MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - oil flller cap steam

Can anyone explain why a lot of steam and some oil are being forced out through the small hole in my oil filler cap? It's a '72 BGT with one of those aftermarket alloy rocker covers and the chrome filler cap with the pinhole.
I understand that the original rocker cover had a breather pipe and that the pinhole is supposed to compensate for none in the alloy type. But really, when I lift the bonnet the steam billowing from the cap is like a pressure-cooker, not to mention the oil its spewing onto the underside of the bonnet, dripping down all over the engine bay.
I have even had to replace the rocker-cover gasket because the pressure blew it out!
The top-end was rebuilt about a year ago. Is there something that was replaced wrong or not fitted which would cause this?. The breather pipes from the crankcase are connected to the Carbs as normal which are SU-HIF's.
On the original set-up, what was the other end of the pipe from the rocker cover connected to? I'm thinking maybe drill my cover add a piece of pipe and re-instate the old system!!
Any comments, ideas?
Ian Doherty

Ian,
Steam from the oil filler cap??? Oil spewing from the top of the engine??? Serious pressurisation! Is the head cracked? The original rocker cover for a 1970 at least had a vented oil filler cap but not a breather pipe.
Neil
Neil22

Normally they were connected to one air cleaner so the engine could breath filtered air. In your case it would just soak the air filter with oil. Neil is correct, something is seriously wrong. How is it running? This is also a classic problem caused if you burn a hole in a piston or distroy the ring grooves in the psiton from severe pinking.
John H

Yeah, it seems that something is seriously wrong with the bottom end of this motor. As John H says this could well be a piston or rings that are causing the problem. I would be inclined to do a compression test or preferrably a leak down test to determine exactly where the problem lies.
Iain MacKintosh

I would start with the simple things first and check for a blown head gasket (and keep my fingers crossed as that is a cheap and easy thing to fix). Cheers - Dave
David DuBois

It does seem like severe pressurisation of the crankcase, and a blown head gasket if you are getting steam, i.e. the top end not the bottom.

Only pre-64 non-positive crankcase ventilation systems, and 1970-on North American spec engines with a charcoal cannister, had a vent on the rocker cover, and neither of those had a ventilated oil filler cap but a solid one. Pre 64 cars had a hose from the rocker cover to one air cleaner can, and another from the front cover dangling down, it was hit and miss as to which way air flowed, if at all.

With positive ventilation systems (post-64) the inlet manifold sucked on the crankcase either via a gulp valve (up to 69) or the carbs (70 and on). Non charcoal canister cars let air in to the crankcase via a restricted and filtered oil filler cap. Charcoal cannister cars had a restricted port on the back of the rocker cover, the filter was in the canister, and that let air in on those cars.
Paul Hunt 2

Just to keep terminology straightand prevent confusion, a gulp valve is used with the air injection system on the post 70s MGBs. The valve used from 64 to 69 MGBs is a Positive Crankcase Ventilation valve (PCV valve). Cheers - Dave
David DuBois

Ian, Are you from Limerick by any chance?

Brian Griffin.
B. G. Griffin

If this is a head or head gasket problem there would be difficulty in keeping the water in the radiator as this too would be pressurised and blowing off through the overflow pipe. In the absence of this I would still be inclined th favour a bottom end problem. Nevertheless a compression or leakdown test will reveal the source of the problem.
Iain MacKintosh

Thanks Dave - got my valves in a twist.

Iain - still don't see where a bottom end problem would cause steam, or even oil vapour. I'd expect a problem there to be knocking or rumbling. Unless this is a terminology thing again - 'bottom end' to me just involves the crank and its bearings.
Paul Hunt 2

This thread was discussed between 18/02/2008 and 23/02/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.