In the early 2000s, BMW implemented a “clutch delay valve” (CDV) or “clutch lock valve” in many of their models. In cars without this valve, the clutch fluid passes through the clutch line normally when you depress or let up on the clutch. Thus, the clutch performs exactly as your foot tells it to; if you dump the clutch, the pressure plate jumps onto the flywheel and off you go. However, on these BMWs with the CDV in place, the action of the clutch is damped. Basically, these valves have narrow openings for the hydraulic fluid to pass through. By narrowing the opening, they slow the rate at which the fluid will pass through the piping, thus slowing the action of the clutch. So, if you dump the clutch in one of these BMWs, the clutch will slip for a bit, as the action is slowed.
The idea behind these valves is to protect the drive train from shock and create a smoother drive. However, this only helps with people who are inexperienced with driving manual-transmission cars. For those who are experienced, the CDV can make smooth shifting more difficult, as you have much less control over the clutch action. Also, because the CDV allows for more clutch slippage, clutch life is considerably shortened. For this reason, many BMW enthusiasts will “delete” the CDV. I also decided to delete the CDV in my E39 BMW; hopefully my driving will become more smooth and not less, as the latter would be indicative of inexperience.
In my case, I bought a CDV-delete valve from Bavarian Autosport. On their website, they call this part an “unlock valve.” Basically, it has the same dimensions as the OEM part but has a larger interior diameter, thus allowing the fluid to pass through at a normal rate.
I jacked up the front of my car and placed my jack stands underneath. I left my jack on the center jack point just for extra safety in case the jack stands failed. I crawled under the car and located the CDV that I would be replacing. However, as I was trying to remove the old CDV from the clutch line and master cylinder, I snapped the clutch line. The threaded end of the clutch line had seized in the CDV, and thus when I put too much pressure on it, it snapped. Now, I am waiting on the replacement clutch line. I will post an update when I finish the job and a review of the CDV-delete.
Update: I finally got my E39 up and running again yesterday. With the snow storms we’ve been having recently, my package with the part I needed had been delayed. It got here yesterday and I installed it. To do this, there is a simple clip-mechanism that holds the hard part of the clutch line (the new part I had gotten) to the soft line. I undid this clip and attached the new part. Then, I installed the CDV-delete in the master cylinder. Finally, I attached the hard line to the CDV-delete.
Next, I had to bleed the clutch. I was a bit surprised to push down on the clutch pedal to find it had hardly any resistance, but I did lose a lot of fluid in the process. So, I ended up adding fluid, pumping the clutch, adding more fluid, pumping the clutch more, etc, until I began to feel some resistance at the end of the clutch travel. I kept doing this for a while until there was a normal-feeling amount of resistance. Next, I got a helper to pump the clutch ten times and then hold it down on the last pump, where I would open the bleed valve on the master cylinder and close it again before they released the clutch. The idea behind this is to purge any air from the system. If the helper releases the clutch when the valve is open, it can actually draw air back into the system. This is why it is important for the clutch to be held down while the valve is opened and closed. After doing this about six times, the clutch felt normal. It was time to take it for a spin.
As I drove out of the garage, I immediately felt a different in the clutch feel. It felt much sharper and more direct. Out on the road, it felt easier to modulate the clutch and starts were smoother. I tried a couple of launches as well; I couldn’t resist. Before deleting the CDV, when launching this car, I would often find myself slipping the clutch more than I had wanted to and the engine would rev too high. I don’t ever do hard launches, but just accelerating onto a big road quickly, I felt as if the clutch slipped too much. Now, after the CDV delete, the clutch slips much less and the car gets moving much more quickly.
So, in conclusion, this is a worthwhile mod. I’ve only driven a few miles since I deleted the CDV, but I can definitely feel the difference. I’m not sure of the logic behind having a CDV in the first place—BMW knew well enough not to install them in the M-cars, so why did they install them in the regular cars? Who knows, I’m just glad I deleted mine and am looking forward to testing it out more.
Exhaust Fumes 