I've got an '03 MCS that I drove for a couple years before having the JCW kit installed. A few months after having the kit installed (back in '05) the bypass valve return spring failed, leaving the valve open and effectively negating the supercharger. I didn't know it at the time, just knew that the car felt really sluggish. The local dealer said nothing was wrong. The dealer that installed the kit had a clue, replaced the valve, and the car was back to rocket status. It's been feeling a little sluggish again the last few months. Not nearly as bad as before, and if I floor it it feels strong, but the car feels heavy and not eager to rev. I'm also checking my gas mileage, as I feel like I'm not getting as good mileage as I usually do. I asked about the bypass valve a while back when I was having some other stuff checked/done, but they said it was A-OK. I'm going to have it checked again pretty soon.

I've seen a couple other posts about bad bypass valves from people with JCW cars. Is this a common problem with JCW cars? Any past threads that I should read regarding bypass valve issues?

 

JCW and the BPV have no relationship other than they both are installed on the engine. Thus, there is nothing about the JCW retrofit that should affect the BPV...except the installer.

Two things to check:

(1) the top vacuum hose may have worked itself loose from the top of the BPV if it wasn't securely fastened when the BPV was replaced;

(2) if you have the JCW intake installed, the vacuum line that activates the flap in the airbox is taken from the same vacuum source as the BPV -- a "Y" connector is put in the vacuum line near the BPV -- check that connection.

HTH,

Theo