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MG MGB Technical - Heavy Breathing !!!

Hi, Bought a MGB GT LE last year which has had little use, I am getting alot of vapour coming from the oil filler cap on the top of the engine. The car ahs had very little use in the last 3 years, 80K miles, 180lb compression down all 4 cylinders. I have taken the breather pipes off of the carbs and is breathing heavy through these as well, so would assume that the tappet cover filters are not clocked. The engine runs very well but, i would like to do know ??
Ian Sanders

You could have some stuck rings, from the period of inactivity, causing excessive blow-by. I'm not familiar with the crankcase ventilation system used on UK cars, but it could also be that the fittings that vent the crankcase are restricted, forcing the excess pressure out through the valve cover.
R. L Carleen

R.L. ...isn't that the front tappet cover that has the breather tube in it? Heard of taking it off and blowing through it to see if plugged then either cutting open and replacing wire mesh or flushing with solvent of some type to clean out.
J.T. Bamford

It was on my early 3 main engine, don't know what they do in the UK.
R. L Carleen

There shouldn't be that much blowby if you've got 180psi compression.

Is it possible you're seeing moisture cooking off out of the crankcase/oil from a long period of inactivity?

Do you see the vapor when everything is hooked up and the engine running, or only after you shut it off?

If it's running, then you must have a blockage in your breather system/PCV valve.

Steve

Steve Aichele

No PCV on an LE, and if you have the pipes off at the carbs and having puffing out of them the breather system is clear and it must be coming from blow-by ... *if* it really is flow and not just the effect of two pistons going up as two come down. This can be heard at the oil filler aperture (cap removed, carb pipes connected) as a thrumming. You should be able to feel a slight suction if you place the palm of your hand or a sheet of paper over the aperture, and this suction is coming from the carbs proving the breather is clear. Removing the cap should also cause the idle speed to drop slightly. But if you can feel *pressure* at the oil filler hole then this can only come from blow-by. You should have a ventilated oil filler cap, which is a maintenance replacement item. These can get blocked which will prevent proper breathing, but does not cause pressure in the crankcase. Some vapour coming out of the filler aperture when a hot engine is switched *off* is normal.
Paul Hunt

Hi,

It looks like blowback then, as i posted originally i have very good compressions, i thought with blowback i would be down on compression. If it is due to sticky rings, certainly when i removed the rocker cover i had alot of sludge in that area. The car was very badly out of tune as well when i got it (timing was out by 11 degrees and was running very hot (not overheating) but the cockpit was unbearable. Since correcting and having it tuned, the car and me are alot happier. Is it a sump off job, or do you think it may improve with regular oil changes / flushing ?
Ian Sanders

It's maybe worth running this motor for a bit before putting it on death row. Take the plugs out and put say a tablespoonful of Redex into each cylinder, leave it overnight and then put the vehicle back into service. You will have clouds of white smoke for a while but ignore that. Also use a quantity of Redex in the fuel say half pint for a full tank. This will also produce some smoke but is your best chance of freeing off any sticking rings etc. As a last resort it may be useful to do a blow down test so that you can determine that the pressure definitely does leak to sump.

At 80k it seems that the engine might be nearing a rebuild. It's more than a sump off and I would say its engine out time for a rebore new pistons and crank grind.
Iain MacKintosh

Ian, has the oil been changed recently?

Never heard of Redex but I suspect it is a top cylinder lubricant/penetrating agent like Marvel Mystery Oil.

My two-pence:

Next weekend run it up to operating temp and add 1Qt of parrafin (kerosene) and let it IDLE at fast tick-over for 10 min. Drain and refill with your favorite oil (don't forget the filter).
The parrafin will loosen the sludge quite well and probably help if you have sticky rings (which I doubt with the 180# cylinder pressure).
A little nasty oil/sediment/parrafin will remain in the system but not enough to cause trouble (not like the old oil was!).

I think you have various nasties boiling off and need to get them out!

Go for a nice long hot run and stretch 'er legs.

Mike!
mike!

Hi, I used the car and done about 700 miles with a couple of oil changes, used flushing oil. Must admit didn't give redex a thought. Will give it a go this weekend. Will let you know the outcome.
Ian Sanders

"Flushing" oil could explain the vapors as they have light-weight solvents in them that could be boiling off.

Mike!
mike!

Ian, One thing you seem to have omitted is to tell us how much oil the engine used in the 700 miles./
Iain MacKintosh

I have hardly noticed any oil usage. Certainly not enough to require topping up.
Ian Sanders

Ian, your compression is good, oil consumption negligible I don 't think you have too much to worry about unless of course you are going to be doing high mileages. In any event engines do tend to breath from the filler cap but this vapour should be clear, i.e no heavy fuming. If you havent seen this sort of thing before you might be worrying needlessly. Perhaps an opinion from an experienced person would be in order and if my geography is correct you might not be too far away from Chris Betson at Octerine Services
Iain MacKintosh

This thread was discussed between 27/07/2004 and 09/08/2004

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