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Removal of the Check Valve / Clutch Delay Valve (CDV)

Symptom: slow clutch engagement on manual transmission models including F30, F10, E9x, E8x, E53, E46, E39, E36 M3 and Z3M.

Models: F30, F10, E9x, E8x, E53, E46, E39, Z4, E36 M3, Z8, and Z3M.

Many owners of the cars listed above have noticed that they will get an excessive amount of clutch slippage and poor clutch engagement feel. This problem is most noticeable in situations calling for hard acceleration or quick shifting.

The cause is an added check valve (BMW calls it a "lock valve") in the clutch hydraulic circuit that is intended to slow the engagement of the clutch as you release the pedal. This hydraulic restrictor device is intended to smooth out the feel of shifts and potentially reduce the shock load on the drivetrain. The reality is it causes a high rate of wear on the clutch and excessive heat buildup in the flywheel leading to early failure.

Lock valve:




A little-known detail about the 1996-1999 M3/MZ3 3.2l (including Euro cars) is that they too have this part. Similar to the affliction of the later cars, with even greater effect on cars with aftermarket lightweight flywheels. Due to the quick-revving nature of these setups, the slow-down of the check valve causes momentary excessive clutch slippage. A particular example is a "power shift" where the revs hang for a moment before fully engaging. M3s with the 3.0l engine do not have this part, nor do any other E36.

The two possible locations:

(click for a larger view)



Replacement of the valve with the no-restriction replacement Unlock Valve in these cars results in very crisp and firm shifts. With the lightweight flywheel, the engagement is so positive and quick that passengers are pushed into the seat with greater force than ever before. Replacement is as simple as unscrewing the original lock valve from the slave cylinder line, replacing it with the Unlock Valve, and bleeding the slave hydraulics.

New Unlock Valve:


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