted freeze (core) plugs warped, worn, or damaged parts Use a pressure tester to locate leaks

Engine Blowby

Engine Vacuum Leaks May produce a hissing sound Affect idle quality more than high-speed conditions leaks represent a greater percentage of engine airflow at idle than at high speed

Engine Exhaust Leaks Produce a clicking sound Causes: leaking exhaust gaskets a warped exhaust manifold loose manifold bolts

The leakage path can be easily seen after exhaust manifold removal Engine Exhaust Leaks The leakage path can be easily seen after exhaust manifold removal

Engine Smoking

Abnormal Engine Noises Indicate part wear or damage Use a stethoscope to find internal noises A piece of hose can be used place one end next to your ear, use the other end to probe around the engine

Compression Test Measures the amount of pressure produced during the compression stroke Performed any time symptoms point to cylinder pressure leakage rough idle popping noise in intake or exhaust blue exhaust smoke excess blowby

Combustion Leakage

Compression Testing (Gasoline Engine) Remove all spark plugs Block open the throttle Disable the ignition and fuel injection Screw the compression tester into a spark plug hole Crank the engine through 4–6 compression strokes, noting the readings Repeat for each cylinder

Compression Tester

Installing Adapter

Connect Compression Gauge

Cranking Engine

Replacing Plugs and Wires

Compression Testing (Diesel Engine) Use a diesel compression gauge that reads up to approximately 600 psi (4000 kPa) Remove the injectors or the glow plugs Install the tester in the recommended hole Disable the injection pump Crank the engine, noting the readings Repeat for each cylinder

Wet Compression Test Performed if a cylinder fails a compression test Helps isolate cylinder and ring problems from valve problems Oil is squirted into the cylinder before the compression test if the pressure rises, the rings and cylinder are suspect if the pressure does not rise, the valves are suspect

Wet Compression Test A. Perform dry test B. Squirt oil into cylinder C. Measure again

Compression Test Results Refer to service manual for specifications Typical gasoline engines 125–175 psi (860–1200 kPa) Typical diesel engines 275–400 psi (1900–2750 kPa) Maximum variation 10–15% between cylinders

Cylinder Leakage Tester Measures the amount of air leakage out of a cylinder Shop air pressure is forced into the cylinder on Top Dead Center Pressure gauge reads percentage of leakage out of the cylinder Typical maximum leakage is 20%

Cylinder Leakage Tester If leakage exceeds specifications, listen at various points to isolate leakage Oil filler cap—ring leakage Throttle body—intake valve leakage Exhaust pipe—exhaust valve leakage Radiator bubbles—head gasket or cracked head, block

Engine Repair Is Needed Decide What Type of Engine Repair Is Needed After performing inspections and tests, decide what part or parts must be repaired or replaced Evaluate your pre-teardown diagnosis If you still can’t determine the problem, partially disassemble the engine for further inspection

Evaluating Engine Mechanical Problems

Valve Train Problems

Causes an engine miss, a popping sound at the throttle body or exhaust Burned Valve Causes an engine miss, a popping sound at the throttle body or exhaust

Worn Valve Guides and Stems Causes tapping noise, oil consumption, spark plug fouling, or stem breakage

Leaking Valve Stem Seal Lets oil drain through the clearance between the stem and guide Oil will be pulled into the intake port and burned Causes blue exhaust smoke, especially after startup

Valve Breakage Caused by valve stem fatigue or by a broken or weak valve spring Usually causes severe piston and cylinder damage

Stuck Valve Valve stem rusts or corrodes and locks in the valve guide May happen when the engine sits in storage

Valve Float Excess engine speed, weakened valve springs, or lifter problems cause the valves to remain partially open Usually occurs at higher engine speeds Engine may miss, pop, or backfire

Causes reduced compression and power Worn Timing Chain Causes reduced compression and power

Worn Timing Belt May break, jump off its sprockets, or skip a few teeth Severe lack of power, no-start, and valve damage can result

Camshaft Problems Worn cam lobes Worn journals Broken cams Worn distributor drive gear Loose or worn fuel pump drive eccentric

Reduces valve lift and power output Cam Lobe Wear Reduces valve lift and power output

Rocker Arm and Push Rod Problems May cause clatter (light tapping noise) Rocker arms may wear Push rods may be bent To check, roll the push rods on a flat bench

Excess clearance can produce noise Valve Train Clearance Excess clearance can produce noise

Hydraulic Lifter Problems Worn or defective hydraulic lifters may produce valve clatter To check, remove the valve cover Try adjusting the valves If adjustment will not quiet the noise, check for valve train wear If the valve train passes, the lifter is bad Low oil pressure can cause lifter noise

Loud, metallic knock that is loudest when the engine is cold Piston Knock (Slap) Loud, metallic knock that is loudest when the engine is cold

Piston Pin Knock Too much clearance between the piston pin and the pin bore or connecting rod bushing Makes a double knock The noise does not change much with engine load

Worn Rings and Cylinder

Worn Ring Grooves

Burned Piston Results from prolonged preignition or detonation Causes low compression, blowby, smoking, and rough idle

Burned Piston

Crankshaft Problems Journal wear Main bearing wear Rod bearing wear Low oil pressure

Rod Bearing Knock Light, regular rapping noise with the engine floating Loudest after warm-up Caused by wear and excessive rod bearing-to-crankshaft clearance To locate, short out or disconnect the spark plug wires, one at a time The knocking bearing may quiet down when its cylinder is disabled

Main Bearing Knock Similar to rod bearing knock, but slightly deeper in pitch More pronounced when the engine is under load Worn bearings and journals are letting the crankshaft move up and down Usually reduces oil pressure

Main Bearing Knock To verify, remove the oil pan and pressure test the lubrication system Excessive oil flowing out of one or more of the main bearings implies too much bearing clearance If the crankshaft is not worn, the bearing inserts may be replaced

Excess Crankshaft End Play Caused by a worn main thrust bearing May produce a deep knock when applying or releasing the clutch On an automatic transmission, a single thud or knock may occur during acceleration or deceleration

Broken Engine Mounts Allow the engine to move in the vehicle To check: open the hood engage the parking brake shift transmission into drive or into gear with the brakes on, slowly increase engine speed or release the clutch pedal if the engine moves excessively, the mounts may be broken

Broken Engine Mounts

Troubleshooting Charts Service Manual Troubleshooting Charts Charts list possible problems and needed repairs Refer to the chart when you have difficulty locating or correcting an engine mechanical problem