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by Dave Zeckhausen


 Theory of Operation - What is a CDV?

A 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.

Due to this factory device, the clutch engages the flywheel slowly, regardless of how quickly you lift off the clutch pedal.  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 first to second gear shift may be jerky, leading passengers to question your skill, as their heads bob back and forth!  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, which is how we first noticed this evil device on our own 540i 6-Speed.  Perhaps they figured this shifting behavior would be unacceptable to M drivers, while the rest of their customers wouldn't 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 installing the wrong part, each CDV type has a different visual appearance.  (See Figures 3 & 4 below)  The effect on the car is the same, however.  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 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 causes it to leak and may also leave bits of metal behind, potentially damaging the seals in the clutch slave cylinder.

We developed a technique for removing the interior valve and spring without damaging the tapered seats.   In addition to offering modified CDVs for sale, Zeckhausen Racing provides a free service to modify valves 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 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.  Jack stands can collapse if not positioned properly, and a floor jack can fail suddenly, without warning.  Please use both a floor jack and a pair of jack stands 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.

Raise the front of the car and 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 as a safety backup, in case a jackstand should fail or slip.  You can be seriously injured or even killed if you are careless doing this!

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

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 must 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.

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 attached at the junction of a 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 fitting as soon as you loosen it from the CDV.

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

Once you've loosened the 11mm fitting, use a stubby, open-end wrench to remove it the rest of the way.  It will go much faster than continuing to use the longer flare wrench.

Install the modified CDV in its place, being careful not to cross-thread it.  Start by hand, and make sure it is engaging properly before using a wrench.  When the fitting is hand snug, tighten it the rest of the way with the 11mm flare wrench.

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, ensuring the rear chamber is completely full.

If you have a pressure bleeder, connect it to the reservoir and set to 20-25 psi.

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 the bleeder open for 4 to 5 seconds, as air bubbles are purged from the system.  Do not keep it open much longer or you'll run the reservoir dry.

If you don't have a pressure bleeder, you'll need an assistant.  As before, fill the reservoir.  Get under the car and 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 by hand.  Open the bleed screw while your assistant is pushing down and close it while they are lifting.  Keep testing the clutch to see if it has returned to full firmness.  Once the 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, leave it set at 20 psi and watch for leaks.

* An alternative to replacing the CDV is to eliminate it entirely.  There are two reasons why you might not want to do this. 

1. On some models, the steel hard line must be bent to a new angle, if the CDV is deleted.  If the line is not bent perfectly, it's difficult to engage the treads without stripping them.

2. It's not uncommon for an overzealous service writer to blame modifications for unrelated failures.  Rather than argue with a 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 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.  BMW 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 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 process of acquiring and 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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