BMW 335d Maintenance Most People Don’t Know About

If you don’t think diesel-engined cars can be fun, you’ve probably never driven a BMW 335d. The six-cylinder turbo in this car makes close to 300 hp and over 400 lb-ft of torque yet can achieve 35 mpg on the highway. No car is ever perfect, though, and the Achilles Heel of the 335d is a problem known as carbon buildup or CBU.

We recently had a customer bring an older, higher mileage 335d into the EurAuto Shop in Plano, TX., because it wasn’t running well. Here’s what we found, why it’s a problem, what causes it, and what we did to fix it.

Carbon Buildup

The problem with this car was a buildup of hard, black carbon material throughout the intake system. This buildup had coated all the passageways and bores air comes in through, reducing their effective diameter.

Buildup throughout the intake system is like having your airway constricted: it makes it harder for the engine to breathe. That reduces power, causes a lumpy or uneven idle, and increases fuel consumption. Eventually, the ‘Check Engine” light comes on, and some fault codes are set that we can read with our scan tool. Usually, one of these will indicate a problem with the Mass Flow Sensor. The sensor measures how much air is flowing into the engine. It will also set a code if there’s too little.

What Causes CBU?

It’s believed the black carbon deposits are a combination of soot and oil. The soot comes from the exhaust gas recirculation (EGR) system that diesel engines use to reduce emissions.

These deposits work by sending some exhaust gas back through the engine. This burns some of the more harmful components of the exhaust, which cleans up what comes out of the tailpipe.

That leaves the question of how oil gets into the intake. There are several theories. Some say it’s the crankcase ventilation system returning oil that gets past the piston rings. Others say it’s from turbocharger lubrication, and yet others think it’s from the valve stem seals. In reality, it’s probably a combination of all three.

There is, however, another theory. The theory holds that poor-quality fuel is the root of the problem. Whether or not that’s correct, it’s advisable to buy good quality fuel rather than the least expensive you can find.

Repairing an Engine With a CBU Problem

The job starts with removing the intake manifold and exposing the areas where the buildup occurs. The buildup is then scraped away as best we can, after which we blast walnut shells at what remains. (Walnut shells are hard enough to wear away the CBU, but they’re also brittle, so they break down into a fine dust that will burn up in the engine.)

Once we’ve restored all the surfaces to their as-new condition, we replace the gaskets in the parts we’ve taken off and put everything back together. It’s not a quick job, which leads customers to ask if there’s something about BMW diesel maintenance they’re overlooking.

CBU Prevention

Regular oil changes and following the factory-recommended maintenance procedures will keep your BMW running well, but they won’t stop CBU entirely. In all honesty, if you drive a 335d, you should probably accept that having CBU removed, maybe every 50 – 100,000 miles, is part of looking after it. Consider it the price of that fantastic combination of performance and fuel economy.

If you suspect your BMW 335d is suffering some CBU, bring it to us. We’re in Plano, TX., and we specialize in European auto repair. We’ll take a look and tell you what it needs.