Initial fuel prime question

[Rich Z]

The description of the relevant tables (B3201, B3202, B3203) mentions that a prime pulse of fuel will be delivered when the ignition key is first turned to the "on" position. Does that mean when the starter is actually engaged or just when the key is turned to the position to where power is applied to the PCM? The reason I ask is because I am trying to firm up the start-up and I tend to turn the key on and allow it to set for a couple of seconds to get the fuel pressure up before actually starting the car. Cold start up is kind of weak, so I'm trying to figure out how to get a more solid sounding start up.

And along the same lines, at what point is the engine actually considered by the PCM to be "started" and the "after-start" tables kick in?

[joecar]

Hi Rich,

B3201 description:

When the ignition key is first turned to the on position the PCM will deliver a prime pulse of fuel.
This table defines the amount of fuel (in Grams) to be injected.

See also:
- {B3202} Initial Fuel Prime Delay

B3202 description:

Before the {B3201} "Initial Fuel Prime" pulse is delivered, the PCM will delay the prime pulse for this long after key on.

[joecar]

See B0106...

I don't know how else the PCM determines it's now in running mode.

[Rich Z]

See B0106...

I don't know how else the PCM determines it's now in running mode.

Hmm, in my tune, there is a value of "8" in that location. Since there are 24 reference pulses per crankcase revolution, my PCM switches from "cranking" to "running" after the crankshaft has only made a third of a revolution? That doesn't sound right to me.

[Rich Z]

Hi Rich,

B3201 description:


B3202 description:

Yeah, I read those descriptions, but I'm unclear about what they are telling me. Sorry, I like to understand the "why" behind things.

I guess I'm comparing this function to the accelerator pumps in the olden days with carburetors. You used to pump the gas pedal a few times before starting the engine to squirt fuel into the intake manifold. Obviously this would provide fuel for all 8 intake runners going to each cylinder. So in this case of EFI, do ALL the injectors provide a prime pulse of fuel merely by turning the key to the "on" position before the engine is even cranking over? Or does the engine actually need to start cranking before each cylinder gets that priming pulse in sequence?

B3201 appears to control the amount of fuel delivered in the prime pulse.
B3202 appears to control the delay between the ignition key "on" and when the pulse actually takes place.
B3203 isn't as clear to me, but seems to be an anti-flooding control to keep prime pulses from being delivered if the car doesn't start on the first and subsequent tries without allowing some time to elapse between those pulses.

Does increasing the amount of fuel substantially for the fuel prime pulse (B3201) help significantly with getting the engine to start with the minimum amount of cranking over? Anyone ever played with this?

And, about B3202. Is there any benefit to putting in any sort of delay here? I'm a bit confused about how this works. I'm presuming that once the engine is cranking, the injectors are providing fuel to each cylinder in sequence, so if the delay is longer than the time I move the key to engage the starter, does that cancel the priming function? As I mentioned, I normally turn the ignition key to "on" and then wait a few seconds for the fuel pressure to stabilize. Yeah, I know I probably don't need to, but it's gotten to be a habit to check the "vital signs" before engaging the starter. So would, then, a delay in this priming pulse be better to put the priming pulse closer to when I actually engage the starter? Or is this completely insignificant?

[vcls1]

hey the way i understand it to work is when the ignition is switched to on, prime takes place but that is only to pressurise the fuel rail, injector pulse does not take place until crank rotation hits the determined value of crank trigger sinusoidal pulse signals which you had said was set at 8 being 1/3 of a crank rotation.
As far as your startup is concerned i would be looking into IAC and commanded fuel when cranking to try and get it to burst into life a little better.

[hog]

There is alot going on during this time.

