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by Dave Zeckhausen


 Theory of Operation - What is a CDV?

The Clutch Delay Valve (CDV) is a one-way restrictor, located between the clutch slave and clutch master cylinders.  It slows the engagement of the clutch, similar to a damped record player tone-arm, gently lowering the needle to the surface of a record.

Because of this factory device, no matter how quickly you lift off the clutch pedal, the clutch engages the flywheel at a slow rate.  In theory, it reduces driveline shock, if an inexperienced driver dumps the clutch.  In practice, it prematurely wears the clutch disc and transforms experienced drivers into novices who cannot shift smoothly.  During parallel parking, the delay can be infuriating, causing constant clutch slippage.  During hard acceleration, the slippage shortens the life of your clutch.  During normal driving, the shift from first to second gear will be jerky, leading passengers to question your skill.  As the driver, you can see your passengers' heads bobbing back and forth every shift!  Yes, in their minds, they ARE laughing at you.  


Thankfully, there is an easy solution in the form of Zeckhausen Racing's modified CDVs.  More about that later.

BMW E39 Lock Valve (CDV) Exploded diagram

Figure 1.   E39 5-Series CDV screws directly into clutch slave cylinder

BMW E46 Lock Valve (CDV) Exploded diagram

Figure 2.   E46 3-Series CDV connects between hard line and flex line in clutch hydraulic circuit


Interestingly, BMW did not install a CDV on the E39 M5.  The shifting behavior would be unacceptable to these high performance customers.  BMW does not seem to think the rest of their customers who shift their own gears will notice.  They were very wrong!

Since BMW models have different clutches, they utilize different CDVs with various valve openings and springs.  To reduce the odds of a factory worker installing the wrong part, each CDV has a different visual appearance.  (See Figures 3 & 4 below)  The effect is the same.  The driving experience is degraded.

CDV assortment 1

Figure 3.       E39 & E60 5-Series and Z8 CDVs with hex-shaped bases

Clutch Delay Valve Assortment 2                     

Figure 4.   Z3,      Z4/E46 3-Series,  E46 M3,  E90 3-Series

The solution is to replace* the CDV with a modified valve, with the interior parts carefully removed. We do not drill these valves.  Drilling damages the taper at both ends of the valve.  It does not seal via the threads, it seals at the tapered seats.  Drilling a CDV will cause it to leak and may also leave bits of metal behind, potentially damaging the rubber seals in the clutch slave cylinder.

We developed a technique for removing the interior valve and spring without damaging the tapered seats.   Zeckhausen Racing provides a free service to modify CDVs which are mailed to us (within the USA only).  See below for details.


Benefits of replacing your CDV with a modified unit

After the stock CDV is replaced with one of our modified units, you will be able to shift gears in your BMW like a "normal" car.  No more vague, inconsistent clutch engagement.  No more jerky 1-2 shifts.  You can actually chirp the tires when shifting from 1st to 2nd gear!  Parallel parking becomes a breeze and your clutch will love you for it and last longer.  Best of all, your passengers will stop making fun of your driving skills.


Directions for Installation of Modified CDV


Safety Warning:

Working on your own car can be dangerous.  Even quality jack stands can collapse if not positioned properly, and a floor jack can fail suddenly and without warning.  You can be seriously injured or killed if you do not follow proper safety procedures.  Please use both a floor jack and a pair of jack stands to support your car so a failure of any single support is less likely to result in the car falling on you!  Zeckhausen Racing assumes no liability, expressed or implied, for injuries or damage as a result of following these instructions.

Jack up the front of the car using a jack point specified in your service manual, and gently lower it onto a pair of jackstands, leaving enough room to crawl under the car just forward of the shifter.  Leave the jack in place to provide a safety backup in case one of the jackstands should fail or slip.  You can be seriously injured or killed if you are careless doing this!

Locate the CDV on the driver's side of the transmission case.

The drawing in Figure 1 and the photo in Figure 5 shows a 540i CDV.  The Z8 CDV is identical.  The replacement of the E39 525i, 528i, and 530i CDVs is slightly more difficult, since a support bracket should be unbolted before the CDV can be easily disconnected.  The E60 5 Series (2004+) requires the removal of plastic underbody panels to gain access to the CDV, so you may prefer to take the car to a shop equipped with a lift.

BMW 540i CDV

Figure 5. CDV location for 1997-2003 540i and 1999-2003 Z8

The CDV on the Z4, E46 3-Series, and X3 3.0 models does not screw directly into the slave cylinder.  Instead it is located at the opposite end of the hard line from the slave cylinder, at the junction of the steel hard line and rubber line.   See the drawing in Figure 2 and the photo in Figure 6


Figure 6. CDV location for E46 3-Series (M3 shown)

Place a drip pan under the car, since brake fluid will leak from the hydraulic fitting as soon as you loosen it from the CDV.  Using a hose pinch-off clamp will minimize fluid leakage and make the job of bleeding the system easier, however you must be careful not to damage the hose.  It is suggested you forgo the clamp and just accept that brake fluid is going to run down your arm.

Flare WrenchUse an open-end wrench (14mm or 17mm, depending on BMW model) to hold CDV steady and an 11mm flare wrench to loosen hard line fitting. An open-end wrench may strip the 11mm fitting. 

Once you've broken the 11mm fitting loose, use a stubby, open-end wrench to remove it the rest of the way.  It will go faster.

Install the modified CDV in its place, being careful not to cross-thread it.

Start the threads of hard-line fitting into the new CDV by hand, and make sure it is engaging properly before using a wrench.

When the fitting is snug, use the 11mm flare wrench to finish tightening it.

