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MG Midget and Sprite Technical - PCV valve

Went for a 40 mile trip today, and found on return oil pouring out the back of the engine (again). In fact out of anywhere it could. To recap, I have a 998 engine with the 1098 breather system, ie 1098 valve cover and breather cap, 1098 front tappet cover with oil separator, plus an extra breather on the redundant fuel pump position. The 2 breather pipes go via a Y piece to a standard 1098 PCV valve on the inlet manifold. See photo.

I just tested the PCV valve in the standard way - take off oil filler cap, engine note should rise - and it does exactly that. The diaphragm is intact and pliable. Second photo is the plunger. Should it be this shape? The crinkly plastic bit is rock hard.

I had the oil loss right down to an occasional drip, so something has failed somewhere. The compressions are all 190-200 psi hot and WOT, so the engine is in good health otherwise.

I'm reluctant to replace the whole PCV valve as it's very expensive and it might not help. Should I try one from another car? I have seen the Datsun 1200 one recommended. This is so disappointing as the car ran so well today, after months of work in lockdown.

Les






L B Rose

Les,

The crinkly plastic bit should be soft and flat since it's rubber. Item 33 on the Moss US web site below. That's is probably the leading cause of your problem since it should flex during engine running.

https://mossmotors.com/mg-midget-austin-healey-sprite/fuel-intake-emissions/emission-control
Martin

What Martian said. Spot on. Shouldn't be crinkly and should be nice and flexy.
Greybeard

Les,
Yes the diaphragm should be soft and flexible without the wrinkles !
I am not sure the PCV you have is the original Spridget one. All the one’s I’ve seen / owned have had a flat top (not raised as yours) - see the Moss picture.
Not that should be a problem but check before ordering spares.

R.
richard b

Thanks Martin, I thought as much. I can't find anywhere in the UK to buy the plunger separately, unless any of you guys knows better. I will have to bodge it until I can find one.

Richard, this isn't the diaphragm it's the plunger that sits under the diaphragm.

Les
L B Rose

Les, I was talking about the external metal top of your PCV canister being raised and not flat as the usual Spridget ones i've seen - see the Moss illustration that Martin linked.

Thats why I though it maybe off another car.

R.
richard b

Finally found it in the archives !

Les, if you look at the thread in the M&S General Archive 2019 'PCV Valve Wanted' there are a couple of pics of an original one I had.

R.
richard b

Thanks Richard

From the discussion that appears to be a 1275 one, whereas mine came off a 10CC engine. It's stamped FVP2001/03.

Can anyone explain how this thing is supposed to work? When the plunger is depressed it stops before the plate with the rubber on it meets the housing. I can't see what the rubber is for.

Les
L B Rose

I don't know how yours works as I can't see all the bits or how it goes together but the original works like this...

It's essentially a pressure differential valve. The cover has a small hole in it to ensure the top of the diaphragm is at atmospheric pressure. When the manifold is in vacuum (at idle) it should suck all the air/gas out of the crankcase and then pulls against the rubber diaphragm causing the valve to close. At part throttle the vacuum in the manifold drops allowing the valve to open and that in turn allows crankcase gasses to be drawn in to the engine to reduce the amount of oil gasses escaping to the atmosphere.

TBH it's a complete nightmare to get a working breather system as it needs so many things to be in "tune".
First up you need a healthy engine without excessive blow by. Then you need to a allow a small amount of air to enter via the oil filler cap. Too much and you won't be able to pull enough vacuum to close the valve, too little (unlikely) and the valve will always be closed. Then you need to ensure all the other sources of air intake have working seals e.g. dip stick etc.

If it were me, I would just remove it, fit a filler cap that allows a lot of air in or gas out such that the crankcase is always at atmosphere and live with a little oil drip and possible smell of oil when stationary.

Best of...
MGmike

M McAndrew

Thanks Mike. I understand what it's intended to do, I just don't grasp how the design achieves it. There is a sprung valve in the base of the unit into which the plunger fits, but the latter has no effect on that as it's too short. The disc part of the plunger leaves a big gap all round so what that's for beats me. The engine should not have excessive blow-by as the compressions are healthy and even, and there is no oil mist from the valve cover. Years ago I just vented it to atmosphere via a catch tank and I might go back to that. A new PCV valve is about £60 which is ridiculous for something so hit or miss. I've always had drips from the rear bearing, despite an oilseal kit. A mystery why it's suddenly so much worse now.

