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MG MGB Technical - Oil filler cap - crank case vent?

I noticed this the other day on my 1972 roadster. With the engine running, taking the oil filler cap off increased the engine rev's to about 1000rpm. Replacing the cap dropped the rev's to around 800rpm, but the engine sounded a bit on the rough side (so slight I've never really noticed before)
So it sounds like my cap is blocked, I need a new one, but technically, can anyone tell me what's happening inside the engine with a blocked cap.

Thanks

Colin.......
Colin M

Usually taking off the oil cap produces a slower rough idle - have you checked your gulp valve?

rn
RN Lipow

Gulp valve? sounds like something to do with emissions related equipment, which fortunately, I don't have.
Thanks anyway.
Colin M

The PCV system used on your car needs a source of outside makeup air, which on your car is drawn through the cap and "filtered" by the bit of foam gauze. The crankcase is vented through the tube on the oil seperator located on the front tappet chest cover via the "Y" hose arrangement to ports on each of the carbs. The negative pressure (i.e suction) at the carbs draws the fumes from the crankcase and burns them in the combustion process. New air must be drawn into the engine, and this comes through the cap. If the cap is blocked the air will be drawn into the engine from other (valve cover gaskets and seals, side cover gaskets) sources. If the hoses are old and cracked, and the cap is blocked, I suppose this would work in reverse as air is drawn through the hoses and pushed out through the various old, dry gaskets, manifested in oil leaks all around.
Andrew Blackley

Colin,

The cap may or may not be vented. Engine series 18GA thru 18GH use a vented cap. 18J-18V use a non-vented cap per my Haynes manual. Your 1972 model should have a non-vented cap, and should draw it's air through the evaporative control cannister. This is the black plastic can located on the firewall. The hose from the cannister goes to the rocked arm cover. I am not 100% certain if this is the same in non-US cars.

My Haynes manual says: "The crankcase emission control can be easily tested as follows. Run the engine at it's normal idling speed and undo the oil filler cap. This allows an unrestricted flow of air into the engine and it's speed should therefore increase and show signs of a weaker mixture."

You may be running slightly rich if the idle is a bit lumpy with the cap secured.

Cheers, Doug
Doug Keene

Doug: Collin is, I believe, from the Midlands, England, hence his car has no evaporative cannister and should have the same PCV system as the early NA MkII cars.
Andrew Blackley

Doug is correct - you should have a sealed oil filler cap. You should also have a small curved pipe coming of the rocker cover at the rear. This is supposed to be connected to the charcoal canister (if you still have one - I removed this from my '73). This curved pipe has a small orifice built into the end. This is where the engine draws outside air for crankcase venting. The orifice provides for a smaller flow than would happen through the filler cap. Coupled with the PCV properly connected to the inlet manifold, the crankcase runs at a slightly negative pressure.

Make sure your canister (if you have one) is not blocked. If you don't have one, ensure that the orifice on the curved pipe is not blocked.

By the way, if you no longer have the canister it is a VERY good idea to install some sort of small filter on the end of the curved pipe (I use a small round K&N) because the engine is continuosly drawing air through the orifice. No filter equals crud and dirt in the crankcase.

Hope this helps.
Richard Smith

As such, removing the oil filler cap on *any* UK car will alter the revs, PCV valve or no. It is normal.
Paul Hunt

Thanks all for your help. As my car is not a USA model most of the advice re-charcoal canisters etc doesn't apply to me, but Mr Andrew Blackley, Ohio, USA described my system perfectly. After some probing with a torch I can see the pipe coming form the tappet chest, via a 'Y'piece to the carb's. So it now makes sense that my cap is blocked and this is somehow causing a problem with the intake mixture (too rich) So when I take the cap off, more air enters the engine and leans out the mix' a little.

As an extra, I read on a different BB of an RS turbo owner who was worried that when he removed his oil filler cap, the engine cut out! A number of people replied telling him it was not unusual for these cars!

Thanks everyone,

Colin M.......
Colin Muir

This thread was discussed between 13/03/2003 and 14/03/2003

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