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Engine guides and mods

Top Breather Pipe Repair

If your top breather has failed and you are having problems changing it, repair it instead.

Modification Details

Repaired Top Breather Pipe

Here is the top breather pipe found on the 450 Fortwo and Roadster. The black puck is a one-way (check) valve.
It allows vapor from the crank case to be sucked into the engine but stops any boost air from passing
into the crank case which can push oil out of the lower breather pipe and into the turbo.

If you have air conditioning, replacing this pipe is hard so instead of replacing it, just replace
the broken part. In this case, we will be replacing the one way valve with a better one.

Photos are shown with the pipe off the car for clarity.

Cut through the plastic pipe just behind the check valve.

You'll need a replacement check valve and some rubber or silicone tubing.

The 1st section of tubing needs to be 4.5cm long with an 8mm inside diameter.
Look on the check valve for an arrow denoting air flow. This short hose needs to go at the base of the arrow.

Another piece of 8mm inside diameter hosing goes on the other end. This piece is 7cm long.

Push the short section of pipe over the tube, it'll push on about 2.5cm.

You can also reuse the black rubber hose from the old breather pipe if you wanted to.

Double check that the check valve is fitted on the correct way around. Try blowing
into the straight tube. If it's correctly fitted, you will not be able to do this.


45mm and 70mm long pieces of hose with an 8mm inside diameter. Click
A check valve suitable for air with 10mm barbed hose connections. Click

Both of these will be available on eBay.

Problems With The Metal Valves

As we all know, metal expands and contracts with temperature. The problem occurs when these types of
valves get very cold (freezing outside temperatures). They shrink and can cause the valve disc inside
to become deformed or stuck. Generally they stick open, let boost through until the heat of the air unfreezes it.
That cycle occurs over and over and you may not notice it happening.

If you are going to use a valve such as this, I would recommend lagging it (insulating it) with something.
Either pipe insulation or something similar just to stop the freezing temperatures reaching the valve body.

Niall B's Fix

Niall had a different idea for a fix. He took a replacement breather pipe but used the other end for the fix.

He took the rubber elbow that normally goes down the back of the engine and disonnected it from the rest of the pipe.

He then pulled out the failed valve and cut the plastic pipe just after the 1st bend behind the valve.

The elbow is then pushed between the existing plastic pipe and the replaced valve. He fitted
pipe clips however this isn't necessary as there is no pressure in the pipe behind the valve.

Handmade Top Breather Pipe

Mister Bridger from the forum came up with a radical idea. Because he has air conditioning, 
replacing this breather is a much harder task. Originally Stuart thought about just changing 
the valve but found that it was hard to get off and thought that the pipe may split.

He decided to make his own breather pipe that would make it easier next time around.

Mister Bridger's Story

A trip to a plumbers merchant and for £1.41 i bought 1m of 10mm soft copper pipe.
On the way home I bought 1.5 m of Net curtain wire for £2.48 at Homebase. After taking
a look at the original, I bent the copper pipe as close to the original in shape as possible.

I cut the end going onto the valve a little shorter for 2 reasons.

I wanted to have some flexxing to be allowed at the join to the valve as 
it is very hard to get every angle spot on when bending the pipe by hand.

It makes it easier to pull the valve off when the time comes to change it.

Now for the easy bit. Make sure you change the valve before an oil change is due. Pull off the old valve. 

Squirt some brake cleaner fluid/carb cleaner/ or WD 40 down the pipe. Then insert the net curtain 
wire and push it back and forth to move any crud. Give it a final squirt and all the fluid and crud 
goes into the sump and is sucked out with the old oil.

WARNING- make sure you do not loose the straw of whichever cleaner you
use down the pipe. A good tip is to apply a large bit of BLU TAK to the straw
and this will stop it going down the pipe.

Extra Info For Bending Copper Pipe

You have 4 ways to bend the copper pipe to the required shape.

Carefully bend it by hand - be careful as you can kink the pipe.
Pack the pipe with sand and hand bend it - be sure to clean the pipe out after.
Use a proper mandrel pipe bender - expensive if you can find the right size.
Use a pipe bending spring - if you can find one the right size.


If you have air conditioning, plan to keep your smart for a long time and don't fancy 
lowering the engine more than once, this is a good alternative. Once the DIY top
breather pipe has been installed in position, the valve can be swapped in minutes which 
tends to be the part that fails. Saving you from mucking about behind the A/C pipes again.

Click if Info Helpful

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