Troubleshooting a Non-Starting Engine
A technician is diagnosing a gasoline engine that cranks strongly but will not start. The battery measures 12.4 V at rest and 10.8 V while cranking. The starter turns the engine at normal speed. There is spark at the plugs, and the plugs are dry after several cranking attempts. The fuel gauge reads one-quarter tank. The fuel pump runs briefly when the key is turned to ON, but the technician cannot hear it running during cranking. The fuel filter was replaced two weeks ago. Ambient temperature is 35°F.
1. Given the symptoms, rank the most likely causes and explain the reasoning behind the ranking.
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Cranking is strong (10.8 V while cranking is acceptable), so the battery and starter are not the primary fault. Spark is present, so ignition is at least partially working. The decisive clue is dry plugs after cranking: fuel is not reaching the cylinders. The most likely causes are (1) fuel pump not running during cranking - possibly a pump relay or control signal issue, since it primes at key-on but may not get power during crank; (2) a restriction in the fuel line or a failed pump; (3) no injector pulse, though that is less likely if the pump primes normally. The recently replaced filter makes a clogged filter less likely but not impossible if debris was introduced.
2. Describe a step-by-step test to distinguish a fuel-delivery problem from a control/signal problem without replacing parts.
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- Check for fuel pressure at the rail with a gauge during cranking; if pressure is within spec, delivery is likely fine and the issue is control (injector pulse). 2) If pressure is low or absent, verify power and ground at the pump connector during cranking - a test light or meter tells whether the pump is being commanded. 3) If power is present but the pump does not run, the pump is suspect. 4) If power is absent during crank but present at key-on, inspect the relay and the crank-signal path to the pump control. 5) Use a noid light or scope on an injector to confirm pulse. This isolates delivery versus command before any replacement.
3. How does the 35°F ambient temperature change your interpretation of the dry plugs?
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Cold ambient makes cold-start enrichment important. Dry plugs in cold weather could also mean the engine is not receiving the extra fuel it needs for cold starting, or that the fuel is not vaporizing well. However, dry plugs after several cranking attempts still point most strongly to no fuel delivery rather than a mixture-quality issue, because even a lean cold start would typically leave some fuel residue. Cold temperature mainly reinforces checking that the cold-start enrichment path and pump operation are functioning.