Crankcase ventilation and brake servo vacuum

[HRA]

For a while now I've been wondering how oil is getting on top of the rocker cover. There's usually a bit around the screws that hold it down, and a trail down the side of the engine to the cross-member. At first I was blaming myself for slopping oil as I top-up, but more recently I've been taking more care and cleaning-up any dribbles.

I think I've traced the source to the oil-filler cap. It's the sort without a pipe connection to the intake manifold. I suspect oil is spraying around under the cover, sometimes hitting the mesh inside the cap and then dripping down onto the rocker-cover. I've checked that the mesh is there clean except for a coating of oil.

That led me to look into crankcase ventilation and read-up on it online. Clearly my engine’s ventilation is set-up in a non-standard way. I’ve got the oil-separator on the left-hand side of the engine and this is piped direct to the air-intake filter. I can’t see a PCV valve in the line unless it’s incorporated into the oil-separator. If a PCV valve is present it isn’t doing much – I can blow and suck down the pipe so there’s no restriction or one-way valve.

My plan is to clean the oil-separator and refit it, buy and fit a PCV valve, and then pipe it to the intake manifold. That should give me some “positive” to the crankcase ventilation! However, I’ve only got one vacuum port on the inlet manifold, and that’s used for the brake servo. What’s my best option? Should I “T” the crankcase ventilation into the brake servo pipe, or will this reduce servo efficiency? Is there any advantage to T-ing one into the other or vice-versa? Or a “Y”-piece even?

Alternatively, should I go “old school” and fabricate a pipe from the oil-separator that rises above the level of the head-gasket and then turns around to end below sump-level? I’m sure that’s all I had on my MGB years ago. Will the air rushing past the end of the pipe be sufficient to efficiently pull blow-by fumes out of the engine and suck clean air in through the filler-cap?

As ever, any advice or insight would be much appreciated!

[Burdekin]

The way you have it would be the same flow direction as a B20. You could also just leave as is and add a catch pot from the rocker cover cap and let it collect the oil residue rather than put it back through the engine.

[Derek UK]

A new seal for you filler cap might be all you need. Sounds like an early B18. Set up your pipework as original. No need to mix and match systems.

[HRA]

The engine is a B20. As it is set up at the moment there is no "forced" or "positive" crankcase ventilation. The hose from the oil separator goes to the back of the K&N air-filter on the carburettor. The oil filler-cap is the type that doesn't have a pipe coming out of the top; it just vents to air on the underside. So there's nothing actually sucking air/fumes out of the crankcase. I suppose any blow-by fumes just condense inside the crank-case and dilute the oil.

According to Ron Kwas's website (https://www.sw-em.com/pcv_diagrams_and_notes.htm) either the oil-separator or the oil filler-cap should be piped to the inlet manifold. My problem is that I only have one pipe-fitting on the inlet manifold and that's already connected to the brake servo. I don't want to do anything that might compromise braking efficiency...

[Ron Kwas]

HRA;

It sounds as though the original plumbed Oil Filler Cap of the B20 (Carbed OR FI) has been replaced with an earlier non-plumbed one...there's actually some evacuation of crankcase gases happening due to the slight vacuum inside the Air Filter...but at higher revs, where more blow-by gasses occur, this is maybe being overwhelmed and so some fumes and oil droplets are being pushed out of the vented Oil Filler Cap, making the mess...if your engine is not really tired and making massive amounts of blow-by, and needing a rebuild, you could just return things back to their original configuration, so that there is a minor vacuum in the Crankcase to pull out all gasses, where the Brake booster is directly connected to the Intake Manifold, and PCV Sys is "T"d into that as well, as shown on the linked page.

Cheers

[HRA]

Thanks Ron,

So it's quite OK to T into the brake-servo hose? On the manifold side of the non-return valve of course! Presumably the PCV valve will restrict the flow from crankcase to manifold sufficiently that the brake servo will still experience a good "vacuum".

My brakes are fine with the servo working, but on the few occasions I've rolled the car downhill without the engine running I've felt I needed both feet on the brake pedal!

[Ron Kwas]

HRA;

There is no "flow" from the Brake Assist unit, and only minimal flow, as you correctly note, flow limited by PCV Valve.

"I've rolled the car downhill without the engine running I've felt I needed both feet on the brake pedal"...you're probably used to the assist when engine is "helping".

Cheers

[HRA]

The last of the bits and pieces arrived in the post this morning, so I took the afternoon off to sort-out my PCV.

The new PCV valve has a fat spigot and a thin one. I can blow air through the fat end but not through the thin, so I connected the fat end to the oil-separator with a short length of reinforced-wall hose. I think that makes sense - I want to suck fumes out of the crank-case but in the event of a back-fire I don't want flames getting into the crank-case. The spigot coming out of the oil-separator is also "fat", so it makes sense to fit fat-to-fat via a larger diameter hose.

I fitted a short length of reinforced wall hose to the inlet manifold, and joined this to a Y-piece.

I connected one arm of the Y-piece to the thin end of the PCV valve.

I connected the other arm of the Y-piece to the brake-servo via a one-way valve. I made sure to connect the one-way valve the right way round.

It all looked good - new hose throughout and jubilee clips on every connection. The car started fine so I went for a spin. I soon found that I had no servo-assist on the brakes. So rather than a spirited drive in the country I went gently round the block and back to my garage.

I disconnected the line from the Y-piece to the PCV valve and fitted a short length of plugged hose to the Y-piece instead. I started the engine and the servo is working fine - effective braking with minimal force on the pedal.

Clearly there's plenty of vacuum from the manifold when I've just got the servo connected. However, with the PCV valve also connected to the manifold there's less vacuum so the servo doesn't work.

What have I done wrong, or what have I mis-understood?

[Burdekin]

This is my 142 with the original B20 setup.

[HRA]

Thanks Burdekin.

I'm running my PCV in the opposite direction. Sucking from the crankcase and venting via the oil-cap. But that should work - it's how PCV was plumbed on B18 Amazons.

The other difference is that I only have one vacuum spigot on my inlet manifold. Hence the Y-piece so I can connect both servo and PCV valve.

So why does my PCV valve let so much air into the inlet manifold that there's not enough vacuum for the servo...?

I couldn't see it in the pictures, but I take it you have a PCV valve fitted between the manifold and oil-filler cap?