Mine is a 97 SE (2.5L), if that matters.
1) I'm wondering which sensor signals are monitored by the PCM for its adjustments to ignition timing. Also wondering (roughly) in what ways the timing is adjusted for each of those signals. I'm hoping for something like this:
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Answers So Far (Please Correct / Improve)
2) Yes, ignition timing is controlled entirely by the ECU.
1) Consensus seems to be that there is an ignition timing "base map", with values that depend upon two variables: RPMs and MAP (Manifold Air Pressure). Then, signals from the IAT (Intake Air Temp) and ECT (Engine Coolant Temp) are monitored to slightly offset the base-map values. They are effectively lookup tables for fine-tuning the values of the base map. Finally, signal from TPS (Throttle Position Sensor) is also used for more minor adjustments. In order of significance, it may look something like this:
1) I'm wondering which sensor signals are monitored by the PCM for its adjustments to ignition timing. Also wondering (roughly) in what ways the timing is adjusted for each of those signals. I'm hoping for something like this:
- MAP sensor:
- At idle (high vacuum), the timing is slightly [advanced/delayed] so that [such and such happens].
- As vacuum pressure decreases (approaching atmospheric), the timing is [advanced/delayed] so that [such and such happens].
- RPMs:
- At idle ...
- As RPMs increase ...
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Answers So Far (Please Correct / Improve)
2) Yes, ignition timing is controlled entirely by the ECU.
- "The 2.5L engine distributor does not have built-in centrifugal or vacuum assisted advances. Ignition timing/advance is controlled by the Electronic Control Unit (ECU). Because ignition timing is controlled by the ECU, base ignition timing is not adjustable on any of these engines. The distributor is locked in place by an ear on the distributor housing that a hold down bolt passes through when the distributor is installed. Because the distributor position is locked when installed, its rotational position cannot be changed. DO NOT attempt to modify the distributor housing to get distributor rotation. [...] Distributor position will have no effect on ignition timing. The position of the distributor will determine fuel synchronization only."
1) Consensus seems to be that there is an ignition timing "base map", with values that depend upon two variables: RPMs and MAP (Manifold Air Pressure). Then, signals from the IAT (Intake Air Temp) and ECT (Engine Coolant Temp) are monitored to slightly offset the base-map values. They are effectively lookup tables for fine-tuning the values of the base map. Finally, signal from TPS (Throttle Position Sensor) is also used for more minor adjustments. In order of significance, it may look something like this:
Final Timing = Base Timing (RPM & MAP) + IAT Offset + ECT Offset + TPS Adjustment
Base Map
- RPMs: "Engine Speed"
- @freedom_in_4low suggests below that timing will generally advance with increase with RPM. Since the engine is moving faster, ignition needs to initiate a few degrees earlier to ignite the charge at the right point in the rotation.
- I've read elsewhere that this is true for moderate increases in RPM, but that at higher RPMs or when under high load, timing might be delayed slightly. This would be a protective measure against knocking, which is more likely to occur at high engine speeds and high loads.
- MAP: "Manifold Air Pressure"
- Increased MAP (Lower Vacuum): Delays timing under higher load to prevent knocking.
- Decreased MAP (Higher Vacuum): Advances timing during low load for improved efficiency.
- Note that there appear to be two base maps for ignition timing, depending upon whether or not the EGR is active. ("Exhaust Gas Recirculation" reduces nitrogen oxide emissions by recirculating a portion of exhaust back to the engine. It lowers combustion temperature in the process. It is deactivated in conditions like heavy load or cold starts.)
- When deactivated, the base table used is called "Main Spark (EGR Off)".
- When active, the base table used is called "Warm Spark".
- The "Main Spark" base map shows a wider range of values, and also includes decimal (half-degree) values. When EGR is off, ignition timing needs to be more precisely controlled to prevent knocking, due to higher temperatures."
Offsets:
- IAT:
- At colder intake temperatures, ignition timing is slightly advanced. Colder air is denser, and flame propagation occurs more slowly. To ensure that combustion gives peak pressure at the right point in the cycle (15-ish degrees after top-dead-center), the process has to be initiated a bit earlier.
- At warmer intake temperatures, ignition timing is slightly delayed. Warmer air is less dense, and flame propagation occurs more rapidly. To ensure that combustion gives peak pressure at the right point in the cycle, the process has to be initiated a bit later.
- ECT:
- At lower engine temps, the PCM advances ignition timing to help the engine warm up, and to stabilize idling.
- At higher/operating engine temps, the timing is adjusted back to normal values.
- At highest engine temps, timing is delayed slightly, to prevent overheating and/or knocking.
Adjustment:
- TPS:
- Some sources suggest that TPS is the third variable of the base map. However, it seems to play a more minor role. For instance, HP doesn't seem to show TPS in the base map (see here and here).
- What's below is hypothetical, coming from ChatGPT 4.0. Eager to hear feedback from others.
- Rapid Changes in Acceleration or Deceleration:
- Sudden Acceleration (Rapid Increase in Throttle Opening): PCM may advance the timing briefly to provide a quicker throttle response and avoid hesitation
- Sudden Deceleration (Rapid Decrease in Throttle Opening): PCM may delay the timing to reduce the risk of backfiring and to improve engine braking.
- Idle and Part-Throttle Situations:
- Idle: During idle, the TPS helps the PCM stabilize the engine's idle speed. Although the MAP sensor primarily controls idle speed, the TPS ensures consistency in throttle opening, and timing may be slightly advanced to maintain stability.
- Part-Throttle (Cruising): At steady-state cruising, the PCM may advance the timing to improve fuel efficiency, especially when the throttle opening is relatively small and engine load is low.
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