GEN3 PCM Cranking Parameters Definitions and Effects

[DetRedWingsFan]

Does anyone know where I can find the definitions and effects of the cranking parameters for a GEN3 PCM? I have searched and searched and can find same for every other parameter except cranking.

Reason I ask is that I'm having an issue only with cranking. Sometimes engine starts instantly, other times difficult to start but if I stop cranking,shut key off and retry, it will start readily 99% of the time. The remaining 1% might require fiddling with the throttle to get it started but I can always get it started. Given that sometimes it starts instantly, I'd like it to start instantly 100% of the time.

Engine is a 1996 Corvette LT1 upgraded to a 24x GEN3 PCM system with individual coils, cam, throttle body, headers, injectors, etc.
Other than cranking, the car runs almost perfect, I have spent a lot of time tuning and warmup, idle, open loop, closed loop, part throttle and WOT all are darn good. I run it at track day events and it is really fun to drive and quick.

I understand most of the cranking parameters and most of the why but not enough to solve the cranking issue. If anyone has a link, pdf, doc or a tuning guide that would explain the GEN3 PCM cranking parameters, it would be appreciated.

[blindsquirrel]

To rule out a fuel pressure issue, turn the key to RUN, wait for it to time out, turn off, then crank. Let it run a full prime cycle.

[Folken]

Have you checked your fuel pressure? If you never dropped the tank or are still usuing the stock fuel pump they usually lag and come back a little when they fail.

[DetRedWingsFan]

To rule out a fuel pressure issue, turn the key to RUN, wait for it to time out, turn off, then crank. Let it run a full prime cycle.

Thanks, I tried lots of variations of that and didn't seem to make any difference. Was hoping that was the problem as it would have been an easy fix. I generally smelled fuel when it is hard to start. Clears up once it does start.

[DetRedWingsFan]

Have you checked your fuel pressure? If you never dropped the tank or are still usuing the stock fuel pump they usually lag and come back a little when they fail.

Thanks, Fuel pressure checked good (gauged it at the rail) and no issues at 6000 RPM in 3rd or 4th gears.

[gtstorey]

Are you still running the stock regulator and if so how did you account for the pressure difference when on the throttle? I don’t think this would be a 1:1 regulator since the pressure generally is just 2 different numbers with nothing in between. Not saying this is the problem but just curious.

[DetRedWingsFan]

Are you still running the stock regulator and if so how did you account for the pressure difference when on the throttle? I don’t think this would be a 1:1 regulator since the pressure generally is just 2 different numbers with nothing in between. Not saying this is the problem but just curious.

Thanks. Stock regulator and it is vacuum referenced. My thought is that if it were a fuel pressure problem, it should show up every time when cranking but it doesn't and all other conditions besides cranking are great.

However, based on the feedback here, I am going to add a fuel pressure sender to the rail and monitor the pressure with the MPVI2+. And also replace the current fuel pressure regulator.

[gtstorey]

Maybe you had to skew things to much to make up for the fuel/injector data not being quiet correct to get to run everywhere else that it has the cranking fuel off. I'm really throwing out a guess, I just happen to have a 1996 w/ LT4 and have wondered how the fuel would work in a swap. I don't remember the exact numbers but the fuel pressure will change from something like 40psi at partial throttle to 50+ at open throttle. This is by design at matches the service manual. I cam check manual with the actual numbers tonight.

Are you running stock injectors? If so, that data might be incorrect also.

[Alvin]

Does anyone know where I can find the definitions and effects of the cranking parameters for a GEN3 PCM? I have searched and searched and can find same for every other parameter except cranking.

Reason I ask is that I'm having an issue only with cranking. Sometimes engine starts instantly, other times difficult to start but if I stop cranking,shut key off and retry, it will start readily 99% of the time. The remaining 1% might require fiddling with the throttle to get it started but I can always get it started. Given that sometimes it starts instantly, I'd like it to start instantly 100% of the time.

