ARAP long crank alternate fix - Third Generation F-Body Message Boards

rhuarc31 · 10-10-2007, 08:03 PM

Read many times on the board how when switching to the arap code people had trouble with extended crank times. The prevailing fix alway seemed to be to take the "Crank Fuel PW Multiplier vs. Ref. Pulse" table up from zero several places lower on the table. Now I tried this fix, and it did indeed seem to help a bit, but being the pain in the ... uhhh I mean perfectionist that I am, I wasn't satisfied. So I started thinking about it and looking through the bin and had a couple thoughts.

First is that in a carbureted car when we look for cold starts being easy we restrict air flow - "choke". In a fuel injected car, everyone seems to add fuel. Probably because there is no choke as we know it on fuel injected cars, yet adding fuel is easy. Both accomplish the same thing, a richer mixture for firing the vehicle.

Second was that in this day and age with gas prices so high, the last thing I wanted to do, especially with an already thirsty 400+ cubic inch small block was add fuel anywhere I didn't have to.

So my brain started percolating trying to come up with a way to add a choke to the set up to restrict air flow. Came up with some real screwy ideas let me tell you.

Then a simple thought hit me. We already have a device for adding or restricting air flow in addition to the throttle blades, the IAC. But surely this starts closed and opens up to add air as needed, right? Fat chance buddy, when I checked the iac, start up position is ALL THE WAY OPEN!. What the hell kind of sense does that make? So I checked some other '165 bins, and guess what, they were all that way. Maybe all of you knew this already, but I'd never really thought to look at that parameter before, or think about it at all really. I'd always added fuel with the rest of the crowd.

So knowing that restricting the air works great on a carb, I cut the IAC start up position in half to 80. And when I cranked the key, BEHOLD, the car started instantly, despite the 58 degree weather we had this morning. I was actually able to take fuel out of my start up enrichment and cranking tables, which will make me much happier. Heck I was so happy I must of started the car 50 times today just to see it start so fast.

Now using my head, I sure wouldn't cut the IAC down to 0 at start up, but a good middle ground where it can react in each direction as it needs to makes a whole lot more sense than all the way open to me. Maybe this will help the rest of ya, maybe not, but thought I'd share.

John

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1tpi · 10-10-2007, 09:03 PM

I have AUJP_8D maybe I will try that, what do you think

RednGold86Z · 10-10-2007, 09:39 PM

Quote:

Originally Posted by 1tpi

I have AUJP_8D maybe I will try that, what do you think

It's not quite right in a few ways.
First, a carb's choke causes much more fuel to get sucked in, and thus richer.
Second, a closed IAC will reduce the cranking RPM (bad idea).
Third, the right way is to set the IAC to get a desired RPM overshoot, AND set the fueling properly to wet the port wall, cylinder wall, and have enough left over to burn richer than stoich. After the overshoot, it should settle to the idle in a very controlled manner.

Lowering the IAC effectively can richen the cranking mixture (although only a little), via reducing the air. It cannot however get enough crank RPMs with a weak battery, or get enough RPM increase rate and overshoot to ensure a proper start when cold (if you overdo it). Most modern systems will allow IAC to have a different cranking position vs coolant temp, to avoid the common GM flare-up. GM was very conservative in their calibrations to ensure long lifetime starting. ARAP has pitifully small PWs for cranking if I remember correctly, but I think every DRP gives an injection during cranking, so mult by 4 or 8 (depending on how you mentally view PWs).

Let it breath enough to start quickly. Give it enough fuel to start quickly. Don't give it so much air that it zings up too high (annoying + high emissions). Use a wideband powered by a separate battery to get the fueling right (like 10-12:1 immediately, then go to warmup a/f).

A good cranking/starting point for the IAC at each temperature is only a few steps (10 warm, 20 cool, 30 very cold) above the idle position soon after starting (with same oil temperatures).

