PCV Valve on the BMW E90 N46B20: Causes of Failure and Replacement
Quick Summary
The crankcase ventilation valve (PCV) on the N46B20 is a frequent failure point. It causes a fluctuating idle, lean-mixture codes, and high oil consumption. The PCV is integrated into the valve cover, which makes replacement more involved. When symptoms appear, don’t put off the repair: ignoring the problem can end in an engine rebuild. This guide will help you diagnose the fault, pick a quality part, and do the replacement.
The Problem
You get in, start the engine—and the idle starts wandering: dipping, then surging. Before long the “Check Engine” light comes on, and oil consumption climbs to 1 quart every 620 miles. A familiar story for owners of the BMW E90 with the N46B20—a 2.0-liter engine rated at 150–170 hp, fitted from 2004 through 2013.
The culprit is a failed PCV valve. This small valve vents crankcase gases from the oil pan back into the intake manifold, maintains vacuum in the crankcase, and keeps oil from being forced out through the seals. When it fails, the engine starts running unevenly, and oil burns heavily in the cylinders.
Symptoms
- Fluctuating idle, especially on a warm engine
- P0171 and P0174 codes (lean mixture)
- High oil consumption—up to 1 quart every 620 miles
- Codes come back after a reset until the root cause is fixed
Causes
The PCV fails when its diaphragm jams up with deposits. Oil vapor and combustion byproducts settle on the diaphragm, it loses elasticity and stops moving. Over time, this locks the valve completely.
When the valve lets in extra air, the engine control unit (DME) tries to compensate by adding fuel, but the system falls out of balance. Crankcase gases laden with oil mist get pushed into the intake manifold and burn in the cylinders—that’s where the high oil consumption comes from.
Ignoring the fault has serious consequences: crankcase pressure builds, oil gets forced past the seals, and the air leak leans out the mixture and causes overheating. In the worst case, you’re looking at cylinder bore scoring and an engine rebuild.
The Fix
Diagnosis
Start by looking up the part number by VIN on RealOEM. Two numbers fit the N46B20: 11617503744 and 11617832777. Important: after 2007 the valve is not interchangeable with the old one, so always verify against your VIN.
Visual inspection: remove the valve cover and check the diaphragm and spring for elasticity. If the diaphragm has hardened or torn, or the spring has gone soft, the valve needs replacing. Thick black deposits are a sure sign of coking.
Cleaning the old valve is not recommended. The diaphragm wears out over time, and even after cleaning it won’t recover its properties. Cleaning only buys time—a few thousand miles down the road, the problem comes back.
Part Selection
| Option | Service Life | Comments |
|---|---|---|
| Genuine BMW valve | 37,000–50,000 miles | Preferred for longevity, matches factory specs exactly |
| Mahle OC237 | Comparable to genuine | Reliable alternative, similar design |
| Topran, SWAG | 6,000–9,000 miles | Low-quality diaphragm and spring, fails quickly |
The genuine valve is the preferred choice. Mahle OC237 is a proven alternative from a well-known engine components manufacturer. No-name brands like Topran and SWAG often fail within 6,000–9,000 miles—the savings turn into a repeat repair.
Step-by-Step Replacement
- Disconnect the battery and remove the engine cover.
- Unplug the connectors and remove the hoses blocking access to the valve cover. Remove the ignition coils and spark plugs.
- Undo the valve cover bolts—there are about 15–20 of them. Work carefully so you don’t damage the gasket.
- Remove the cover to reach the PCV. The valve is held in by clips or screws—undo them and pull out the old unit.
- Install the new PCV, making sure the diaphragm and spring are in place. Reassemble in reverse order.
If you’re not confident in your skills, leave the job to a professional. Assembly mistakes will cause air leaks or damage the valve cover gasket.
Prevention
- Avoid short trips in freezing weather: condensation in the crankcase speeds up diaphragm coking
- Warm the engine up to 90°C at least once a week if you mostly drive short distances in winter
- Check the oil level regularly and change the oil every 4,700 miles using oil that meets the LL-04 spec
- Flush the ventilation system every 37,000 miles with dedicated cleaners
- Install an oil catch can—it traps oil mist and keeps it off the PCV diaphragm
Bottom Line
The PCV on the N46B20 is one of the most troublesome components, but with proper care it lasts. The main thing is not to ignore the symptoms: a fluctuating idle, P0171/P0174 codes, and oil consumption up to 1 quart every 620 miles are grounds for immediate diagnosis. Choose the genuine valve or Mahle OC237, change the oil every 4,700 miles with LL-04 approval, and flush the ventilation system every 37,000 miles. Stay on schedule, and your BMW E90 will reward you with smooth running.