Smiths PCV Valve Weak Mixture (Solved)

[svenedin]

As many of the regulars will know, my 1969 car has a Smiths PCV valve. For those who have not encountered one of these (they are unusual on Minors) I have attached a photo.

The PCV was used for a short time on Minors. I have never been able to work out the exact time period but mostly 1969 cars and possibly late 1968. The PCV was replaced by an even later system whereby the tappet chest oil separator was connected to a special type of HS2 carburettor with a breather inlet.

Many people think that the PCV (positive crankcase ventilation) was introduced due to emissions regulations in the USA but in fact the PCV valve long predates these regulations. It was introduced in conjunction with a change to closed-circuit engine breathing which means that the engine does not vent blow-by gases to atmosphere. Instead, the blow-gas (which are hot, oily and contain water) are sucked into the inlet manifold via a PCV valve and burnt in the engine. All modern cars have PCV valves. Minor engines that originally had closed-circuit breathing are designated 10ME rather than 10MA.

There are many advantages of having better engine breathing. Oil leaks from gaskets and especially from the crankshaft rear scroll are very much reduced or totally eliminated (because the crankcase is kept at slightly negative pressure relative to atmospheric); blow-by fumes are removed from the crankcase which keeps the oil clean and less corrosive (products of combustion contain water and acidic gases); unpleasant fumes are not vented to atmosphere in the engine bay. Mayonnaise (water and oil whipped together by the rockers) is never seen in the rocker cover with this system, even with short journeys.

The Smiths PCV is a very cleverly designed unit. It is designed to maintain a small constant negative pressure in the crankcase regardless of the inlet vacuum which varies widely according to whether the engine is idling (high vacuum) or under load (low vacuum). It does this by altering the flow through the valve with a diaphragm and spring arrangement inside the housing. The system of connecting the oil separator to the carburettor cannot do this and operates at whatever pressure the inlet manifold is at.

Anyway, enough about how great the system is, there are a number of drawbacks. The valve relies on a thin rubber diaphragm. If this develops so much as a pinhole the valve draws in air from the atmosphere and the car runs very lean and rough. In fact, the engine can behave very strangely. Starting a cold engine when there is a hole in the diaphragm gives unexpected behaviour of the choke. The choke makes the mixture richer so one would think this would compensate but the hole in the diaphragm causes the valve to fully open with the same effect of opening the throttle and the engine tries to race. Once you know this can happen you will always carry a spare diaphragm!

Recently I had a very puzzling few weeks with the engine fuel/air mixture. It had been perfect and the spark plugs were the perfect biscuit colour. Then the mixture started to become subtly weak and I just couldn't seem to correct it. I would adjust the carburettor a little richer and all would seem fine but then a few days later it seemed to be weak again and the spark plugs looked white.

I checked the diaphragm in the PCV and it was absolutely fine. I checked all the breather hoses and they were fine too. I did the test in the workshop manual: remove oil filler cap (long vented type is used with a PCV) and the engine revs rose as they should. Most odd. I then proceeded to check everything with the hypothesis that there must be an air leak somewhere on the induction side. I could not find one. I then took the PCV off cleaned it out and replaced the diaphragm. No difference. I checked tappet chest gaskets, rocker gasket, rocker cover bolt rubber washers etc etc etc. No problems found.

In the end I decided to change the long-vented oil filler cap. This is supposed to be a service item anyway. That was the answer! The oil filler cap rubber gasket was not sealing properly from age and so excessive air was being drawn in there. In fact, I think I could hear it when driving the car. If I suddenly came off the accelerator (vacuum rises sharply) I could hear a strange whistle.

So perhaps this is why the Smiths PCV was dropped in favour of the breather inlet HS2 carburettor? It caused too many odd problems!

PS: long-vented is really an incorrect term for that type of oil filler cap. It's not a "vent" it's a restricted inlet for fresh air.


Stephen

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[svenedin]

Just to summarise this as my original post was rather long-winded.

If you have the Smiths PCV valve system, all of the engine external gaskets and seals must be airtight except for the specially restricted air intake in the long-vented oil filler cap. If a gasket or seal lets in air the crankcase pressure rises to atmospheric pressure or closer to atmospheric pressure. Under those circumstances the PCV valve reacts as if there are excessive blow-by gases and opens fully which only draws in more air from the leak. The result is rough running especially at idle and excessive ingestion of oily gases into the engine causing high oil consumption and carbon deposits on spark plugs (there is excessive flow through the tappet chest oil separator and the oil cannot condense out). Don't forget the rubber seal under the cup washer on the tappet chest covers (concave type) on the later engines.

Further details on the workings of the Smiths PCV valve are here: https://mgaguru.com/mgtech/engine/cv103.htm

Stephen

[svenedin]

I picked up a spare Smiths PCV valve recently for £16. If you ever need to service one of these out on the road but the lid is stuck, here's a quick tip. The wire clip is exactly the right diameter to fit in the air hole in the lid to use as a tool.

Stephen

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[philthehill]

A word of warning - the early Smiths PCV are still around and being advertised S/H for sale but spare parts i.e. the diaphragm are no longer available.
Therefore make sure that valve is the later type with the separate diaphragm.
There is no servicing requirements for the Smiths PCV valve - either it works or it does not work. The diaphragm is the part that fails and has to be replaced.

[svenedin]

This is a late one Phil with a replaceable diaphragm (marked 1970). Servicing in the field is replacing a diaphragm with a hole in it which does happen. I always carry a spare. Actually, they do need periodic servicing. The poppet valve in the base can become gummed up with sticky oil residue. This can be cleaned out with carburettor cleaner and pushing the poppet valve gently with a piece of wire but it is better to remove the valve from the car and shake the cleaner through.

Stephen