Where to look next? Feedback/advice?

[Tom]

Here is something to think about. You absolutely CANNOT have a too high idle without enough air to mix with fuel, in order to create the horsepower necessary to raise the idle. This means you either have a vacuum leak, your ISV is open too much of the time or does not close completely, or your throttle plate isn't closed enough, or some combination thereof. If making the mixture more rich increases the idle even more, that just means that the too much air you already had was running lean, and the additional fuel increased the horsepower even more. The DME computer tries real hard to put enough fuel in to match the airflow you have, in order to keep the engine running.

The first step in solving this is to find the source of too much air entering the intake manifold. You squeezed off the ISV intake line, and the RPM's dropped to normal. I don't know if there is a minimum ISV duty cycle, but if there is then intake leakage and/or throttle idle bypass is overwhelming it. If there is no minimum duty cycle (i.e. it can close and stay closed,) then the ISV valve is not working correctly, AND you have too much leakage and/or throttle plate is too open as I would believe Porsche/Bosch would expect at least some activity of the ISV at idle in any case, resulting in a too low idle during your test.

Some of the other comments would lead me to believe the ISV just does not close completely. Tom pointed you to his video of testing the ISV, and you should do this test. Another possibility is too high of a resistance in the close wire path to the DME that causes it to move slowly in the close direction, resulting in it not completely snapping shut before the open signal is given to it. This would not be apparent in a bench test of the ISV.


Fair warning: I have an early car, so my knowledge of late cars is highly influenced by internet disinformation.

Great post. It makes me think of Occam's Razor i.e., -- the simplest or most obvious solution (vacuum leak) is the most likely...

[spacecad3t]

Here is something to think about. You absolutely CANNOT have a too high idle without enough air to mix with fuel, in order to create the horsepower necessary to raise the idle. This means you either have a vacuum leak, your ISV is open too much of the time or does not close completely, or your throttle plate isn't closed enough, or some combination thereof. If making the mixture more rich increases the idle even more, that just means that the too much air you already had was running lean, and the additional fuel increased the horsepower even more. The DME computer tries real hard to put enough fuel in to match the airflow you have, in order to keep the engine running.

The first step in solving this is to find the source of too much air entering the intake manifold. You squeezed off the ISV intake line, and the RPM's dropped to normal. I don't know if there is a minimum ISV duty cycle, but if there is then intake leakage and/or throttle idle bypass is overwhelming it. If there is no minimum duty cycle (i.e. it can close and stay closed,) then the ISV valve is not working correctly, AND you have too much leakage and/or throttle plate is too open as I would believe Porsche/Bosch would expect at least some activity of the ISV at idle in any case, resulting in a too low idle during your test.

Some of the other comments would lead me to believe the ISV just does not close completely. Tom pointed you to his video of testing the ISV, and you should do this test. Another possibility is too high of a resistance in the close wire path to the DME that causes it to move slowly in the close direction, resulting in it not completely snapping shut before the open signal is given to it. This would not be apparent in a bench test of the ISV.


Fair warning: I have an early car, so my knowledge of late cars is highly influenced by internet disinformation.

Great post. It makes me think of Occam's Razor i.e., -- the simplest or most obvious solution (vacuum leak) is the most likely...

Its something. I'll smoke the engine again soon and test the wiring on the ISV. My money is something telling the DME to tell the ISV to stay open, or the wiring itself is...

[barnwerks]

I went back to read the beginning of this and noted the AOS has been vented to atmosphere. Can this lead to false air and elevated warm idle speed?

[spacecad3t]

I went back to read the beginning of this and noted the AOS has been vented to atmosphere. Can this lead to false air and elevated warm idle speed?

I wonder this myself. I have searched nearly all the forums for input here. I have not had a problem on other engines of this displacement and compression... Same with the NA venturi delete... Which I have going on. I however, did not know that the venturi was in the hose, not the plastic Y pipe and have been waiting on a new restrictor to reinstall this configuration and see what happens.

