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Cycling Valve Replacement
 
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Pelican Technical Article:

Cycling Valve Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$125

Talent:

***

Tools:

6mm Allen, flathead screwdriver

Applicable Models:

Porsche 944 Turbo (1986-89)

Parts Required:

Cycling valve

Hot Tip:

Work gently with the hoses

Performance Gain:

Proper power

Complementary Modification:

Check spark plugs

If you are having a problem reaching full boost on your Porsche 951 there may be a chance your cycling valve has failed and is stuck in the fully closed position. A cycling valve in the fully closed position will limit your boost to approximately 1.2 bar.

The cycling valve is located under the intake manifold and servicing or removing it means removing the manifold. Please see our article on intake manifold removal for additional assistance.

There are three lines connected to the ports on the cycling valve. One port is connected to the inlet side of the turbocharger between the air flow meter and the turbocharger, one to the compressed side of the turbocharger via a banjo bolt on the discharge to intercooler pipe, and the last line goes to the waste gate diaphragm. The cycling valve is normally in the "open" position, which allows the pressure from the turbocharger discharge to be bled back to the turbocharger inlet. This is possible because the port on the cycling valve coming from the turbocharger discharge has an orifice in it. When the cycling valve moves toward the "closed" position, it is closing off the port to the turbocharger inlet. This permits pressure from the turbocharger outlet to pass through the cycling valve to the line going to the waste gate. When this happens it causes pressure to build up on top of the waste gate diaphragm. When the combined pressure on top of the diaphragm and exhaust pressure against the valve seat overcome the spring pressure in the diaphragm, the waste gate will open bypassing or "dumping" the exhaust flow around the turbocharger and limiting boost pressure.

The cycling valve is controlled by the KLR Unit (Knock Regulator Unit). When the boost level reaches the computer's preset limit, the KLR sends a signal to the cycling valve to close. This allows exhaust pressure to be directed to the waste gate and the waste gate to open. If the cycling valve fails, loses power, or a problem occurs in the KLR unit, the cycling valve will fail in the fully closed position. This limits boost to approximately 1.2 bar.

The cycling valve is located under the intake manifold below the number three and four intake runners (red arrow).
Figure 1

The cycling valve is located under the intake manifold below the number three and four intake runners (red arrow). You must remove the intake manifold to replace the cycling valve. Please see our article on intake manifold removal for additional assistance

With the manifold removed you can see the cycling valve (red arrow) at the rear of the motor beside the number four spark plug.
Figure 2

With the manifold removed you can see the cycling valve (red arrow) at the rear of the motor beside the number four spark plug.

The cycling valve has three rubber hoses attached to it.
Figure 3

The cycling valve has three rubber hoses attached to it. The red arrow indicates the hose that runs to the waste gate connection. The green arrows indicate the line from the valve to the inlet air pipe side of the turbo. The yellow arrows indicate the line from the valve to the turbo boost side via the banjo bolt. If you are just replacing the cycling valve you do not need to disconnect the hoses from anything other than the valve.

Remove the electrical connection from the top of the valve (red arrow).
Figure 4

Remove the electrical connection from the top of the valve (red arrow).

Use a flathead screwdriver and remove the hose clamps and hoses from the valve (red arrows).
Figure 5

Use a flathead screwdriver and remove the hose clamps and hoses from the valve (red arrows). Separate the hoses from the valve using care as these hoses can get brittle over the years.

There was mixed hardware on our cycling valve and we had to use an Allen and Torx.
Figure 6

There was mixed hardware on our cycling valve and we had to use an Allen and Torx. Use the appropriate tool for the hardware holding your cycling valve in place and remove the two fasteners.

With the hardware removed you can simply remove the valve from the mount and install the new one.
Figure 7

With the hardware removed you can simply remove the valve from the mount and install the new one. Installation is the reverse of removal.


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