fume vent dumping oil

[KeithL]

I would suggest that you will not experience any noticeable problems with the engine.
I have run Minors with the old metal rocker cover oil filler cap, the sealed plastic cap, the breathable plastic cap and aftermarket caps fitted to aftermarket rocker covers and never experienced any noticeable problems but then my engines have always been kept in tip top condition.
Whilst the engine breathing system may not be utilised at its best when having the wrong oil filler cap no permanent or impermanent damage to the engine will be caused by having the wrong oil filer cap as shown by the running of 10,000 miles by your engine.

But still worth changing the cap for the correct one?

[KeithL]

I changed the filler cap for a non-vented one and the other day I was checking everything out and pulled the hose off the air cleaner container and noticed that rather than air being sucked into the rocker cover, air is being blown out of the tube. This seems to be at odds with what has been said before.

Any ideas what is going on?

[paul 300358]

Was it re-bored or just the rings replaced? Have you carried out a compression test?

[oliver90owner]

Without ventilation, crankcase will be under positive pressure when the engine is running. There will always be blow-by, past the pistons, to some degree.

[Bowie69]

Unless it is like a steam train, I probably wouldn't worry about it.

Also, it may feel like it is blowing, but it is quite possibly 'chuffing', in other words pulsing in and out, which feels very like it blowing vapour out.

[KeithL]

Was it re-bored or just the rings replaced? Have you carried out a compression test?

The last compression test was 6,000 miles ago at which time all cylinders were even at 140 psi. They didn't tut and shake their head when they told me that so I assume 140 psi is okay.

[IslipMinor]

Keith,

The early non-closed circuit systems only 'work' when the car is moving. That is the only time when the 'draught' tube that runs down from the front tappet cover sees any movement of air across the end of it, which is what is supposed to create the partial vacuum to draw fresh air into the crankcase, via the air filter, into the rocker cover and crankcase, and then extract the fumes from the crankcase via the draught tube.

When stationary, there is no air movement, thus no extraction, and the crankcase fumes will build up and escape from any orifice available - a vented oil filler cap, the tube running from the rocker cover back to the air filter body, the rear scroll, a worn timing cover seal, the draught tube etc. Almost anywhere but where the fumes are supposed to go, i.e. down the draught tube!

Both the later 1098 closed circuit systems worked reasonably well, and can be retro-fitted to the earlier cars. It may not be the problem on your engine, which has had recent work done on it, but no ventilation system will cope with excessive blow-by of fumes in a worn engine.

Out of interest, how clean is your air filter element? If it has more than a very light staining of oil on it, it is likely that the extraction side of the ventilation system is not working as well as it should. Now that you have fitted a non-vented filler cap, I would replace the air filter element and keep an eye on how clean it stays from 'oily fumes'.

[KeithL]

The air filter element itself is very clean. However, and the reason I started this investigation, is that oil does collect in the air cleaner hosing, run down the side and drip out the bottom. Not a problem as such but it's a bit annoying and I was worried it was symptomatic of a bigger problem. From what you are saying this could start to happen whenever the car is stationary.

Can the draught tube become blocked and how can you check? Presumably if it does then the whole system stops working.

Sorry if I keep asking basic questions but I am not a mechanic - I'm just trying to keep the car running as best it can.

Many thanks.

[Edward1949]

For the breathing system to work, fresh, filtered air enters the rocker cover FROM the air cleaner and is drawn down through the crankcase and OUT through the tappet cover and down the side vent tube.


Trying to get my head around this. What causes the pressure in the rocker cover to be LESS than the pressure in the air cleaner, thus drawing air in , then becoming higher in pressure as it journeys down to be expelled (which suggests higher pressure) from the tappet cover?
I can't see why the entire innards of the engine aren't all at the same pressure, but the suggestion is that there's some factor causing partial vacuum in the rocker cover which turns into positive pressure upon reaching the tappet cover exit pipe. Can somebody explain?

[IslipMinor]

Keith,

The draught tube does not block easily, but may be worthwhile taking the tappet cover off and cleaning it to make sure.

Edward,

All crankcase ventilation systems rely on drawing or sucking fumes out from the crankcase area, which includes the rocker cover, and delivering them somewhere outside the crankcase.

