Ammstar Mercedes-Benz Blog

Can a PCV valve cause rough idling? A Mercedes specialist explains

It’s hard not to notice when your Mercedes-Benz isn’t idling like it should.

While everything may sound normal while you’re in motion, the moment you come to a stop at a red light, the engine starts shuddering violently, and the RPM jumps up and down for no reason at all.

You look down at the dash, only to see that there’s no check engine light, you’ve never had a stall and your temperature and levels are where they should be.

So, what’s behind your Mercedes’ rough idle? Is it…

  • Bad fuel?
  • Worn spark plugs?
  • Burnt-out valves??

It could be any of these issues… if you drive an AMG vehicle with the well known V8 6.3 litre 156 engine however, it could be something else entirely – specifically, a problem with your engines ventilation system.

Whatever the issue is, get it looked at by your local Mercedes-Benz specialist in Melbourne!

Could the cause be a broken Mercedes PCV valve?

Blow by gasses, the partially burned fuel-air mixture that slips through the piston rings and into the crankcase, is unavoidable. Even in highly-engineered vehicles with tight tolerances like your Mercedes-Benz aren’t immune.

The PCV (positive crankcase ventilation) system exists to reduce pressure in the crankcase caused by blow by, removing it before pressure can build up to a point where your engine’s seals start to fail.

A properly functioning PCV system protects your Mercedes-Benz from a range of engine issues such as excess oil consumption, engine oil degradation, blown gaskets and seals, oil leaks and an incorrect fuel-air ratio.

The intake manifold operates at a different pressure to the outside atmosphere, creating a natural vacuum. When the PCV valve is open, the intake vacuum draws the blow by out of the crankcase.

These vapours are then mixed into the fuel-air mixture back in the combustion chamber, where it’s fully burned, maximising efficiency and reducing emissions.

Don’t worry, the system is designed so that it only enters the combustion chamber while your car is idling – hence why so many Mercedes-Benz owners report roughness seemingly vanishing once they hit the accelerator!

While there are many consequences to a faulty PCV system, today, our Mercedes-Benz specialists in Melbourne will be focusing on how a faulty PCV system – in particular, a faulty Mercedes PCV valve – can lead to rough idling.

The link between PCV valve failure and rough idling

The PCV valve is key to your PCV system. Of course, that also means that issues with the valve can lead to the entire system failing.

Most PCV systems use simple spring-loaded valves, usually made of plastic. Over the years, it isn’t uncommon for these parts to degrade and wear out due to heat and the high pressures Mercedes-Benz engines tend to generate.

In addition to failure with the valve itself, your Mercedes PCV valve may become jammed by a buildup of fuel deposits, locking it in an open or shut position.

A PCV valve that’s stuck open will allow too much air to enter the intake manifold, leading to a leaner idling fuel-air mixture. Stuck closed leads to a build up of pressure in the crank case which creates a variety of oil leaks!

And this in turn can lead to a range of issues, including a rough idle.

Rough idling with the M156 engine

In addition to rough idling, PCV problems with AMG models using the M156 engine are also prone to other ssues, one of which is squealing noises at idle. The squealing noise is produced as air is being forced out the rear main oil seal on the crankshaft.

This particular V8 engine is used in many AMG models produced from 2006 and onwards, including:

  • W204 C63 AMG
  • C209 CLK63 AMG
  • W211 E63 AMG
  • W164 ML63 AMG
  • C219 CLS63 AMG
  • W221 S63 AMG
  • R230 SL63 AMG

Like with many engines, AMG models aren’t immune from PCV failure. For AMG vehicles however, a faulty PCV valve can often come with other issues in addition to a rough idle. In some instances, this may also be accompanied by strange noises.

One of the most common is a squealing noise while idling, though some owners have reported issues including whistling and sometimes even rattling.

How often should you get your Mercedes PCV system inspected?

Many car manufacturers aren’t very strict about PCV valve replacements, with few including them on their service guides at all. Mercedes-Benz is one of them.

And as they aren’t listed as a maintenance item, they are often completely overlooked by generalist mechanics and dealerships alike!

Since they aren’t listed in the service guide, there isn’t really a set time or distance interval tied to PCV valve replacement.

And because the PCV system pulls air and blowby gases into the intake manifold, it has a similar effect on fuel-air mixture as a vacuum leak. As a result, PCV problems are often misdiagnosed.

To be on the safe side, we suggest having it inspected with each service – that way, your Mercedes-Benz mechanic can identify faults as they arise, and recommend replacement if required

In order to keep your Mercedes-Benz running smoothly, it’s important that every piece of your engine is in tip-top condition. That includes often ignored parts like your PCV valve.

Our Mercedes-Benz specialists in Melbourne make it their mission to offer superior service for your vehicle. In addition to offering a superior price to the dealership, that also means inspecting and replacing parts and systems that often get overlooked, like your PCV system.

Entrust your vehicle to a Mercedes-Benz specialist in Melbourne

Driven to deliver better service

If you own a Mercedes-Benz vehicle – especially if it’s an Mercedes-AMG powered by an M156 engine – you want it looked after by a mechanic that knows it inside-out, and who will treat it with the care it deserves.

With 20 years of experience specialising in German brands like Mercedes-Benz, Ammstar is the team to call!

As an independent Mercedes-Benz mechanic, we operate independently of the dealership network. This allows us considerable freedom to:

  • Adjust your Mercedes-Benz service
  • Choose alternate suppliers (without compromising quality, of course)
  • Use our own more efficient and cost-effective procedures

We aim to provide you a better service, full stop – and we aim to offer a lower Mercedes-Benz service cost while we’re at it!

If your AMG (or any Mercedes-Benz model) is experiencing rough idling, bring it in to our Mercedes-Benz specialists in Melbourne. Using our unbeatable knowledge and specialist systems, we’ll fix the issue and keep your Mercedes-Benz running.

You want a better Mercedes-Benz service that won’t cost a fortune. To get it, simply get in touch with Ammstar today.

Alternatively, click here to get in touch online!

2020-05-13T03:06:19+00:00April 30th, 2020|News|