Stalling in Dashpot Mode

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Stalling in Dashpot Mode

Unread postby wickedsnake00 » Tue Oct 02, 2018 7:29 am

Background: CVAF1 strategy, 03 Cobra engine, HPX-E MAF, FR500 cams, ported Eaton w/twin 66s. Stock TB and IAC. Tuned with SCT Advantage III.

I have an issue with it settling to idle, sometimes stalling out. I ran a hot throttle snap test, sitting stationary. I let it sit for awhile at idle, integrator at 0, then blipped the throttle. Idle speed control mode goes to -1, indicating dashpot. Desired air mass at idle and idle air valve duty cycle jump up and start to decay with dashpot. RPM drops, but around 74.5s (~7 seconds after the throttle snap), it hits the desired idle rpm, even though desired air mass is above what it should be at stable hot idle (0.94 vs 0.82 at idle). Idle air continues to drop, and so does RPM rather quickly down to 200, almost to the point of stalling, briefly recovers, and then stalls. All this time it's still in dashpot, not base idle.

It looks like the dashpot function is essentially an open loop system where it decays from the pre-position down to the isc neutral idle air value at the dashpot decay rate, no RPM feedback or anything. That said, is it undershooting due to the decay rate, or is it something else that needs to be adjusted? I know that regardless it needs to be leaned out a lot more at idle- I adjusted injector timing, and need to compensate for it a lot more. It's hanging in the 13s to high 12s. And the MAF signal is noisy, but there's not anything I can do about that.

For dashpot decay rate (FN879) values I'm using close to your values-
16 - 0.04
0.75 - 0.02
0.5 - 0.01
0.25 - 0.005
0 - 0.0015

I also assume that the left column in that table represents the delta from base idle airflow. Is that correct?
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Re: Stalling in Dashpot Mode

Unread postby decipha » Tue Oct 02, 2018 8:08 am

not surprising at all the 4v engines are very dashpot needy engines. You need to kick up the minimum dashpot value to allow idle feedback. I think they call it the dashpot for zeroing the entry timer or something along those lines should be easy to find. Set it to 0.500 should fix her on up.

Be sure to set the minimum mph for idle control to 25mph and the dashpot min mph to 0.5mph and hysteresis to 12 mph. That's very important. And it would be wise to reduce the dashpot decay significantly below 0.250 dashpot. I usually set it to a decay rate of 0.0007

the left values are the dashpot value the right is the decay rate
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Re: Stalling in Dashpot Mode

Unread postby wickedsnake00 » Sat Oct 06, 2018 7:41 pm

Awesome help.

Now is that to say that the "dashpot for zeroing entry timer" represents the amount of dashpot where it transitions from dashpot control to idle control? I haven't been able to find a parameter description for it anywhere. The default value was 0.02, which seems pretty crazy if that's the case.

I made most of the changes, but didn't find any related to hysteresis in dashpot or idle.

As for results it seems to have eliminated the issue when warmed up at least; no more than a 50-100 RPM undershoot. I've still got some tweaking to do, and logging it will help me determine just what needs to be changed.
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Re: Stalling in Dashpot Mode

Unread postby decipha » Sun Oct 07, 2018 12:03 am

yes that's what it does

no surprise sct is prob missing the hysteresis parameter

sounds like you need more dashpot, go kick up the pre position / max dashpot function and slow down the decay
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Re: Stalling in Dashpot Mode

Unread postby mvancopp » Thu Oct 11, 2018 1:22 pm

I was having a similar issue and the prior posts solved the dipping idle, and stalling, when shifting or coming directly out of gear to coast. However if you take your foot off the gas and let the engine drag for more than a few seconds and then press in the clutch the same deep RPM dive occurs and sometimes stalls. The RPM dip is greatest if you are in a low gear (1st or 2nd) and release the gas pedal and allow the RPMs to drop to around 1800 RPMs, taking 2 - 3 seconds, and then press in the clutch.
Do you have any pointers on what to look for under these conditions?

Thanks for any help you can provide.

FYI: I also am using SCT Advantage and could not find the hysteresis setting.
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Re: Stalling in Dashpot Mode

Unread postby decipha » Thu Oct 11, 2018 7:41 pm

set minmph to 25 mph should fix that all up

minmph is the minimum speed to allow idle spark control
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Re: Stalling in Dashpot Mode

Unread postby mvancopp » Sat Oct 13, 2018 9:00 am

Thanks for the reply.

The idle minmph is already et to 25. I followed your suggestions in the second post, your first reply, of this thread.
Do you have any other suggestions or suggestion for logging that may help point in the right direction.

Thank you,
Mark
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Re: Stalling in Dashpot Mode

Unread postby decipha » Sun Oct 14, 2018 8:19 am

make sure idlrpm is high enough 650 or so that it goes in to idle spark feedback well before its at a low rpm so spark doesnt drop and the bottom falls out
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91 4r70w - Skarlett
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93 auto - Danika
94 5spd Rionda
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98 Camaro SS - Bounquisha
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00 Explorer 5L - Bernyce
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Re: Stalling in Dashpot Mode

Unread postby wickedsnake00 » Sun Mar 24, 2019 3:45 pm

Bringing this back from the dead. Finally got to give the updated tune a shot with the nicer weather.

