Having a car that cranks but refuses to start is a common and frustrating issue for many car owners. When you’ve ruled out the obvious – like a dead battery or empty fuel tank – and you’re still faced with an engine that spins but doesn’t fire, the hunt for the culprit begins. One particularly perplexing scenario arises when you’re using a diagnostic scanner and suspect an ignition problem, specifically a lack of spark. Let’s delve into a real-world example of this issue and explore potential diagnostic paths, focusing on situations where you might be investigating for sparks, especially in relation to your scanner port and a no-crank situation – or more accurately, a cranks but won’t start scenario.
Consider the case of a 2008 Peugeot 207 1.4 VTI that underwent recent engine work, including a new head gasket and timing chain. Prior to this work, the car was running, even driven into the garage. However, post-repairs, the engine cranks energetically but stubbornly refuses to start. This immediately points to a potential issue introduced during the repair process, or a pre-existing condition exacerbated by the work.
Initial checks confirm the basics are in order: the engine cranks with good compression, and the fuel injectors are indeed squirting fuel. Power is reaching the ignition coils during cranking – this particular Peugeot utilizes individual coil-on-plug units. Despite these seemingly positive signs, the crucial element for combustion – spark – is absent. This absence of spark, even with power to the coils, strongly suggests a control problem.
The immediate suspicion might fall on timing issues, especially given the recent timing chain replacement. Incorrect timing can certainly prevent spark. However, a careful check reveals that the timing marks are correctly aligned. Furthermore, the complete absence of spark, not even an errant or late spark, points away from a simple timing misalignment.
To further investigate, a bi-directional scanner is employed. This tool allows for direct command of various engine components, including the ignition coils. When the scanner is used to command individual coils to spark, the result is the same: power is present at the coil, but no spark is generated. Crucially, even direct command via the scanner fails to produce a spark.
The fact that the fuel injectors are firing suggests that the engine control unit (ECU) is receiving at least some timing input, likely from the crankshaft position sensor. However, the inability to trigger a spark, even when directly commanded through the scanner, casts a long shadow of suspicion over the ECU itself – often referred to as the Powertrain Control Module (PCM).
Could a faulty PCM be the reason for the missing spark? The scanner’s inability to command a spark, despite confirming power to the coils, strongly points in this direction. This scenario also tends to rule out simple input sensor failures. If a sensor like the crankshaft or camshaft sensor was completely dead, the fuel injectors might also not be firing. Since the injectors are active, but the coils are not, even under direct scanner command, the PCM’s output stage for ignition control becomes the prime suspect.
While variable valve timing (VVT), present on both intake and exhaust camshafts in this engine, is a sophisticated system, it’s less likely to directly cause a complete absence of spark. VVT issues typically manifest as poor running, performance problems, or fault codes related to valve timing, rather than a complete ignition failure. And in this case, no fault codes are stored in the system, adding another layer of mystery.
One diagnostic step that could provide further insight is attempting to bypass the PCM’s control of the ignition coil. Introducing a direct control signal to a coil could help determine if the coil itself is functional and if the issue truly lies within the PCM’s control circuitry. However, this type of bypass testing requires caution and a good understanding of automotive electrical systems to avoid damage.
In conclusion, when faced with a “car won’t crank, no spark” situation, particularly when diagnostic efforts using a scanner port reveal a persistent lack of spark even under direct command, the PCM emerges as a highly probable area of fault. While input sensors and timing should not be completely discounted, the scanner’s diagnostic capability in this scenario strongly focuses attention on the PCM’s ability to control the ignition system. Further, more in-depth diagnostics, potentially including PCM testing or replacement, might be necessary to resolve this no-start condition.
