An ISV experiment
08-15-2017, 02:22 PM
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Racer
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An ISV experiment
This is a long post but please bear with me.
Everyone knows the 964 should have fuel adaptation after it loses battery power. This is especially true for early C4s. My car's low RPM throttle response was horrible. Blipping the throttle to downshift below 3000 RPM just didn't work. I had used the openly available software with interface cables to perform adaptation. It said it worked, but not much changed. As I drove the car for a long time like this so I got very familiar with its behavior.
When I'd lift the throttle it seemed as though the engine just died. RPM would drop like a rock until it got below 850 then the ECU would try to recover with the resulting dreaded throttle bounce. I tried a new ISV. No change. So I now had a spare ISV so I tried a fix I read about somewhere related to stalling that occurred with a LWF.
The ISV has a set screw that adjusts the minimum opening of the bypass valve. This would set the lowest RPM and then leave it to the ECU to tweak it up. The set screw is normally epoxied over. The suggestion was to clean out the epoxy and increase the minimum valve opening to get the RPM closer to the set point.
I did this. I increased the minimum opening. No change. Increased some more. No change. Even more. Nothing.
So what the hell was going on? With my original ISV I noticed that if you rotated the solenoid entirely around to its opposite lock, the bypass was closed. Then I thought, the ISV is wired backwards! When the ECU gets the throttle closed signal, it should drive the solenoid against the minimum opening set point. If its wired backwards, it would drive it to opposite lock completely closing the air and as a result starving cylinders for fuel. This would explain the why my adjustment didn't work and why I would get a bog when I tried to blip the throttle.
I want to test this theory, but I need to get a meter on the ISV to cable connector. Ideally, I'd fabricate an extension cable so that I could also easily swap the polarity. I'd test the theory by measuring voltages at some high RPM, then close the throttle and watch the voltages. I could really use a data acquisition system for this as some voltage changes will be rapid.
When I have the recorded voltages I can apply them to me old ISV to see what the valve is doing. If my theory is right, then I should be able to just swap the extension cable and see what happens (after I disconnect battery to reset the ECU).
My question is: Does anyone have connectors or plugs I can use to make the extension cable? Or know where to get them?
Thanks for taking the time to read all this.
Dan
Everyone knows the 964 should have fuel adaptation after it loses battery power. This is especially true for early C4s. My car's low RPM throttle response was horrible. Blipping the throttle to downshift below 3000 RPM just didn't work. I had used the openly available software with interface cables to perform adaptation. It said it worked, but not much changed. As I drove the car for a long time like this so I got very familiar with its behavior.
When I'd lift the throttle it seemed as though the engine just died. RPM would drop like a rock until it got below 850 then the ECU would try to recover with the resulting dreaded throttle bounce. I tried a new ISV. No change. So I now had a spare ISV so I tried a fix I read about somewhere related to stalling that occurred with a LWF.
The ISV has a set screw that adjusts the minimum opening of the bypass valve. This would set the lowest RPM and then leave it to the ECU to tweak it up. The set screw is normally epoxied over. The suggestion was to clean out the epoxy and increase the minimum valve opening to get the RPM closer to the set point.
I did this. I increased the minimum opening. No change. Increased some more. No change. Even more. Nothing.
So what the hell was going on? With my original ISV I noticed that if you rotated the solenoid entirely around to its opposite lock, the bypass was closed. Then I thought, the ISV is wired backwards! When the ECU gets the throttle closed signal, it should drive the solenoid against the minimum opening set point. If its wired backwards, it would drive it to opposite lock completely closing the air and as a result starving cylinders for fuel. This would explain the why my adjustment didn't work and why I would get a bog when I tried to blip the throttle.
I want to test this theory, but I need to get a meter on the ISV to cable connector. Ideally, I'd fabricate an extension cable so that I could also easily swap the polarity. I'd test the theory by measuring voltages at some high RPM, then close the throttle and watch the voltages. I could really use a data acquisition system for this as some voltage changes will be rapid.
When I have the recorded voltages I can apply them to me old ISV to see what the valve is doing. If my theory is right, then I should be able to just swap the extension cable and see what happens (after I disconnect battery to reset the ECU).
My question is: Does anyone have connectors or plugs I can use to make the extension cable? Or know where to get them?
Thanks for taking the time to read all this.
Dan
Last edited by dlpalumbo; 08-15-2017 at 02:23 PM. Reason: Typo in title
08-17-2017, 01:59 PM
#2
Maybe there is an easier way to check the harness wiring - the wiring diagram shows Pin 1 of the ISV (Red/White) being directly connected to ground (same ground as the injectors). If you disconnect the DME connector it should be easy to see whether pin 1 or pin 2 is grounded. It's probably worth checking that Pin 2 of the ISV is connected to pin 4 of the DME connector and that all the contacts are in good shape.
Good luck!
Good luck!