I have found tables for:

1st crank pulse fuel mass vs. coolant temp,
2nd crank pulse vs coolant temp
2nd crank pulse delay time vs. coolant temp (measured in crank pulses) 6 pulses was stock
Prime Pulse Delay after powerup vs. Coolant Temp (seconds)=0.3-0.1seconds depending on temp
Prime Pulse Enable Minimum Runtime vs Coolant Temp (seconds) 10 seconds
Crank Fuel Barometer Gain Multiplier vs Baro (multipliers 0.650-1.000)
Prime Pulse Fuel Mass vs. Coolant Temp. (0.300-0.018 grams)
Normal Crank Fuel/Air Equivalent Ratio vs. Coolant Temp. vs. Time
Extended Crank Fuel/Air Equiv. Ratio vs. Coolant Temp. vs Time
End of Injection Timing Trim Pulse vs .Coolant temp (Low Resolution Periods)
End Of Injection Timing vs. Coolant Temp (Low Resolution Periods)
Prime Fuel Disable Engine RPM


Not sure if its possible on the Vettes, but on the trucks if you have the key in the run position, and you get under the truck and grab the crank pulley and slowly turn the engine over(slow enough to let the compression bleed off so you arent fighting it) eventually all of a sudden the fuel pump will cycle. It will cycle for 3 seconds stock IIRC(there are PCM tables that control this pump prime length) and it will cycle twice for every complete rotation of the crank, so 4 pulses in my case and 24 pulses in your case.

It would be easy enought to see when the injectors allow a prime volume of fuel by monitoring their power supply. If they pulse when you turn the key to run, then that is the prime pulse volume. If you dont detect any injector activity upon Key On, then you know that the Prime pulse volume must occur when you continue from the "Run" to the "Crank" position.

Rich, B3203 simply holds back the Secondary Prime event until that timer has been met, provided there is a continued no start. Once the timer calibration has been met, the PCM allow the secondary prime pulse. If this table wasnt present, the PCM would tell the injectors to continually give the engine its secondary prime pulse volume, every 6 pulsecounts(or whatever the 2nd crank pulse delay time-measured in pulsecounts is entered as a value) until either the engine started or the engine flooded out.
So primarily you have the primary injecor prime pulses, if that doesnt work, after a certain amount of crank pulses you'll get the secondary injector prime volume, then if it hasnt started, the actual TIME counter kicks in and wont allow more fuel until the TIME counter has expired. These are interwoven and overlapped, all in an attempt to prevent flooding.

Then if it does flood, you hold the accelerator on the floor(which kills the injector pulsewidths by forcing the ECM/PCM into "CLear Flooded Engine Mode". Once you crank the engine over for a few seconds without any fuel being added, you can release the accelerator whgich allows the crank/startup tables to be followed again thus allowing fueling for startup.

peace
Hog

[Rich Z]

Well, OK, that all makes sense to me.

I've actually had pretty good success fiddling around with the tune, and the car starts with the engine just barely cranking, which is the way I want it. And it's not just some feeble start, neither, like one cylinder starts firing and the others decide they might as well too. I turn the ignition key to ON, and let it set for a couple of seconds, then engage the starter and it starts up firmly in about a second.

I took baby steps in the changes I made, so it took me a few days to get it where I want it. So I'm satisfied with what I've got now and can move on to some other minor problems.

The most annoying of which is that when I start the car, I usually let it sit for a while, as I am logging with the scanner and like to just see how things are looking while the engine temps start warming. It idles real well, but when I back out of the garage and take my foot off of the gas pedal to drift into the driveway, the engine has a very pronounced surge to it. When I stop, there is no surge, and when I start moving forward, even taking my foot off of the gas to drift at the same 2mph, there is no surging. Only seems to happen when going in reverse out of the garage. Does the PCM really even know that the car is going in reverse? If so, does it use different tables for that condition?

Along the same lines, does the PCM know when the clutch is depressed? Since there are tables indicating that certain tables and values are used depending on whether the drivetrain is "in gear" or at "idle/park", I guess there must be some way for the PCM to know this. Honestly I haven't yet looked for a PID that indicates this and I've never looked for any sort of sensor actually on the clutch engagement system itself. So how does the PCM know whether you are "in gear" or not?

[IJ.]

There will be a Clutch switch for Cruise control and start inhibit, I'm assuming the PCM can see this?

[joecar]

Yes, the PCM see the the CPP (clutch pedal position) switch, it uses this to choose between the P/N tables or the IG (in-gear) tables.