1tpi · 10-10-2007, 10:11 PM

Thanks, lets see what happens

rhuarc31 · 10-11-2007, 08:50 AM

You're Right Redngold about all the choke stuff of course, but we both are, yes the choke does restrict airflow, which causes the carb to suck more fuel by creating a greater vacuum. And of course when it's really cold you have to pump it a few times to shoot extra fuel in from the accelerator pump.
You'd be suprised, however, at how much more air the IAC flows at 160 than it does at 80, basically the same as a good sized vacuum leak, which can lean out a mixture and cause hard starting.
The mix numbers for startup you gave are about right. All this allowed me to do was go from having to use the 10:1 number to a little over the 12:1 number. I get a nice even start up at about 12.4:1 on the WB. It'll even start up leaner than that, but then as the decay kicks in I'll get an unsteady idle for a bit. Nothing that'll stall it or anything, just me wanting it perfect again.
As for lowering the crank rpms, I'm not going to argue that, what I am going to say is that if you're starting system is weak enough for it to make that much difference, you need to fix that before you start worrying about extended crank times. There is, though, a setting to offset IAC steps depending on battery voltage.
I am going to have to disagree with you on the "wet the port wall, cylinder wall, and have enough left over to burn richer than stoich". If at all possible I would never put enough fuel in to wet down the cylinder walls. Starting is hard enough on moving parts without washing down the cylinder walls. If I have to put that much fuel in something else is wrong IMHO.
Another interesting note is that there is an IAC multiplyer VS coolant temp - basically a percentage- that in colder temp's does get lower, reducing IAC steps. So apparently GM did think that the IAC could help act as a choking agent.
Also, don't know about you, but an IAC position of 80 IS about 30 more than my idle point, so it's right on the money with what you were saying.

In reality though, we could argue it back and forth all day, and both bring up all kinds of good points. Bottom line though is that it worked for me. Enough that I was even able to lean out the mix a bit - at least at temps 50* F and up. Does that mean it will work for everyone? No, just like any of the other tricks we bat around. But if someones looking for something to try it gives them one more alternative.

RednGold86Z · 10-11-2007, 11:25 AM

Quote:

Originally Posted by rhuarc31

I am going to have to disagree with you on the "wet the port wall, cylinder wall, and have enough left over to burn richer than stoich". If at all possible I would never put enough fuel in to wet down the cylinder walls. Starting is hard enough on moving parts without washing down the cylinder walls. If I have to put that much fuel in something else is wrong IMHO.
Another interesting note is that there is an IAC multiplyer VS coolant temp - basically a percentage- that in colder temp's does get lower, reducing IAC steps. So apparently GM did think that the IAC could help act as a choking agent.

The engine doesn't give you the choice to wet or not wet the walls at cold start. It WILL simply suck that fuel out of the air, and if you don't give it enough, it won't have enough left to make a stable burn (be it lean, misfire, or stall). If you were to "audit" the fuel during cold startup, you'll see at first that much has to go to the port walls, the cylinder walls, and piston top. Then as combustion starts and continues, that combustion chamber portion of the fuel drops rapidly. This IS the main reason for afterstart enrichment. Most of it goes through the engine as HC's. During the warmup phase, some will contribute to enrichment = CO (as in: why won't it idle well at 14.7:1 when cold, but fine when warm = residuals are burning at the end of the combustion and show up on a lambda meter, but didn't contribute to the power stroke).

The IAC does need to be more OPEN during cold start. I've done dozens to hundreds of -40 to -15 C cold starts (it's my job

). There's simply no other way to counter all the extreme load from the oil. Richness is also necessary, but an engine runs on 3 things - Air (= IAC), Fuel/Air near the plug, and well timed spark with enough energy to ignite the Air/Fuel.

I'm not arguing. I'm just pointing out the correct way to think about this. "Choking" is a carb crutch, and doesn't apply well to EFI.

rhuarc31 · 10-11-2007, 12:41 PM

Actually I think the problem is that we're talkin about to different situations. I'm talking about the extended cranking problem the arap was known to have in pretty much all temperatures. I'm not worried about - 40 and -16 starts, If it's that cold, I'm not going anywhere, much less starting the firebird

Heck, I'm probably never gonna start it if its below 40 Farenheight. So I'm not worried or thinking about cold starts in general.
As for choking not being the right term for EFI, well maybe, but it's sure a hell of a lot easier to type than cold start enrichment, and for most people it gets the point across.

rockit · 10-11-2007, 05:39 PM

If i didn't have enough fuel at startup after modding the pulswidth vs ref puls table, would it cause the car to almost stall after it almost started? i have this issue now with my car. It used to start hard as ARAP tends to, i searched ,found the fix and did it but now the car cranks for a second, starts for a split second then the rpms drop for another split second then either the car starts(most of the time) or it stalls(once every five starts maybe). if it DOES stall i have to crank for like 4 seconds before it will start again.

i noticed ARAP has more iac startup steps then the other 165 bins, about 20 i believe. might have been there to account for the long startup time.