Trying to get pre-crank injector pulses

[kilohertz]

Hi guys,

I read somewhere over a month ago that someone had managed to get pre crank injector pulses but I can't find the thread.

Could someone please help me with the code or method to get a few pre crank pulses on the TBI ECM 1227747?

Thank you.

[kilohertz]

A little more research... I found it looks like the pre-crank pulses were done on the second gen ECMs, '93-'95??

Does anyone have an ECM that has the pre crank pulses, that could share a BIN code so I could try to find it and implement it in the 1227747?

thanks!

[Six_Shooter]

You mean like a Prime pulse? All GM ECMs do that AFAIK.

[kilohertz]

You mean like a Prime pulse? All GM ECMs do that AFAIK.

Exactly! Mine doesn't, and I have tried 3 different BCCs.

My injectors won't pulse until it's cranking, and with the convoluted intake path of the Flathead, I would like to have some fuel sitting on the butterflies ready to get sucked in.

Thanks!

[In-Tech]

The '7747 is pretty archaic and I don't think there is a prime pulse, from memory. If your injector/cu in constant is correct(very critical)...then all you should have to tailor is IAC counts during crank. I realize this is a bit vague. Bottom line is it will take a bit of work since you are starting from scratch basically. It seems like you are well on your way.

[kilohertz]

The '7747 is pretty archaic

Ha! But miles ahead of a Holley 94 carb.

If your injector/cu in constant is correct(very critical)...then all you should have to tailor is IAC counts during crank. I realize this is a bit vague. Bottom line is it will take a bit of work since you are starting from scratch basically. It seems like you are well on your way.

Thanks Carl. Yes I have moved from 2.8L injectors to 4.3L and the math comes out to BPW of 126 for the 3.9L Flathead with BSFC of .65 and 100HP. I have played with the IAC park position but didn't realize it was that important...I will continue to fine tune it. Appreciate the info.

Cheers

[tayto]

make sure the crank wire actually has voltage while switch is in crank. also that the various ecm power feeds dont lose power while cranking. I would start with a 4.3L calibration vs a modified 2.8L bin myself.

[kilohertz]

Thanks Tayto,

I am using BCC ASDX which is '88 5.7L Sync fueling mode. Yes, there is crank voltage on the crank wire and the other main Vcc power as well. The ECM is working properly, what I am trying to do is get a few injector pulses before I hit the starter, right after key on.

Cheers

[tayto]

i dont think any GM EFI does that, nor would you want it to.

[steveo]

you definitely wouldn't want it to do that just at key on without warning and at zero throttle, that would be a mistake.

remember with a TBI engine your injectors are before the throttle plate, so unless the engine is drawing vacuum or the plates are open, you're just spraying atomized fuel at a brick wall which will just fill your engine bay with fuel fumes.

you just need to get used to the fact that you need to turn over a TBI engine a half dozen times for it to start sometimes even when it's properly tuned.

i've seen a hack for TBI where hitting full throttle will give you a shot of fuel without the engine running, simulating what the accelerator pump on a carb does when you pump the gas). it was a long time ago. this at least squirts the fuel into the manifold. is that what you're talking about ?

[kilohertz]

you just need to get used to the fact that you need to turn over a TBI engine a half dozen times for it to start sometimes even when it's properly tuned.

Thanks guys, I appreciate the good info, I will cease to pursue this any further. I know it id primitive fuel injection, I think I am expecting too much from it.

i've seen a hack for TBI where hitting full throttle will give you a shot of fuel without the engine running, simulating what the accelerator pump on a carb does when you pump the gas). it was a long time ago. this at least squirts the fuel into the manifold. is that what you're talking about ?

Yes, that was it exactly.

I also should realize that I am trying to get the startup perfected, for a scenario that will only present itself once in a blue moon. This truck will be a summer crusier and will hardly ever see temps below 5C, so the cold start I am working on...well, a few seconds of cranking won't hurt anything.

I have it pretty well dialed in now anyway, for the cold start, still working on perfecting the IAC park and choke settings...I figure I better do this while it is still bloody cold here...Shop was -2C tonight.

I really am enjoying learning about this, although I'm about 10 years behind when it was "hot stuff". Appreciate the help, even though it's out of fashion.

Cheers

[tayto]

holding the throttle 80% or more will initiate "clear flood mode", this will still pulse the injectors but at an extremely lean mixture, 20:1 IIRC.

[kilohertz]

Thanks Taylor,

I just got back in from the shop after testing my latest tune, and I have pretty much dialed in for now. Shop was 2C and it fired right up and idled properly at 1000 RPM, slowly dropped to 900 after 7 seconds then slowly dropped over the next minute as it warmed up, to the target 700 RPM at 75C and above. No surging or bouncing off the idle saver, it just ran nicely. I have decided the pre start pulse isn't required.

Shop is up to 12C and I am heading back out to weld up a temporary exhaust so I can properly install the O2 sensor for some CL tuning. I have 2 exhaust stubs with bungs from a Vortec engine so I can put NBO2 on one side and WBO2 on the other. I have an Innovate LC-2 and it will be nice to see the WB data.

Here is this mornings data log file.

Cheers