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94cobra69ss396 · 12-15-2013, 08:33 AM

Time for an update. So even though the EVAP was full of fuel, it was not the cause of the stumble I was feeling at a cruise. I thought that it might have been caused by the magnetic pickup in the distributor since I'd already tested the coil and ignition module and it wasn't either. I called up GTR and asked if they had a distributor I could borrow. They said that the only one they had was on their race car but that I was more than welcome to borrow it. So I ran over to their shop and pulled it off the car. I installed it on the Cobra the following day and took it for a test drive. That wasn't the issue.

I swapped my distributor back in and took theirs back to them. While there I was talking to Ricardo about the issue and said my next thought was something Steve had mentioned to me when he did the tune which was that the location of the bypass valve. With the UPR intake tube the location moved from directly in front of the blower inlet to the front of the tube which is about 4 inches behind the MAF. This position can cause air to re-enter the MAF from the backside causing the stumble. Ricardo said an easy way to test it is to just remove the supercharger belt so I tried it. But it still had a light stumble. It wasn't nearly as bad but I could still feel it.

Since it did improve it I decided that I was going to move the bypass location. I borrow a plastic welder from a friend and started practicing on another intake tube I have. However, I just couldn't get it to hold like I wanted. My next thought was that instead of moving it that I could use some aluminum tubing inside the intake tube to direct the air towards the blower. So I pulled the bypass valve off to see what the diameter of the hole was. While I had it off I tested the bypass valve and found that it was bad. When I tried to create vacuum with my vacuum pump it just leaked. The diaphragm must be cracked which was causing it not to open and would creat a vacuum leak. That explains why I still had a stumble without the blower belt on.

I called GTR to see if they had a replacement in stock but they didn't. I then went online to see if any parts store carried the Bosch bypass valve I had and found that a factory Audi/VW/Saab valve was an upgrade to the one that I had. I went to Ontario VW and picked one up. Sure enough, that was the issue!

12-15-2013, 08:33 AM	#7
94cobra69ss396 Fast & Filthy Join Date: Jun 2008 Posts: 3,840 Tweet	Time for an update. So even though the EVAP was full of fuel, it was not the cause of the stumble I was feeling at a cruise. I thought that it might have been caused by the magnetic pickup in the distributor since I'd already tested the coil and ignition module and it wasn't either. I called up GTR and asked if they had a distributor I could borrow. They said that the only one they had was on their race car but that I was more than welcome to borrow it. So I ran over to their shop and pulled it off the car. I installed it on the Cobra the following day and took it for a test drive. That wasn't the issue. I swapped my distributor back in and took theirs back to them. While there I was talking to Ricardo about the issue and said my next thought was something Steve had mentioned to me when he did the tune which was that the location of the bypass valve. With the UPR intake tube the location moved from directly in front of the blower inlet to the front of the tube which is about 4 inches behind the MAF. This position can cause air to re-enter the MAF from the backside causing the stumble. Ricardo said an easy way to test it is to just remove the supercharger belt so I tried it. But it still had a light stumble. It wasn't nearly as bad but I could still feel it. Since it did improve it I decided that I was going to move the bypass location. I borrow a plastic welder from a friend and started practicing on another intake tube I have. However, I just couldn't get it to hold like I wanted. My next thought was that instead of moving it that I could use some aluminum tubing inside the intake tube to direct the air towards the blower. So I pulled the bypass valve off to see what the diameter of the hole was. While I had it off I tested the bypass valve and found that it was bad. When I tried to create vacuum with my vacuum pump it just leaked. The diaphragm must be cracked which was causing it not to open and would creat a vacuum leak. That explains why I still had a stumble without the blower belt on. I called GTR to see if they had a replacement in stock but they didn't. I then went online to see if any parts store carried the Bosch bypass valve I had and found that a factory Audi/VW/Saab valve was an upgrade to the one that I had. I went to Ontario VW and picked one up. Sure enough, that was the issue!