The early 'draught tube' system uses the forward motion of the car to pass air across the end of the tube down by the sump, to create a very slight vacuum in the draught tube, into the crankcase via the tappet cover, up into the rocker cover and then into the air filter case, which is the source of the fresh air that replaces the fumes. The fumes exit from the draught tube directly into the atmosphere, which the system was replaced by the 'closed circuit' design as an early contribution to pollution.

The later 'closed circuit' systems use either the PCV valve on the inlet manifold, or the port on the side of the SU carburettor, to create the partial vacuum, with the source of fresh air for these system being via the vented oil filler cap, which contains a filter within it. The fumes exit through the PCV valve and are ingested by the engine.

Does that explain it better?

[Edward1949]

Keith,

The draught tube does not block easily, but may be worthwhile taking the tappet cover off and cleaning it to make sure.

Edward,

All crankcase ventilation systems rely on drawing or sucking fumes out from the crankcase area, which includes the rocker cover, and delivering them somewhere outside the crankcase.

The early 'draught tube' system uses the forward motion of the car to pass air across the end of the tube down by the sump, to create a very slight vacuum in the draught tube, into the crankcase via the tappet cover, up into the rocker cover and then into the air filter case, which is the source of the fresh air that replaces the fumes.

The later 'closed circuit' systems use either the PCV valve on the inlet manifold, or the port on the side of the SU carburettor, to create the partial vacuum, with the source of fresh air for these system being via the vented oil filler cap, which contains a filter within it.

Does that explain it better?

Thanks for this explanation. I've always assumed that unless an engine is in absolutely pristine condition it will always suffer some piston blow-by resulting in positive pressure throughout the engine, including the rocker cover. I assumed that the rocker cover breather pipe was to relieve this pressure into the engine's air intake. So you're saying that instead of blowing out through this pipe it's sucking it in?

[IslipMinor]

Keith,

Yes, the rocker cover either connects to the source of replacement fresh air from the air filter housing in the earlier 'draught tube' systems, or provides the direct source of fresh air, via the vented filler cap, in the later 'closed circuit' systems, which has no connection to the air filter.

Once in the engine, the fresh air is drawn down into the crankcase and either out into the atmosphere through the draught tube on the front tappet cover, or out through the flame trap canister, which on the 1098 is on the front tappet cover, or on 1275 engines on the timing cover, and finally through the PCV valve and ingested by the engine.

If the flow is reversed, the oily fumes will start to clog up the air filter element, and if a vented filler cap is fitted, can make a mess on the top of the rocker cover.

When the engine is idling, if you remove the non-vented oil filler cap on the early system, the engine speed is unaltered and fumes can escape from the oil filler hole. On the later 'closed circuit' systems, the idle speed should increase slightly as more air passes into the engine, but the fumes should continue to be drawn into the ventilation system. If any escape through the oil filler hole, it very likely indicates excess blow-by past the piston rings.

I have added a couple of comments to the previous post to clarify where the fumes are designed to end up!

[KeithL]

The draught tube does not block easily, but may be worthwhile taking the tappet cover off and cleaning it to make sure.

Should there be anything in the draught tube? I tried pushing a thin piece of wire up it this morning and it seemed to hit an obstruction around the join in the tube.

Is taking the tappet cover off, or more to the point putting it back on, a big job? Would I need a new gasket, and if so is cork or neoprene better? How tight do you do the bolt up? Anything else I need to watch for?

That having been said I ran the engine earlier and once again air was blowing out of the rocker cover so I stuck a vacuum cleaner nozzle at the bottom of the draught tube and air seemed to stop coming out of the rocker cover so some air is passing through the draught tube.

[Mogs]

I'd always thought the fumes were sucked into the air filter (or via the vent tube in the side of some carbs), too. Very helpful info Gents. Back in the 1970s, my 1275 Midget at the time had the PCV fitment and after a complete engine rebuild, the car routinely blew a quick puff of oil smoke from the exhaust, momentarily, about five minutes into any journey, and was fine after that, with very good good compression and no other oil burning or loss. I never cured it.