As expected, increasing the dashpot min mph and entry for zeroing value got it to kick out of dashpot mode into idle control mode much earlier. That's the good news.

Bad news is at a stop while the RPM is coming down right around 1100 RPM it'll still drop off sharply and die. Desired idle speed at that point is usually around 960-970. Dashpot is around 0.25-0.27.

Since the idle controller isn't giving me the behavior I want, is my best bet to roll back to earlier settings (set dashpot for zeroing entry timer back down to 0.20), and really reduce the decay rate around 0.25? It seems like I can shed RPM as quick as I want at higher dashpot values, but once around 0.25 it needs to decay really slowly. Or is there another approach I should be looking at?
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Re: Stalling in Dashpot Mode

Unread postby decipha » Sun Mar 24, 2019 4:00 pm

the dashpot for zeroing entry timer is DASCTL, it needs to be set to 0.500 in all tunes reducing it is guaranteed to cause stalls

from .25 on down you should always have dashpot decay at 0.0007 in all tunes regardless

port your tune file and i'll take a look
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94 5spd Rionda
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Re: Stalling in Dashpot Mode

Unread postby wickedsnake00 » Mon Mar 25, 2019 8:52 am

Unfortunately I can't post the tune file since I'm using SCT Advantage, although after the EPA lawsuit thing and building a 2.3T car a QH is probably in the near future. I can post up any pertinent tables if desired.

I'll have to go back and check my dashpot decay settings. The notes I have in front of me show my decay rate is twice your recommendation of 0.0007 lb/min/sec at 0.25 lb/min but I'll have to check my tune this evening to verify if that's what I tested with.

Care to explain DASCTL a little more to me? I thought that it represented the threshold at which the control mode switches from dashpot to idle control. I set it to 0.5 as you recommended, and around that dashpot value I see the idle speed control mode go to idle (1, IIRC). I thought that at this point the idle controller would be in control, and it would ignore dashpot items like decay rate and strictly control it as if it's just idling. Is any form of dashpot still active in that region?
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Re: Stalling in Dashpot Mode

Unread postby decipha » Tue Mar 26, 2019 2:29 am

u can post the mtf file or pm it to me if u want

it doesnt ignore dashpot it just allows idle control as well
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90 (4x 5spds) - Dante, Ruby, Daja, Ava
91 4r70w - Skarlett
92 (2x) 5spd & auto - Bianqa, Andrea
93 auto - Danika
94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
00 Lincoln LS - Luanda
98 Camaro SS - Bounquisha
02 Harley F-150 - Sasasha
03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
07 GMC 2500HD 6L - Veranafer

Re: Stalling in Dashpot Mode

Unread postby wickedsnake00 » Tue Mar 26, 2019 10:06 am

That is interesting to know.

I've attached the .mtf file for you to peruse. It does look like the decay rate could be reduced further at lower dashpot flow rates.
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Re: Stalling in Dashpot Mode

Unread postby wickedsnake00 » Wed Apr 17, 2019 9:40 am

Adjusted the decay rate. Marked improvement.

Not a 100% fix but I think I've got another factor at play causing the remainder of the issue in specific conditions. Complete tangent, but when it's relatively warm, both ambient and/or ECT it's perfect. At cold startup/idle it's hanging too rich, much richer than the cold fuel table and cold enleanment table. Clears up with higher load, higher airflow, or higher ECT. I think the fuel puddle model needs adjustment in that range, which isn't surprising given it probably still has the stock 2V CVAF1 values for it.
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Re: Stalling in Dashpot Mode

Unread postby decipha » Fri Apr 19, 2019 10:08 pm

you have some significant problems with that tune

first your injectors are clipped

http://www.efidynotuning.com/fuel.htm#ahisl

i cut your injectors, maf and engine size in half and reorganized the maf in order to dial it in properly, you will need to re-dial in the maf correctly this time

you do not have the load with failed maf table dialed in, nor did you dial in startup enrichment fuel, i half assed both of them

i simplified idle air and half assed it as well

half assed dashpot decay since you had it wayyyy too fast down low and up top

your spark table is not efficient at all, i take it your running a low grade fuel? i seen you had the tip in spark retard active too but very poorly executed. I removed that you may need to add it back in if needed.

this should give you a big push in the right direction to help you out
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decipha
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Posts: 16984
Joined: Mon Jul 15, 2013 5:29 pm
Location: New Orleans, LA
Name: Michael Ponthieux
Vehicle Information: Supercoupin' x10
90 (4x 5spds) - Dante, Ruby, Daja, Ava
91 4r70w - Skarlett
92 (2x) 5spd & auto - Bianqa, Andrea
93 auto - Danika
94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
00 Lincoln LS - Luanda
98 Camaro SS - Bounquisha
02 Harley F-150 - Sasasha
03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
07 GMC 2500HD 6L - Veranafer


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