Engine Start Tuning Questions | AEM
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spikeygg
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Engine Start Tuning Questions

Hi guys, now that it's cold outside again I'm taking a more aggressive approach to tuning my cold-start but I'm realizing that I've got some questions. I'm trying to understand the specifics around the start tables, specifically:

  • Start Extra vs. Temp table
  • Warm Up Enrichment table

I see in the "Function Explanation" it says: "Depending on the coolant temp, the cranking injection pulse will be increased by a percentage (Start Extra vs Temp). The duration of this extra cranking fuel is based on temperature and is pulled from the Start Extra Decay table." It sounds to me like this table applies during cranking, when the RPM is below the Crank Max RPM, is that true?

 

Also, in the "Function Explanation" it says: "Once running, additional fuel percentage may be injected based on coolant temperature. This is pulled from the Warm Up Enrichment table." This makes it sound like the Warm Up Enrichment table doesn't apply until the RPM is above the Crank Max RPM, is that true?

 

Thanks,

-Greg

Kurt
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All the fuel trims are active

All the fuel trims are active during cranking.  Boost, AIT, Warm Up, Start Extra, Throttle, Baro, Gear, etc.... these are all multipliers to the base injector PW value.  That base PW will either come from the Crank Injector Time table or the main fuel map.  It will switch from one to the other at Crank Max RPM.  Watch the channel Start Extra and you'll see it reduce to zero over the afterstart time you have set.  

These modifiers apply to the sequential injections during cranking and running.  The options Crank Inject All affects how the initial pulse is applied.  Initial pulse does not have any modifiers.  It's just the PW from that table sent to all the injectors right after sync.  Sequential injections after.

spikeygg
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Thanks for the response Kurt,

Thanks for the response Kurt, it's a strange problem I have. The car starts up perfectly when it's warm or hot but when it gets cold outside I see the injector pulse width do a kind of step function on the handoff from cranking to engine running. You can see it in red in this screen shot: https://www.dropbox.com/s/pph558dv27zibjn/ColdStart.png

I tried to link an img to it here but it looks like it's not going to work...

Cold Start @ 43F

 

Anyway, looking at the shape of the engine speed waveform (brown), it kind of bogs out when that transition happens. I think it's going too rich and then recovering.

Any ideas how to smooth this out without affecting my warm/hot starts?

Also, every start seems to throw my o2s into a dive to 10-11 AFR, is this normal or do I have something screwed up? Seems to me like when it transitions to engine running mode, you're still pretty high up in the load window until the engine can start to suck against the closed throttle plate but I'm not sure if that is combatible.

 

Thanks for the help.

Kurt
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Look at what your main fuel

Look at what your main fuel map PW is at that load at 500rpm and that's where the base PW comes from when getting past the crank exit rpm.  Boost Fuel Trim, Coolant Trim, AIT Trim will scale from that number.  You can log every one of the modifiers that get you to the final injector PW, you just need to add them to your tab.  I'd go through and set them all to the fastest update rate too.  That jump is due to the base number in your crank table being different than the base number in your main fuel map at that load.  You can make changes to the main fuel map in that area and you'll see that behavior change.  

Another issue I've noticed is the reading from flex fuel sensors can fluctuate on startup.  This is due to there being no fuel in the line at first and the sensor will read zero % ethanol.  You'll see it start to step up as fuel reaches it.  When I install a FF sensor, I'll add a second route for the return fuel and place the sensor at a low point on that line.  This way the sensor will always have some fuel sitting in it.  This will also help with air bubbles distorting the reading.

 

spikeygg
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Thanks Kurt, this is helpful.

Thanks Kurt, this is helpful. I tried to log the trims today, I noticed that there are a couple varieties.

For example:

  • Fuel Trim (AIT)
  • Fuel Trim AIT

both exist. Which do you recommend? When i look at the one with parenthsis it looks like the Warm-up is to blame. It's weird though: my coolant temperature isn't changing but the Fuel Trim (Warm-up) is -- does that make sense? Also, I noticed that there are Fuel Trim 1 through 12, not sure what these do maybe injector-specific fuel trims?

Kurt
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Channels in parenthese show

Channels in parenthese show an injector PW value.  The order you will see each multiplier follow is: TPS, AIT, Warm-up, Boost, Baro, Start, Alt, Nitrous, Gear, Idle, User Fuel.  The channels without parenthese show what their current % contribution is.  The Item Explanation on the right side of the screen does a pretty good job of describing all the tables, options, and channels!