Forge dump valve

[youngscottie]

hi all
Just wondering if anyone has ever had problems with a forge dump valve
I sold 1 to a fellow member and it's not working
The diaphragm is not split or spring broken
At idle vacuum opens the valve but it stays open it won't close when the throttle is applied
I know the valve works as I've seen it with my own eyes on another car
The buyer already has a different brand one on there and that works
Anything quirky on a c1j I should know about?

[Trevhib]

Did it work on your GTT ok? It can get stuck open due to sticky residues/build-up but that used to be an issue with the Forge piston type, not the diaphragm type. Did he send you a picture of it fitted?

[youngscottie]

Did it work on your GTT ok? It can get stuck open due to sticky residues/build-up but that used to be an issue with the Forge piston type, not the diaphragm type. Did he send you a picture of it fitted?

I've never ran a dumpvalve on my gt trev (not my cup of tea)
I've seen this one working before recently
Member has opened it and diaphragm and spring ok
Bit baffled really have offered refund

[Ian S]

The spring should close it.

So there must be a maintained vacuum to hold it open.

The diaphragm type only close when there is sufficient lack of vacuum, so at part throttle it my stay open.

That's why I had the double piston type. They remain shut unless there is sufficient boost pressure as well as vacuum. IIRC, mine would vent above 6psi of boost (when the manifold was at vacuum).

[youngscottie]

The spring should close it.

So there must be a maintained vacuum to hold it open.

The diaphragm type only close when there is sufficient lack of vacuum, so at part throttle it my stay open.

That's why I had the double piston type. They remain shut unless there is sufficient boost pressure as well as vacuum. IIRC, mine would vent above 6psi of boost (when the manifold was at vacuum).

From what I can gather vacuum is pulling it open and it won't close at all even under boost
Vaguely remember something about people running 6mm vac pipe and using a direct port into the manifold to work dump valves
Although I never ran one so could be wrong

[Ian S]

The place to T into is the pipe that goes to the MAP sensor in the Renix unit from near the bottom of the carb somewhere.

That should have the same as in the inlet manifold, vacuum or boost.

[Jimmy_GTT]

The spring should close it.

So there must be a maintained vacuum to hold it open.

The diaphragm type only close when there is sufficient lack of vacuum, so at part throttle it my stay open.

That's why I had the double piston type. They remain shut unless there is sufficient boost pressure as well as vacuum. IIRC, mine would vent above 6psi of boost (when the manifold was at vacuum).

Isn't there a manual boost controller connected to the dump valve line?
They used to use a ball and spring construction which might cause a maintained vacuum.

[Ian S]

Maybe that's what happened in this case? It's been T'd in between such a bleed valve and the wastegate actuator? Though the bleeds valves I saw and used did not have a spring and ball.

Normally the wastegate is not joined to the MAP sensor line. If you were bleeding off air or the actuator leaked, then the MAP sensor would have a wrong pressure for the inlet manifold.

There is another port on the side of the carb which operates the wastegate, via a bleed valve if fitted, from above the throttle plate. If you're operating the wastegate from below the throttle plate, then you can't control the boost on part throttle.