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924-UK
Joined: 14 Nov 2023
Posts: 28
Location: UK
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 Posted: Wed Dec 11, 2024 12:33 am Post subject: 1984 924 NA High Idle |
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Slowly getting the old girl together. She now starts OK (ish) and stops (kind of) but I'm struggling with a high idle.
I've had the timing and fuelling checked and adjusted but she wants to idle at 1800rpm when warm and the air screw is all the way in!
I did read that clamping the vacuum hose from the manifold to the air intake can identify some sort of a vacuum leak and I tried this and the idle drops to below 800rpm.
I sprayed some carb cleaner on all the vacuum pipe joints, the brake booster, intake manifold gasket etc. and it had no effect.
I can obviously reroute the vacuum piping to fix this temporarily but I'm wondering if there should be a check valve from the manifold to the air intake boot to prevent air being added to the intake?
Any suggestions welcome.
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morghen
Joined: 21 Jan 2005
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Location: Romania
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| Posted: Wed Dec 11, 2024 7:26 am Post subject: |
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Which hose did you clamp?
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924-UK
Joined: 14 Nov 2023
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Location: UK
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| Posted: Wed Dec 11, 2024 8:15 am Post subject: |
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| morghen wrote: | | Which hose did you clamp? |
From the inlet manifold to the airbox (which also splits to the brake booster from memory)
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morghen
Joined: 21 Jan 2005
Posts: 9095
Location: Romania
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| Posted: Wed Dec 11, 2024 5:51 pm Post subject: |
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Brake booster diaphragm is dead? Try pulling some vacuum into the booster alone to check if the diaphragm is broken or that one way valve going into the booster isn't cracked.
There is a strange plastic thing that splits the hoses that go from the manifold to the booster and the airbox/inlet tract, check that it is intact.
You can actually remove that and cap it off, cap off the port that hose goes to as well. You just need a hose from the manifold that goes to the brake booster.
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924-UK
Joined: 14 Nov 2023
Posts: 28
Location: UK
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| Posted: Wed Dec 11, 2024 8:41 pm Post subject: |
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| morghen wrote: | Brake booster diaphragm is dead? Try pulling some vacuum into the booster alone to check if the diaphragm is broken or that one way valve going into the booster isn't cracked.
There is a strange plastic thing that splits the hoses that go from the manifold to the booster and the airbox/inlet tract, check that it is intact.
You can actually remove that and cap it off, cap off the port that hose goes to as well. You just need a hose from the manifold that goes to the brake booster. |
Thank you. I will try that.
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Raize
Joined: 18 Sep 2013
Posts: 447
Location: Scotland
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| Posted: Thu Dec 12, 2024 5:44 am Post subject: |
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Mine always had that problem so I got rid of the hose to airbox and just had the hose go from brake booster to manifold.
I remember noticing that clamping the hose on the brake booster leg had no effect so doesn't seem to have been ruptured booster diaphragm.
Was impossible to get the idle down even with idle screw all the way in. |
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924-UK
Joined: 14 Nov 2023
Posts: 28
Location: UK
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| Posted: Thu Dec 12, 2024 6:41 am Post subject: |
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| Raize wrote: | Mine always had that problem so I got rid of the hose to airbox and just had the hose go from brake booster to manifold.
I remember noticing that clamping the hose on the brake booster leg had no effect so doesn't seem to have been ruptured booster diaphragm.
Was impossible to get the idle down even with idle screw all the way in. |
Ah, that's interesting as I clamped that pipe first and nothing happened to the idle on mine. It was only when I clamped the pipe directly out of the manifold that the idle dropped.
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Raize
Joined: 18 Sep 2013
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Location: Scotland
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| Posted: Thu Dec 12, 2024 8:35 am Post subject: |
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| 924-UK wrote: | | Raize wrote: | Mine always had that problem so I got rid of the hose to airbox and just had the hose go from brake booster to manifold.
I remember noticing that clamping the hose on the brake booster leg had no effect so doesn't seem to have been ruptured booster diaphragm.
Was impossible to get the idle down even with idle screw all the way in. |
Ah, that's interesting as I clamped that pipe first and nothing happened to the idle on mine. It was only when I clamped the pipe directly out of the manifold that the idle dropped. |
I honestly don't see what Porsche was trying to do when they made it this way. You'd only ever have venturi effect on that tube if you have a lot of airflow through the tube, but the only time you'd have a lot of airflow through that tube is if the throttle is closed so you have a ton of intake vacuum anyway, so what's the point? |
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morghen
Joined: 21 Jan 2005
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Location: Romania
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| Posted: Thu Dec 12, 2024 8:48 pm Post subject: |
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I guess they tried to protect the booster assy...? or have a safe guard for vacuum for some other reason..maybe for the k-jet?
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