Les
L B Rose

I have an oil seal kit installed on the 1275 and it seems to somewhat help contain the oil that makes it past the scroll. I have replaced the seal several times and only once did it seem to be completely effective.

The problem is the taper on the crank flange surface. The seal is designed for a flat, perpendicular surface, not conical. For the seal trip to bear effectively, the crank flange would need to be cut so that it is a flat surface, at which point the seal in the kit would need to be replaced with one that is a smaller diameter.

Regarding PCV, I have installed a Mazda PCV valve on the B that feeds into an intake manifold port. The Moss kit uses a generic PC valve, (Dodge I think but dodgyin any case) and the car consumed oil excessively. The Mazda valve seems to be spot on as there is no noticeable oil consumption (1 Qt + 5,000 miles). The vacuum line to the manifold is also a small diameter, reduced to fit the small diameter intake port on the Weber manifold. In any case, it has worked out well. Like the smaller A series engines, the PCV circuit on the B is vented from a tappet cover port.



Glenn Mallory

Glenn, I have the Morris Minor Centre oilseal kit and this destroys the scroll effect because the top half of it is replaced with an aluminium part for the seal carrier. So I'm totally reliant on an effective lipseal. The crankshaft flange on which it runs has a Speedi-Sleeve so it's square and true, and the right size. But every time I replace the lipseal it only lasts a month before the leak starts up again. I have looked at reverting to the scroll, by having the bearing cap machined tighter and then line bored (same for the upper half of the scroll), but the engineer says he'd have to take an awful lot off the ends of the caps and the sides would not close up enough. Expensive and no guarantee it would work. What is the answer for a 60 year old casting? I've been trying for 15 years and never find a solution.

Les
L B Rose

its a case of luck I think. I still have the scroll seal and its just the occasional drip. I have retained the negative crankcase pressure set up. ie the timing cover is connected to the carbs. fortunately no oil suck either!!!

Bob Beaumont

There was an interesting alternative crankshaft seal arrangement described in MASCOT by Steve Kiel a couple of years ago. He devised a means of fitting a face contact seal that bears axially at the rear of the crank. His idea works on the argument that the lip type seals fail because the lip wears rapidly with any slight eccentricity as the crank revolves, whereas his face contact seal can easily adapt to any fore and aft movement of the crank. He used a standard readily available seal and there was a small amount of machining needed.

From memory, as described in the magazine it sounds like a fairly simple DIY project and I think he has tried this out on a 1098 engine. Its certainly an interesting idea. I wonder how it is now performing as the miles increase.
GuyW

I just took the PCV system off and I'm considering going back to atmospheric breathing. As I explained, I had the 1098 breather with oil separator on the front tappet cover, and a home made breather and separator on the redundant fuel pump point. I found that by blowing through both of these there was quite a lot of back pressure. It's not really feasible to clean them, is it? I'm thinking of fitting the 945 tappet cover with breather pipe, and venting this and the other one to a catch tank. Maybe also go back to the 948 valve cover so I have 3 breathers to atmosphere. I'll try this with a plastic bottle and if it's better I can fit one of those nice metal catch tanks. What do you guys think?
L B Rose

One last try with the PCV valve. I managed to clean the oil separators and they now flow well. I bought a new diaphragm and flat cover. But the new plunger I bought does not have any rubber on it. The old one had that hardened crinkly stuff, this one is naked metal. They assure me at Moss that this is correct. Do you folks agree?

I will have to, as they say, suck it and see.

Les





L B Rose

Yes, they should be just metal - any crinkly stuff is wrong!
Chris at Octarine Services

There should however be a spring under the plate.
Chris at Octarine Services

Yes I've got the spring. Haven't tried it yet as the bonnet is masked up for spraying and I'm still waiting for the correct paint!

Les
L B Rose

I went for a 50 mile run today. To recap, I had cleaned the oil separators and filler cap gauze, and fitted a new PCV diaphragm, plunger, and flat cap with correct clip. I can find no evidence of oil leaks. Towards the end of the run the clutch felt smoother. The oil is probably burning off. Maybe I have fixed this at last.

Les
L B Rose

This thread was discussed between 11/07/2020 and 14/08/2020

MG Midget and Sprite Technical index

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