Anonymous asked in Cars & TransportationMaintenance & Repairs · 8 years ago

How is 'ignition advance' made in electronic injection controlled systems in the throtle body?

i.e. Pontiac Trans Sport 3.1 van's engine.


been there.. - You are right; I should have said: " "..ignition advance" in electronic injection controlled systems THAT HAVE a throttle body", as a difference with those that have a "rail" injection line. Thanks.

Update 2:

monkeybo... - Of course, yet the ECM receives input from fuel pressure, air flow mass (or pressure), etc. In all this, the EGR and the solenoid vacuum switch (which receives the signal from the ECM) have a bearing in it, AND I DO NOT KNOW HOW!. Thanks.

Update 3:

Stpaulgu... - Then (I believe) throttle position is changed from analogue (resistance) to digital (for the ECM) via a Analogue to Digital converter. What is the work done by the EGR and its solenoid controlled vacuum switch (which receives signal from the ECM) then? Because they are all tied to the vacuum source of the throttle body's plenum? Thanks.

4 Answers

  • 8 years ago
    Favorite Answer

    It's not.

    Done through the ECM/PCM.

    Edit: So this is a true "wet flow" TBI system with injectors in the actual throttle body? I am unaware of a MAF TBI setup from GM, so I will assume MAP, but reference MAF a bit.

    That vintage, there MIGHT be some input from an ESC (Electronic Spark Control) module, but you don't mention one. Without the wiring manual in front of me, I can't be quite certain, but originally spark retard was handled through a separate ESC module that fed the ECM/PCM (going to say ECM form here on out, just for simplicity of typing) a signal to retard timing if knock was detected.

    But outside of unusual conditions (knock) the ECM uses a timing map (when viewed, it's a rectangular table, as in a spreadsheet, with RPM on one axis, and load on the other). The timing map is based on RPM and depending on whether MAP or MAF system a calculation of various inputs to determine engine load, or simply manifold vacuum with MAP. At a given RPM and engine load, the ECM timing table cross references those two inputs, and whatever "cell" the RPM and load cross at, is the timing given to the engine. Here is an example of a GM timing map:

    On early GM EFI systems with a distributor, base timing (what the distributor is set at initially, with no ECM control/input) is factored into the timing tables in the ECM programming. If the base timing (on the core support/hood sticker) is say 2*, the ECM already "knows" this, and if the timing table calls for 30* at X load and Y RPM, it "knows" there is already 2* to factor in.

    That's timing in a very small nutshell in a standard GM EFI system. In regards to throttle body though, there is very little to do with timing. As I said, with MAP, that is what is used to determine engine load, not TPS. TPS is unusable for this, as at 25% throttle for instance, every gear would have a different engine load, and the vehicle speed would vary based on the angle of the roadway, again if the throttle was never moved. TPS is used as a diagnostic aid (ECM: "TPS says 100% throttle, but engine load is staying very low, something is wrong, trigger check engine light"), to indicate to the ECM that fuel flow from the injectors needs modified, to control the torque converter lockup in non-completely PCM controlled transmissions, and is used for things like deceleration fuel cutoff, power enrichment (think pump shot in a carb), etc. NORMALLY those parameters have some spark advance or retard tied to them that modifies the "base" timing map I described before, as slamming the throttle to the floor requires different timing and fueling than gradually applying throttle.

    The ECM on any GM EFI setup I know has no input from fuel pressure. It "knows" (based on factory programming in the PROM) what the fuel pressure is, that is set by the fuel pressure regulator. It knows fuel flow based on the fuel injector lb/hr rating. That shouldn't ever change, that is what the regulator is for.

    EGR application (again, governed by the ECM) DOES require modification of timing, as it changes the air/fuel density (less of both since EGR is inert), but this is normally "seen" with a MAP setup. EGR is a vacuum leak, so when EGR is commanded, the MAP changes, and timing changes accordingly.

    I am unfamiliar with the solenoid vacuum switch. Is that tied into the EGR? I'm used to hearing "EGR solenoid" or "EVRV" in regards to EGR control. If that isn't what you are talking about, if you can explain it better, I would be better able to help.

    Depending on your vintage of vehicle, you could probably spend some time over on in the forums, as early (OBD1) TBI and TPI are nearly identical in all aspects (talking 1990-1992 MAP TPI, not 1985-1989 MAF TPI, which is slightly different, but not much), other than the port fuel vs. throttle body. All the same sensors/components are used, just their location changed.

  • 8 years ago

    Woops! The knock sensor triggers the electronic ignition to retard the total crankshaft degrees of ignition timing if the fuel octane isn't high enough.

    When you get a change, remove a throttle body from any car someday and clean it. There are never any fuel lines connected to a throttle body.

    Are you truing to put fuel injector nozzles out of business?

  • 8 years ago

    Ignition timing is done electronically by the engine control computer, not in the throttle body. If sensors detect detonation, the computer alters the timing as needed to eliminate the problem.

  • 8 years ago

    The throttle position sensor tells the computer how far you've opened the throttle plate. Based on that reading and readings from other sensors, the PCM determines spark advance

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