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ISC clamp

belize1334

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Nov 18, 2003
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Bozeman, MT
This is a copy of a topic that I stared on the DSMap board in their section for development ideas. I wanted to throw it out here also and see what people think.

My VR4 has a bizarre behavior that not many people mention but which is, I believe, fundamental to the design of the DSM throttle body. Under normal driving conditions or when at WOT the ECU opens the ISC all the way. This is done so that if you abruptly close the throttle the car won't buck as hard. The problem is that if you go from WOT to closed throttle for a hard shift, the ISC hangs open for about a second which lets a fair amount of fairly dense air past the throttle plate and causes the rpms to spike. At first I thought it was driver error but I've look at my logs and the spike occurs AFTER the closed throttle switch is triggered.

As an example, if I let off hard and engage the clutch at 3,000rpm and 10psi then my rpms jump to 4,100rpm. The higher the boost at let-off the more the spike, the higher the rpm at let off the lower the spike (which makes sense since higher rpm would consume more air naturally so the amount let through the ISC is effectively less significant).

As an experiment, I blocked off my ISC (note that the FIAV was already blocked) and this almost completely eliminated the behavior. I say almost because I still get about 300rpm spike at 3k and 10psi but by 5k it's basically gone. I attribute the fact that it remains to a certain degree to the BISS which still provides a pathway around the throttle plate under all conditions.

I've also done some looking around and this behavior seems to exist in all cars with forced induction that have a BISS and/or ISC which are inline with the compressor. Two examples are Evos and WRXs, both of which have an idle control which draws air from inside the charge path and around the throttle plate. I've looked on their respective forums and both cars have this same reported symptom. As soon as you start looking at cars which are operating in SD and draw their idle air from a separate source (which is never above ambient pressure) then the effect is gone.

I've considered two possible solutions.

1) Modify the throttle-body so that idle air is drawn from either ambient air in the engine bay (works if your in SD) or from a separate path that bypasses the compressor (metered air provided by, say, a line tapped from your intake pipe through a check-valve to a tapped fitting on the ISC housing). The problem here is that it seems hard to accomplish physically, especially for the BISS which is part of the TB and harder to cut/tap/weld than the ISC housing.

2) This is teh haxors come in ... Modify the ECU code to change the ISC behavior. I'm not sure what the best way to affect this would be or whether it's even possible to mess with the code but I'm imagining a scheme like ... an upper limit on ISC steps that falls off with rpm so that once you get to about 2500 the ISC is always closed ... or ... an overwrite that forces the ISC to clamp shut any time the TPS signal goes abruptly to zero above a certain rpm range.

So...that's all I've got on this, except to say that for combustion you need both air and fuel so perhaps another solution would be to make the injectors shut down faster as I've noticed in my logs that my IPW hangs at like 1ms for about a second after the throttle plate closes before they go to zero for coast-down.

Thoughts?
 

prove_it

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Jul 3, 2008
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Sioux Falls, SD
Just convert over to speed density? That's seems to be the most logical solution.
 

belize1334

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Just converting to SD won't do it since there'll still be air getting past the TB and fuel to go with it. I think the issue is that the ISC is open when I let off and charged air is getting through it. I don't see how switching to SD would help unless I also modify the ISC housing so that it pulls air from a non-charged source.

I'm also surprised that most people don't have this issue. My setup isn't so different from other peoples'. If I could find something wrong which is causing the issue then I'd be happy to fix that and proceed.

My logic for the diagnosis goes like this.

Engine rev's up after I close the throttle plate...and it seems to be magnified with boost. This implies that there is air/fuel being supplied after the throttle closes and that the amount of air (at least) is somehow related to the boost pressure. The only source I can see is the charge pipes and IC so air must be moving from there into the manifold. The only way to do that is past the throttle body and the only way past the throttle body is the ISC and BISS.

Am I missing something?
 
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belize1334

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Nov 18, 2003
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Bozeman, MT
Perhaps I could get somebody to post a log of their RPM, TPS, Idle Switch, airflow/boost, injectors and ISC. I could compare it to mine and see if maybe I'm getting some bizarre behavior. I'll post my own log as well. The situation is most dramatic at around 10psi, 3krpm with an abrupt let off of the throttle and clutch to the floor.
 

I believe you are describing the dashpot operation of the ISC, right? Where it drops the rpm's down slowly when you decelerate. The only issue I see is that you really do need the dashpot operation to catch the rpm's, so the motor doesn't die. If the ISC can't react fast enough, it won't catch them in time.
It might not be as difficult as you think to tap a nipple into the FAIV to port air into it from pre-compressor. You would just then have to block off the port on the TB that normally supplies air to the ISC.
 

belize1334

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Nov 18, 2003
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Bozeman, MT
I'm with you on plugging the main tb port...but I feel like this could be easily accomplished with a variation on the ISC block-off plate. I'm still confused though as to why I have this issue and EVOs and WRXs and even supercharged Fords have this issue but apparently nobody else with DSMs does. I thought perhaps that it was my brake booster acting as a charge reservoir but I just checked the check-valve and it's fine.

As far as the dash-pot function, I agree that you don't want to throw it out entirely. That's no better than just blocking the ISC which I personally don't like from a drivability perspective. My thought is to make an upper limit to the ISC steps that fades out as the rpms lower. So, for instance, the upper limit could be Max[0 , Min[100 , (1/5)*(2000-rpm)]]...that is, it's 100 up to 1500, then falls linearly to zero by 2k and they stays at zero from there out. This could be adjusted a little to make sure the ISC has time to respond but I don't think it should take more than the time to fall 500rpm for the ISC to recover it's functionality.
 

belize1334

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Joined
Nov 18, 2003
Messages
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Location
Bozeman, MT
BTW...here's an example log. You can see that I let off at 3,000 rpm and then the ISC slowly tapers from 100 to 40 while the injectors hang at 1.28ms and the rpms spike to 4,000. Note also that this is NOT driver error. I've done logs with the TPS and idle-switch and they're both closed well before the rpms rise.

Maybe the solution is just to force the injectors to shut down when the throttle is closed...but I feel like they're hanging open in anticipation of getting back on the throttle and avoiding the dip-in bog.
 
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