Testing 2CV crank case vacuum with a manometer

The 2CV’s two cylinder boxer engine configuration means the crank case volume changes with each revolution of the crank, reducing as the two pistons come towards each other on the inlet/power stroke and away from each other on the exhaust/compression stroke.

If the crank case was a sealed system this change in volume would cause resistance to the running of the engine.  To prevent this the crank case has a breather that vents through the oil filler and out to the air box.

The breather is fitted with one-way valves that allow the air out of the crank case but not back in thus maintaining a negative pressure.  This negative pressure has the advantage that the oil has a tendency to be drawn back into the engine rather than leak out.

2CV egine breather schematic

To ensure these one-way valves are functioning the crank case vacuum should be tested with a manometer – the pressure differential here is quite low (~0.1 psi) so a normal vacuum gauge typically won’t be sensitive enough to show a reading.  Per the Citroën workshop manual, this vacuum should not drop below 5cm even at high revs.  (At high revs there is less time for the air to be pushed out of the crank case as the pistons move towards each other before they move out again.)

There are various options for a manometer – from the simplest being a loop of tubing up to one made to the same specifications as the original Citroën workshop tool (MR. 630-56/9a) that is available from Burton.  I went with a generic plumber’s manometer from an on-line retailer.

I also added a liquid trap to the engine side of the manometer to prevent water being sucked into the crank case but another option is to use LHM fluid as this won’t contaminate the oil if it does get into the engine.

Manometer used to test 2CV crankcase vacuum

(The water level doesn’t show up too well in these photos so I’ve highlighted it, a bit of dye would mean the level is more obvious but, when you’re actually stood in front of it, seeing the water level isn’t a problem.)

With one end of the manometer open to the atmosphere the other is connected to the inside of the crank case via the dipstick opening.  The 6mm (inside diameter) PVC tubing I’m using wasn’t large enough to seal so some insulating tape wrapped round the end did the job.

2CV manometer dipstick end 'gasket'

Make sure the engine is nicely warmed up then take out the dipstick and connect the manometer to the crank case via the dipstick tube.

Manometer inserted into 2CV crankcase via dipstick aperture

With the engine idling, gently rev it a couple of times to stabilise the fluid level in the manometer.  We’re now ready to take a reading.

At idle, the water level on the engine side should be at least 5cm higher than the side open to the atmosphere – indicating the pressure in the crank case is lower.

Keeping an eye on the water levels, rev the engine all the way up to full throttle – we’re expecting the water level to fall but it should never get low enough to be level with the side open to the atmosphere.

A vacuum of less than 5cm at idle or dropping to zero at high revs indicates that the one-way valve in the breather has failed and needs to be replaced.

Manometer in 2CV engine showing vacuum

I was pleasantly surprised to see that Judith was showing a vacuum of over 30cm , especially as the breather hasn’t been replaced recently – if ever.