Rough idle with forced induction

jwick · 08-22-2014, 01:30 PM

Quote:

Originally Posted by Mitco39

Upon looking at it more closely I have a check I am going to try. I am just going to pull off what I refer to as the primary circuit and vent it to atm. That will stop any chance of this sort of thing happening. Right now I believe all the kits keep this circuit so that they are not blowing out oily emissions all down the car.

What I am referring to as the primary circuit is the purple highlight, green is the secondary with the filters on it currently.

See the pic tells a thousand words. So the primary is the two that tie into the IM right where we tapped for the BOV feed. That must not have as much of an effect on the system as the secondary because all the kits I know of keep that setup.

Mitco39 · 08-22-2014, 01:41 PM

Quote:

Originally Posted by jwick

See the pic tells a thousand words. So the primary is the two that tie into the IM right where we tapped for the BOV feed. That must not have as much of an effect on the system as the secondary because all the kits I know of keep that setup.

yes they tie in there and under idle the manifold is under a vacuum that sucks in crankcase vapors. Now before the kit the crankcase would be filled with air from the intake between the maf and the throttle body. But now this air is just entering the system through those two fiilters and straight into the intake at idle.

jwick · 08-22-2014, 01:55 PM

Quote:

Originally Posted by Mitco39

yes they tie in there and under idle the manifold is under a vacuum that sucks in crankcase vapors. Now before the kit the crankcase would be filled with air from the intake between the maf and the throttle body. But now this air is just entering the system through those two fiilters and straight into the intake at idle.

Just throwing ideas out. Partially for my understanding and partially to see if any trigger some light bulb in your mind that leads us to a solution. I could be completely off but stick with me. So NA it's pulling metered air in from the intake (post-MAF). I would guess the ECU is expecting a certain amount of air to be removed from what it's metering, where in reality (vented) the crankcase is pulling in as much unmetered air as it wants. There is no adjustment for that air not being removed and when you come off throttle it hiccups because it's getting more air than it expects leading to the lean condition. Which is what we see happening.

But let me throw an additional monkey wrench. The GTM kit ties that secondary in pre-MAF, so it has to be something different or in addition to what I stated above or there is something in the tuning that can correct for that.

Mitco39 · 08-22-2014, 02:11 PM

Quote:

Originally Posted by jwick

Just throwing ideas out. Partially for my understanding and partially to see if any trigger some light bulb in your mind that leads us to a solution. I could be completely off but stick with me. So NA it's pulling metered air in from the intake (post-MAF). I would guess the ECU is expecting a certain amount of air to be removed from what it's metering, where in reality (vented) the crankcase is pulling in as much unmetered air as it wants. There is no adjustment for that air not being removed and when you come off throttle it hiccups because it's getting more air than it expects leading to the lean condition. Which is what we see happening.

But let me throw an additional monkey wrench. The GTM kit ties that secondary in pre-MAF, so it has to be something different or in addition to what I stated above or there is something in the tuning that can correct for that.

You could be right, as the car switches from open loop to close loop it could cause a hiccup until it stabilizes itself. Easy way to try and see if this is causing it is to pull both lines and plug the ports in the intake manifold and idle the car and see what happens. You could just vicegrip the lines to and let the secondary system do all the venting. I am going to try that this weekend and see what the car does.

Is that secondary drawn in pre-turbo? It could be that there is enough vac pre turbo to keep the air being drawn through that system to minimum. But your right the same principle applies in this case as well. Its a head scratcher for sure. Ill try and get some time tonight with the car.

jwick · 08-22-2014, 02:21 PM

Quote:

Originally Posted by Mitco39

You could be right, as the car switches from open loop to close loop it could cause a hiccup until it stabilizes itself. Easy way to try and see if this is causing it is to pull both lines and plug the ports in the intake manifold and idle the car and see what happens. You could just vicegrip the lines to and let the secondary system do all the venting. I am going to try that this weekend and see what the car does.

Is that secondary drawn in pre-turbo? It could be that there is enough vac pre turbo to keep the air being drawn through that system to minimum. But your right the same principle applies in this case as well. Its a head scratcher for sure. Ill try and get some time tonight with the car.

As I understand it, yes pre-turbo. My buddy has a GTM TT and he tells me its routed in the intake filter tube before the turbo. I'm almost positive he's correct too because you can clearly see the upper pipe (IC) and that's not where it's tied in.

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