My assumption is that the target idle is based on the vacuum dynamics of the factory system and all of the volume and dampening that provides and by altering that + I have deleted my power steering, have altered the system to a point outside of the adjustable range.

Although, I'm very curious by the fact that with the ISV in the loop, and the idle screw closed, Im at 1200 hot, and with the ISV out of the loop, I can open the idle screw 1.5-2 turns and Im still at 1000-1050rpm.

[johnb]

I went back to read the beginning of this and noted the AOS has been vented to atmosphere. Can this lead to false air and elevated warm idle speed?

I wonder this myself. I have searched nearly all the forums for input here. I have not had a problem on other engines of this displacement and compression... Same with the NA venturi delete... Which I have going on. I however, did not know that the venturi was in the hose, not the plastic Y pipe and have been waiting on a new restrictor to reinstall this configuration and see what happens.

My assumption is that the target idle is based on the vacuum dynamics of the factory system and all of the volume and dampening that provides and by altering that + I have deleted my power steering, have altered the system to a point outside of the adjustable range.

Although, I'm very curious by the fact that with the ISV in the loop, and the idle screw closed, Im at 1200 hot, and with the ISV out of the loop, I can open the idle screw 1.5-2 turns and Im still at 1000-1050rpm.

Is the AOS still connected to the j-boot?

[spacecad3t]

I went back to read the beginning of this and noted the AOS has been vented to atmosphere. Can this lead to false air and elevated warm idle speed?

I wonder this myself. I have searched nearly all the forums for input here. I have not had a problem on other engines of this displacement and compression... Same with the NA venturi delete... Which I have going on. I however, did not know that the venturi was in the hose, not the plastic Y pipe and have been waiting on a new restrictor to reinstall this configuration and see what happens.

My assumption is that the target idle is based on the vacuum dynamics of the factory system and all of the volume and dampening that provides and by altering that + I have deleted my power steering, have altered the system to a point outside of the adjustable range.

Although, I'm very curious by the fact that with the ISV in the loop, and the idle screw closed, Im at 1200 hot, and with the ISV out of the loop, I can open the idle screw 1.5-2 turns and Im still at 1000-1050rpm.

Is the AOS still connected to the j-boot?

No, the jboot is capped there.

[johnb]

Does your car have the fuel vapor purge system? I can't remember the configuration exactly but I have a feeling it's possible for it to leak internally. One valve is connected downstream of the throttle body. The idea is that when you close the throttle, fuel vapor is pulled out of the canister and into the j-boot. So maybe an internal leak there would allow air from the j boot to bypass the throttle plate. It used one of those plastic/rubber diaphragm valves that leak internally quite easily.

[spacecad3t]

Maybe these will help? These are not lined up like in the engine, but the "X" indicates "capped" or taken out of the loop.

IMG_1236.jpeg (1.94 MiB) Viewed 490 times

In this one, Im trying to show that with the ISV inlet capped, the line, ISV, and manifold connection seem to be holding and not leaking... So it was leaking from the jboot junction?

IMG_1237.jpeg (1.64 MiB) Viewed 490 times

I am able to blow on it and feel air leaking! But, I went out and restored the ISV connections with a brass connector, and no change, idling at 1400rpm still. So, to me, it feels like something is commanding the ISV to open too much.

IMG_1238.jpeg (1.49 MiB) Viewed 490 times

[Tom]

You are able to blow on what and feel it leaking?

[spacecad3t]

You are able to blow on what and feel it leaking?

Maybe refresh? I moved the photos inline. The black plastic junction pipe. I think it's OEM, it came with the OEM hoses connected. I capped the 2 ends and blew on one 3rd, the air was coning out of one of the caps, which I had installed. So I introduced that one... but as I mentioned, restoring it all with a brass connector didn't change anything. It would have been awesome if thats all it was.