Now, crawl out from under the car, dry your hands with a paper towel and change your shirt!  It's time to bleed the hydraulic clutch.

Tip for E39 5-Series owners:  The challenging part is finding the brake/clutch fluid reservoir.  BMW hides it under the driver's side microfilter housing.  Remove the housing cover and microfilter.  Unclip the hood sensor from the wiring harness.  Unsnap three plastic tabs on the microfilter housing, using a large screwdriver.  Squeeze the spring clip holding the microfilter housing to the post and pull up.  Wiggle it free.  Voila!  You've just discovered the brake fluid reservoir. 

The hydraulic clutch uses the rear chamber of the reservoir, which is only about 1" wide (front to back).  There is a divider between the clutch and brake chambers so, in the event of a leak in the clutch system, you don't also lose your brakes.  Even though the reservoir looks full, it's possible you've drained the rear chamber.  Use a quality DOT 4 brake fluid, such as ATE TYP 200, and fill the reservoir, making sure the fluid flows over the divider, into the rear chamber.

If you have a pressure bleeder, connect it to the reservoir and adjust the pressure to 20-25 psi.  More than 30 psi and you risk blowing the reservoir apart, which would make a mess!  Brake fluid is not good for painted surfaces.

With pressure bleeder set, crawl under the car with a 7mm box end wrench and a plastic tube or brake bleeder catch bottle.  Remove the rubber cap from the clutch slave cylinder bleed screw and place the wrench over the end, then attach the plastic tube to the nipple.  Turn the wrench about 1/4 turn and hold it for 4 to 5 seconds, as air bubbles are purged from the system.  Do not hold the bleed screw open much longer or you'll run the reservoir dry.

If you don't have a pressure bleeder, you'll have to do this with an assistant.  It will take longer, especially if you've fully emptied the reservoir and introduced air into the system.  As before, fill the reservoir to the top.  Get under the car and follow the same procedure, except this time have an assistant push the clutch pedal to the floor repeatedly.  If there is air in the system, the pedal will drop to the floor and your assistant will need to reach down and pump it up and down.  Open the bleeder screw while your assistant is pushing down and close it while they are lifting the pedal.  Keep testing the clutch to see if it has returned to full firmness.  Once the clutch pedal feels normal, tighten the bleed screw and replace the rubber cap.

Wipe any brake fluid off the CDV and nearby parts and test for leaks by having your assistant push the clutch pedal repeatedly.  If you have a pressure bleeder, simply leave it set at 20 psi and watch for leaks from either end of the CDV.

* An alternative to replacing the CDV is to eliminate it entirely.  There are two reasons why you might not want to do this.  On some models, the steel hard line must be bent to a new angle, if the CDV is deleted.  The bend is slight and there is little risk of damaging the line.  The problem is, if the line is not bent perfectly, it's difficult to get the threads to engage without cross threading them.  This is made harder by the fact the fitting is slippery with brake fluid.

Another reason is some folks are concerned about future warranty issues.  It's not uncommon for an overzealous service writer to blame any modification for unrelated failures.  Rather than try to argue with the service department about whether the CDV deletion was responsible for the air conditioning failure, many folks simply install a modified CDV.  Stock appearance is maintained and the issue of "unauthorized" modifications never comes up.



 Free CDV Modification Service by Mail (USA only)

We do not drill these CDVs, as some have suggested.  Drilling enlarges the openings on the tapered seats at each end, causing leaks. Drilling may also result in loose bits of metal destroying the seals in your clutch slave cylinder.  We have developed a technique for removing the guts of the valve without damaging the tapered seats that allow them to seal properly.  To avoid the risk of a leak and subsequent clutch failure caused by modifying your own CDV, you can have one modified by Zeckhausen Racing.

Rather than paying us, you can send two new CDVs.  We will modify them and send one back. The other will be kept for our local CDV customers and to refresh our inventory of modified valves for sale.

CDV Swap

Send two (new, not used) CDVs, along with a padded, self-addressed, stamped envelope (US Mail only, please) to:

Zeckhausen Racing
Attn: David Zeckhausen
123 US Highway 46
Fairfield, NJ 07004



CDVs are different for each BMW model.  You will need to order the proper part for your car from a dealer.   A word of warning: The dealer will have no idea what a "CDV" is.  The BMW parts CD describes this as a "LOCK VALVE" so that is what you should ask for. BMW part numbers are listed below, as well as the number of fins and the size of the wrench (in millimeters) required to remove it:


BMW Model

Build Date

Part Number




E39 540i



3 14

E39 530i



5 14

E39 528i

up to 9/97


4 14

E39 525i, 528i

after 9/97


4 14

E46 320i, 325i, 330i



1 17

E46 M3



0 17

E90 3-Series, F10 5-Series



0 Round

E60 525i, 535i, 545i, 550i



5 14

E60 530i



4 14

E63 645/650Ci Coupe



5 14

E64 645/650Ci Convertible



5 14

X3 2.5i, 3.0i



1 17

X5 3.0i (E53)



5 14

Z3 2.3i,2.5i,3.0i





Z4 2.5i, 3.0i



1 17

Z4 M Coupe/Roadster



0 17







Modified CDVs Available for Sale

Customers often ask if we can sell them a modified CDV, rather than go through the hassle and delay of sending us a pair of valves to modify. Depending on your BMW model, modified CDVs are available.  To order, click here and select your BMW model.  If a CDV is available for your car, it will be listed among the other parts we have for sale.



Free yourself from the evil Clutch Delay Valve today!


BMW Clinic

CDV clinic participants, Summer 2001


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