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Join Date: Oct 2006
Location: Laguna Vista
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Vapor purge system

Needless to say both of these are not holding vacuum (3 way traffic).



I am trying to make sure I understand how the system works :



When the car is not running and there is a pressure build up in the gas tank (from heat or what ever) the old cars used to just vent to the atmosphere via gas cap or other outlet.
In our case the vapors go down the path 'A' in the the above (the braided hose) to the carbon canister 5. The fuel vapor is captured and the air is let out via 6.

When car is running but not up to temp and has vacuum, this is applied to the temperature control valve 7, but as the valve is not open (due to being cold) nothing happens and vacuum control valve 8 is not on. Valve 12 is not relevant at this point as it has nothing going to it from 8.

When car is warm enough to turn on valve 7 and there is vacuum NOT available on line 9 then valve 8 opens which allows valve 12 to feed the fuel vapors from 5 into the air inlet, thus reclaiming them from the canister.

So in summery:
the system stores the fuel vapors in the carbon during high pressure in the tank.
when car is warm and is actually not idling but driving the vapors are fed back into the air inlet.

Correct ?
If so I don't know if I am ready to drop $650 on the control valve (12).
The vacuum valve 8 is more reasonable $35 or so.

Finally if I go down the delete the system route, I am planing on capping off all vacuum points and eliminating the canister. I will replace it with a purge style one way valve in the wheel well.
Opinions?

Thanks SOx
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Old 07-31-2018, 08:35 AM
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Both of those should hold vacuum on the small nipple, as they are activating a diaphragm.

Get one used that actually works, would be my suggestion.
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'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
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Old 07-31-2018, 09:21 AM
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Thanks Keith,
One more thing...(seems like always one more thing).
The drawing for the vacuum layout floating around shows the check valve for the brake booster in a different place than the PET does ??
See Pics.
Sox


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1986 951"MADDOG" black with maroon interior
All stock with shimmed WG
Old 07-31-2018, 07:11 PM
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Don't believe PET...
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'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
Old 07-31-2018, 07:48 PM
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Ok, but I would think having the check valve in the brake booster would make sense rather in a line that feeds into the intake manifold. One would think that holding vacuum in the brake assist would help with braking when engine is not running ?
Sox
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1986 951"MADDOG" black with maroon interior
All stock with shimmed WG
Old 07-31-2018, 08:36 PM
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Yup after looking at a bunch of engine bay pics, you are correct. Maybe the PET was referring to the check valve that goes in the brake booster it self ?
Thanks
Sox
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1986 951"MADDOG" black with maroon interior
All stock with shimmed WG
Old 07-31-2018, 09:39 PM
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The check valve in question stops boost from filling the vacuum reservoir canister, so vacuum can be preserved for other purposes.
Sox
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Old 08-01-2018, 11:25 AM
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