Disassembly: Startup fuel tables

**Greg E** · 03-21-2012, 12:10 AM

90% of you probably won't ever need to modify these tables but I was curious how the start up routine worked in the ECU. Turns out, there's quite a bit going on when you turn the key. I believe the answer to the issue with "hot starts" on some cars can be fixed in here. As it turns out the fuel table, injector size and latency tables are completely ignored during this time. The ECU simply calculates an IPW based on a few factors.

Once the engine first starts rotating from the starter, there is a single injector pulse that is calculated. The value comes from from a table which I've dubbed "Cranking IPW " and then there is an Adder value applied, then this Primer pulse sent to the injectors. As you can see, the cooler the coolant temps, the longer the injectors are held open.

After this primer pulse is setup, there are various corrections which are applied to it. First one is a multiplier which is looked up based on how fast your starter is cranking the motor. There are two tables and coolant temp determines which tables is used. As you can see, both tables are the same so coolant temp is irrelevant. The coolant temp switch value between these two tables is -4 degF so again....

In a future chrome ROM, I might make use of this for an alcohol switchable map.

So we have the Primer Pulse * RPM Correction Factor = Corrected Primer Pulse.

It's not over yet. There are 2 other compensations still to go!

So as the starter is now cranking the motor, the ECU now counts how many ignition strokes the motor is turning and then starts reducing the Corrected Primer Pulse.

The formula from start to finish is this:

(Base Primer Pulse + Adder value) * RPM Comp / Ignition stroke reduction = Start up IPW

Again the fuel table, injector size and latency tables are completely ignored during this time. This is why you piggy back guy running HUGE injectors may be having hot start issues. Just after the engine has started, there is also a milisecond timer counting down and another enrichment factor is applied based on coolant temperature again (shown in the above screen shot).

One last table the ECU uses is just a simple RPM look up to tell if the motor is actually running on it's own power or still being spun by the starter. Not sure why this is still here. The ECU knows when you have the key turned into the START position. Must be left over from the earlier days before they wired up the starter system to the ECU.

I've been able to run as high as 560cc injectors EVO injectors without really needing to tweak these tables but when you start getting to the 720cc range, you might want to start reducing the base values a bit. I usually just divide them in half and the car starts stock smooth again with no spark plug fouling.

**Alan92RTTT** · 03-21-2012, 07:10 AM

Should this be in the ECU forum?

**J. Fast** · 03-21-2012, 08:35 AM

Originally Posted by Greg E

90% of you probably won't ever need to modify these tables (even when running alcohol) but I was curious how the start up routine worked in the ECU. Once again, the EVO ROM disassembly was my guide thru this and once again, our code is much much simpler.

I'm still picking thru all this and unfortunately, it seems like I have to dissect the entire IPW calculation but here's some progress:

Once the engine first starts rotating from the starter, there is a single injector pulse that is calculated. The value comes from from a table which I've dubbed "Cranking IPW -vs- Coolant Temp" and then sent to the injectors. The fuel table, injector size and latency tables are completely ignored during this time.

Just after the starter is disengaged we get into the mess of incorporating everything. I still don't quite understand it all, but there are 2 start up compensations for IPW that involve coolant temp. One we are familiar with and that's the "Coolant Temp Fuel Enrichment" table which is just an additional percentage added onto the IPW.

The other is an adjustment to something the EVO guys call the "base fuel pulse width". I'm still picking apart how this value is calculated, but this table takes a percentage off that just after the starter disengages.

More to come on this when I get time.

Cranking IPW vs. Coolant is your priming pulse.

Base Pulse Width Startup Comp vs. Coolant Temp is for after the engine starts. You add more fuel right after startup so the engine doesn't stall warms up. You can program an ECU Startup Comp based on a specific number of engine revolutions ( ie. 300 passes thru the cam trigger wheel), or you can set a specific time for startup comp (say 1:00 minute), or you can program and compensate based on coolant temperature. Lots of options. I think the program you're using only allows for a coolant temp adjustment.

**Greg E** · 03-21-2012, 09:05 AM

Originally Posted by J. Fast

Cranking IPW vs. Coolant is your priming pulse.

Base Pulse Width Startup Comp vs. Coolant Temp is for after the engine starts. You add more fuel right after startup so the engine doesn't stall warms up. You can program an ECU Startup Comp based on a specific number of engine revolutions ( ie. 300 passes thru the cam trigger wheel), or you can set a specific time for startup comp (say 1:00 minute), or you can program and compensate based on coolant temperature. Lots of options. I think the program you're using only allows for a coolant temp adjustment.

Primer pulse would be a better term for the first table. I'll rename it.

The program can use any value for adjustments, its just the stock ecu is using coolant temp.

**Greg E** · 10-17-2012, 09:42 PM

*Bump*

I finished this disassembly some time ago. Just now getting around to updating this thread!

**Jesters Deadd** · 10-19-2012, 11:36 AM

I have successfully edited these tables now my 1000cc injs start my car great! And I finally removed the fuel pressure solenoid. I would get flooding issues before. But now it starts like stock hot ot cold... maybe better.

Seems silly Mitsubishi even used that fuel pressure solenoid! They could have saved tons of money if they just twicked these tables in the program and not fitted our cars with those solenoids. But hey we get an extra output to play with.
@ greg I may make my old pin 48 control a shift light now, just til i come up with some thing better to do with it.

BTW: Greg you're the Man! haha

**Greg E** · 10-19-2012, 12:11 PM

Originally Posted by Jesters Deadd

I have successfully edited these tables now my 1000cc injs start my car great! And I finally removed the fuel pressure solenoid. I would get flooding issues before. But now it starts like stock hot ot cold... maybe better.

Seems silly Mitsubishi even used that fuel pressure solenoid! They could have saved tons of money if they just twicked these tables in the program and not fitted our cars with those solenoids. But hey we get an extra output to play with.
@ greg I may make my old pin 48 control a shift light now, just til i come up with some thing better to do with it.

BTW: Greg you're the Man! haha

That was the whole point of the "custom" outputs. To make them do anything your imagination can dream of!