Priming Pulse - Reasonable?

[franklynb]

I'm having trouble sorting out a reasonable set of 'cold start' values.

At current starting temps, which run around 40F, my MS-II 2.1 requires a dozen or more cranking attempts. It tries to fire on the second through nth attempt, but dies in a few [under 10] ignition events. I've raised the ASE to 200% for 1000 cycles, which helps; but still have excessive cranking.

I'm suspecting that my mechanical fuel setup is introducing air into the supply stream. I'm using:

-a 5psi piston electric pump at tank, feeding ...
-a 70 psi rotary electric underhood, attached to rail on a 15" long supply hose
-a 1/2 pint "surge" tank between the two pumps, nearest the high pressure pump
- largish low imp TBI injectors, 650 and 950cc, run alternating, 4 squirts/cycle, with a
[reasonable] idle pulse width around 3-5ms depending on enrichments.
-no check valve inline

I chose this setup to allow the shortest high pressure connection with an [existing] factory banjo & O-ring set of terminations on the hose.

Once started, it runs fair [lean, but I'm working out my VEX/EGO tuning issues on another front]; but I'm suspecting I need more than [the currently programmed] 5.0 ms of priming pulse to get enough time to bring fuel pressure up, and purge air. Don't have a gauge on the rail - yet - but that's on order.

Since PWM% is overridden during cranking, I don't want to risk burning the injectors by slamming a huge [20ms?] priming pulse on each crank initiation. I've added a slope of 2.0ms -> 24.0 ms in the temperature dependent slots, which adds another 13ms -- so if I understand the additions correctly, my initial pulse is ALREADY at 5.0 + 13 = 18ms.

I'm sure there is no straightforward way to predict how large a pulse it takes to overheat a 40F injector at cranking -- but how far can I reasonably push the priming pulse in this situation? My current idle pulses are running 4ms or so; and I've not yet progressed past 10% throttle enrichment at 4K, which tops out around 9ms [all this is PWM'd, of course;
just adding it for reference].

MS-II, N/A 2.6L Montero, NB EGO; Smog Cert is my goal!

[efahl]

I'm having trouble sorting out a reasonable set of 'cold start' values.

Frank,

Do you have a handle on the warm start? If you have a couple of working points, then you can estimate what you'll need for prime/cranking from those.

At current starting temps, which run around 40F,

I thought you said "cold." (It has been very warm here the past couple of weeks, near 30F in the morning.)

Since PWM% is overridden during cranking, I don't want to risk burning the injectors by slamming a huge [20ms?] priming pulse on each crank initiation.

Don't even worry about it, you need to turn the injectors on for seconds or minutes at a time to get them hot enough to damage them, a mere 20-50 ms priming pulse isn't anywhere near the danger zone.

Priming pulse probably has little to do with cold start in your case, due to your fuel system. I would recommend that you jack up the cold cranking number quite a ways, double what you have now and see what that does.

Eric

[franklynb]

I'm having trouble sorting out a reasonable set of 'cold start' values.

Frank,

Do you have a handle on the warm start? If you have a couple of working points, then you can estimate what you'll need for prime/cranking from those.
Eric

Thanks, Eric. I reset the -40f limit to 61ms, and it started on the first crank attempt , although it took perhaps 5 or 6 engine cycles, sputtered a bit, then caught. I've been able to verify that my 170f temp setting of 2.0ms doesn't unduly influence hot starting; so I'll keep sneaking the 61ms down to find a reasonable starting "slope" over the range of "cold" temps.

Or maybe it will just warm up!

Good info. Thanks, again.

--frankb