Mechanical Troubleshooting

When troubleshooting an engine that fails to start or runs poorly, first check its mechanical condition—particularly if the engine has high mileage.

A cylinder compression test is given in 010 General. Also covered there is a brief description of a wet compression test and a cylinder leak down test.

Table b lists the symptoms of common engine mechanical problems, their probable causes and the suggested corrective actions. The bold type indicates the repair groups where applicable test and repair procedures can be found.

Table b. Engine Mechanical Troubleshooting

Symptom

Probable cause and corrective action

1. Engine will not start or run. Starter cranks engine at normal speed.

a. No fuel reaching engine. Check for fuel in tank.

b. Fuel pump not operating. Repair group 201

b. No voltage to spark plugs. Check ignition system. Repair group 280

2. Blue-gray exhaust smoke and oily spark plugs. Indicates oil burning.

a. Contaminated or gasoline-diluted engine oil. Change engine oil and filter. Repair group 030

b. Faulty valve guide seals or valve guides. Replace valve guides oil seals or overhaul cylinder heads. Repair group 150

c. Worn piston rings. Overhaul or replace engine.

3. Pinging or rattling when under load, traveling uphill, or accelerating, especially from low speeds. Indicates pre-ignition or detonation.

a. Fuel octane level too low.

b. Engine running too hot or overheating. Check engine oil cooler system. Repair group 170

c. Air/fuel mixture too lean. Repair group 240

4. Light metallic tapping that varies directly with engine speed. Oil pressure warning light not illuminated.

a. Valve clearances excessive. Repair group 030

b. Low oil pressure or possibly defective warning light circuit. Check engine oil pressure and warning light circuit. Repair group 170

5. Light metallic knock that varies directly with engine speed. Oil pressure warning light blinking or fully illuminated; may be most noticeable during hard stops or cornering.

a. Low oil level. Check and correct oil level.

b. Contaminated or gasoline-diluted engine oil. Change engine oil and filter. Repair group 030

c. Low oil pressure. Worn or faulty oil pump. Oil pump pickup resticted. Check oil pressure. Repair group 170

6. Growling or rumbling that varies with engine rpm. Indicates a bad bearing in an engine-driven accessory.

a. Remove A/C belt and fan/alternator belt. Check for play, bearing roughness, and loose mountings.

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