Engine is a 1995 Corvette LT1 upgraded to a 24x GEN3 PCM system with individual coils, cam, throttle body, headers, injectors, etc.
Other than cranking, the car runs almost perfect, I have spent a lot of time tuning and warmup, idle, open loop, closed loop, part throttle and WOT all are darn good. I run it at Open Track Days at WHRRI and it is really fun to drive and quick.

I understand some of the parameters and some of the why but not enough to solve the cranking issue. If anyone has a link, pdf, doc or a tuning guide that would explain the GEN3 PCM cranking parameters, it would be appreciated.

People have mentioned fuel pressure before. I think it's worth saying again. Watch fuel pressure when it will not start. Occasionally it takes a second prime to get fuel up to the rail.

If not that.. some injectors go bad by leaking. This can cause hard starts.

[anniversaryss]

I don't have a check valve for my pumps so I typically prime once or twice and it starts right up.

[TheMechanic]

I don't have a check valve for my pumps so I typically prime once or twice and it starts right up.

If you want to fix that......
Fuel System>Fuel Pump>Fuel Pump>Prime Time. Set to 5-10 seconds.

[blindsquirrel]

Big difference between starting after one pump prime (like it's supposed to) and two (means the first crank attempt failed to start).

[DetRedWingsFan]

People have mentioned fuel pressure before. I think it's worth saying again. Watch fuel pressure when it will not start. Occasionally it takes a second prime to get fuel up to the rail.

If not that.. some injectors go bad by leaking. This can cause hard starts.

Thanks, your feedback is appreciated.

Engine has new Accel 30lb injectors. A/F ratio is where it should be and O2 sensors indicate PCM operating correctly in closed loop. Engine has no hesitation at WOT, part throttle response is excellent. Even with the bigger cam, injectors and throttle body, it pulls strong from 1600 RPM to 6200 RPM. As I said, engine runs great for all conditions except cranking sometimes but I can always get it started.

I don't think it is fuel pressure as I can smell fuel when it is difficult to start but have ordered fuel rail adapter and sensor so I can monitor fuel pressure. Manual says normal fuel pressure is 41 to 47 psi and I measured 46 psi. Will feed the fuel pressure sensor output into the MPVI2+ A/D input and record it from cranking to WOT at 6200 RPM in 4th gear.

But does anyone know where there are definitions for all the cranking parameters?
I know that the VCM Editor has a definition that says what it is but not what it does;
ex; Fuel Crank - Key-on Soaktimer Modifier: Multiplies Prime pulse based on soaktimer.
I have talked to HP Academy, HP Tuners and other expert tuners but they all say they never have to do anything with Cranking parameters.
I can understand adding fuel if you are cranking for an extended period but what is "soaktime" and why would it affect cranking?

[blindsquirrel]

Accel provides data for their injectors now? Since when?

[DetRedWingsFan]

Accel provides data for their injectors now? Since when?

Injectors are Accel 150830 30 lb/hr. Used that spec to set the injector parameter in the tune.

[blindsquirrel]

For Injector Flow Rate, sure. What about the others, you know, the important ones that affect whether it 'just works' without other adjustments/fudging, or leaves you with a complaint of hard to start?

[blindsquirrel]

And, is that 30lb-hr at the industry standard of 43psi, or at the actual fuel pressure being used in real life?

[Alvin]

Accel has been known for terrible leaky injectors all the way back since when I used to do only chip cars... quite literally 20+ years ago

[gtstorey]

Manual says normal fuel pressure is 41 to 47 psi and I measured 46 psi.

That is a changing pressure from 41 to 47 depending on open throttle vs partial throttle. It will be interesting to see if the pressure shows as anything in-between if you data log. My gauge says pretty much one or the other.

[blindsquirrel]

screenshot.15-03-2023 16.02.50.png

It should be continuously variable, between whatever it is with pure spring pressure alone and however much the highest vacuum (lowest pressure) is able to lift the diaphragm. The PCM can't calculate the correct flow rate/injector pulse needed if it changes at any rate other than 1:1, because it doesn't have a real-time fuel pressure input.