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Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 10:20 am
by Tom
Might be worth just posting some detailed pictures of your engine bay and hose set up... Just in case something jumps out...
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 10:33 am
by spacecad3t
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 2:57 pm
by spacecad3t
Ok, so I haven't had enough time to pull and test the DME plug, but I tested the ISV and it's working. I also tested different connections to the jboot to minimize chance of air leak.
An interesting outcome was that I started the car after testing the ISV and restoring its plumbing, and it idled at 900, but as soon as a blipped the throttle, it was stuck at 1400 again.
With the jboot off, I can confirm the throttle plate is closing, TPS is clicking, its reporting the correct values... If I have time tomorrow, I'll pull the DME plug and check the Temp, TPS, and ISV values at the DME.
Its seems like some combo of the TPS idle signal not reaching the DME, or the DME thinking it needs to command the ISV to remain open.
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 3:01 pm
by Tom
spacecad3t wrote: Tue Nov 11, 2025 2:57 pm
Ok, so I haven't had enough time to pull and test the DME plug, but I tested the ISV and it's working. I also tested different connections to the jboot to minimize chance of air leak.
An interesting outcome was that I started the car after testing the ISV and restoring its plumbing, and it idled at 900, but as soon as a blipped the throttle, it was stuck at 1400 again.
With the jboot off, I can confirm the throttle plate is closing, TPS is clicking, its reporting the correct values... If I have time tomorrow, I'll pull the DME plug and check the Temp, TPS, and ISV values at the DME.
Its seems like some combo of the TPS idle signal not reaching the DME, or the DME thinking it needs to command the ISV to remain open.
Have you tried to force the throttle closed by hand when idling, to make sure it's not sticking after opening?
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 3:08 pm
by spacecad3t
Tom wrote: Tue Nov 11, 2025 3:01 pm
spacecad3t wrote: Tue Nov 11, 2025 2:57 pm
Ok, so I haven't had enough time to pull and test the DME plug, but I tested the ISV and it's working. I also tested different connections to the jboot to minimize chance of air leak.
An interesting outcome was that I started the car after testing the ISV and restoring its plumbing, and it idled at 900, but as soon as a blipped the throttle, it was stuck at 1400 again.
With the jboot off, I can confirm the throttle plate is closing, TPS is clicking, its reporting the correct values... If I have time tomorrow, I'll pull the DME plug and check the Temp, TPS, and ISV values at the DME.
Its seems like some combo of the TPS idle signal not reaching the DME, or the DME thinking it needs to command the ISV to remain open.
Have you tried to force the throttle closed by hand when idling, to make sure it's not sticking after opening?
Yeah, every time I have tested something. It doesn't budge any more closed. The stop and cable are not tightened right now, so I can back them off and on by hand...
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 4:50 pm
by Petethepug
I didn’t read every single post but saw ISV coming up often. They wear and get finicky. Some owners spray them with brake or contact cleaner and that kills them.
Replace the ISV. The bushings inside degrade and cause situations like you’re having. OEM is Bosch # 0280140531.
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 8:20 pm
by 944er
Tom, it would be interesting to know the duty cycle at idle of the ISV of a properly set up car. You know, when you are not busy pushing the belt tension gauge forward.
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 8:31 pm
by Tom
944er wrote: Tue Nov 11, 2025 8:20 pm
Tom, it would be interesting to know the duty cycle at idle of the ISV of a properly set up car. You know, when you are not busy pushing the belt tension gauge forward.
My F9 DME reports duty cycle. I'll see if I can check it this week. I'll also check a spare on the bench to confirm my suspicion that they are never really air tight even when maxed closed...
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 8:33 pm
by johnb
It should be close to 30% when up to temp, since that's the duty cycle when closed loop control is disabled.
Re: Where to look next? Feedback/advice?
Posted: Tue Nov 11, 2025 8:40 pm
by Tom
johnb wrote: Tue Nov 11, 2025 8:33 pm
It should be close to 30% when up to temp, since that's the duty cycle when closed loop control is disabled.
Do you happen to know the frequency of the PWM -- I'm tempted to check how open/closed the valve actually is at 30%...