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MG MGA - Breather / Oil smell

I wonder if someone will have experienced and found a good solution for that oil smell i get each time i keep the engine running above 3.500 rpm.

While driving , oil pressure and water temperature remain both at the right level (ressure 20 to 40 and temp 190° max).
Water level is staying Ok too so there is no overwarming. Compressions are Ok on all 4 cylinder , I have no smoke out of the exhaust,
BUT the oil consumtion is way too high. (2x normal)
I checked for oil leak and could not find any. So it looks like I am "cooking" that oil. I also checked the oil cooler , it is OK too.
My rocker cover was changed by the P.O. to a nice looking aluminium one but without any breather.
Also the nice looking chromed cap is not vented .
So shall I suspect that I am missing a good ventilation circuit into the crankcase ?
If you think this could be the reason for that oil consuming and smell, could you tell me what shall I do ?
marcel

Marcel

The original rocker box did have a breather with a rubber pipe running to one of the air filters. Any fumes being drawn into the engine and burnt.

You probably have a rocker cover off an MGB which took a different route to getting fumes back into the combustion chambers.

Have seen a number of MGA's with this incorrect cover - just because it looks nice, but is really a waste of money. It is mechanically wrong unless some other means of venting have been provided.

Any signs of fumes coming out of the crankcase breather?

Pete
Pete Tipping

Pete, thank you for your quick reply,
Thera are absolutely no fumes/smells at idle coming from the crankcase .
The rocker cover oil cap is tight and not vented. When opening that cap I can see there are fumes that obviously are not eliminated.
May be a vented oil cap could be the cure ?
As it is cheaper than reversing to the original vented rocker cover I ordered one of those vented oil cap and shall see how it works (or not) but I am not confident: it looks like the pressure at idle comes out of crankshaft which shoudl be the opposite if I understand well B. Gaylord's web page.
Marcel

Marcel,
The blowby from your pistons is building up pressure in your crankcase. The vented valve cover was to relieve this pressure. The crankcase pressure in your engine is probably forcing oil past the seals and gaskets. I would especially worry that it was forcing most of it out past the rear of the crankshaft as there is no seal there. This could lead to some problems with the clutch. I would reccomend venting the engine as it was originally intended.
Ed Bell

Oil is being blown out the downdraft tube that comes out of the side cover under the manifolds. Without the breather sucking fumes into the intake the normal blowby forces the venting out the downdraft tube at such a rate that oil goes with it. There is NO way an MGA block could be pressurised enough to push oil past any seal with a open 1/2 inch hole at the downdraft tube. Install the proper valve cover and be done with the problem. A vented cap is a waste of time and $$$
Randy
R J Brown


R.J., Thanks to your answer, I understand better the equilibrium of pressure between downdraft tube and breather. So the oil is pouring out of that tube, that is probably why I cannot see any oil leaks .
OK I am sure you are right about putting back the proper valve cover.
I still will try the vented oil cap prior to spending even more $$$ on the original cover plus breather plus front air filter aso... It's only a 4$ try.
I will let you know.
Marcel

Wow,

This sounds like my story, but I DO have an original valve cover. My engine was rebuilt, good comp, good oil pressure, runs about 190, etc. However, on my last 300 mile trip I used almost 2 qts of oil. The tailpipe is dry, plugs look good, no smoke. Now, I do have one thing that I did; I did remove the hose from the air filter to prevent it from filling them with oil. I wonder if the vaccum from the air filters would help vent some of that blowby and reduce the oil loss as Randy mentioned???

Hmmmm

Tom
Tom Baker

So, Tom you think we need the depression from the carburetor to extract the fumes from the crankshaft, but you did experienced a flow of oil in the air filter if i understand well... so the carburetor was not extracting that oil to be burned in the engine.

On the other hand, I would also like to understand better why when there is a depression in the rocker cover , there is less oil extraction in the downdraft tube.

Not to mention that according to web forums you may have a positive pressure un the rocker shaft at idle , but the pressure becomes negative (sucking air from the hose/air filter) at normal speed.

I am feeling a bit confused now.
marcel

Sort of related to this problem, I have always run my engine (last 8 years) with the tappet vent connected by a Y piece to the rocker vent and then to the front filter - a closed circuit, no oil loss. Works very well.

I will post pictures of my arrangement on my website later on today.

Steve
Steve Gyles

Picture of my breather arrangement under Hot Press at http://freespace.virgin.net/stephen.gyles/index.htm

Steve
Steve Gyles

Got the vented oil cap this morning, tried it . No change, that thing is useless. So carburetor suction must be crucial for creating sufficient depression.
So randy , the final word is yours !
Could you please explain whay why when there is a depression in the rocker cover , there is less oil extraction in the downdraft tube ?
Thanks

Marcel
marcel

Marcel
The vent tube on the original valve cover is at the front near the oil cap. The oil comes in to the head at the rear rocker tower and flows through the rocker shaft to each rocker arm from back to front. I think MG chose that location to minimize oil loss through the vent. The location of the downdraft tube is below the holes where oil drains from the head to lubricate the lifters. The same amount of air drawn through the top will bring less oil with it than through the side. Most modern cars have a baffle arrangement inside the valve covers to minimize oil loss further. Because MG didn't do that there is something that can be done to help. Find a small round metal brush that can be cut off and stuffed into the metal tube in the valve cover that the vent hose hooks on to. The brush will help limit the loss of oil without limiting the air flow to the carb.
To directly answer your question. Does your term depression in valve cover mean suction from the carb creates a vacuum inside the valve cover? If so the answer to you question is that when the air is drawn into the carb from the valve cover it keeps air and oil from being forced out the downdraft tube by stopping or possibly reversing the flow in the downdraft tube.
Any questions ? please ask if this is not clear.
Randy
R J Brown

Randy, its is more than very clear: it's... luminous !
Thank you -very- much .
And also thanks for the "trick" of the small metal brush: sure I will use it when re-installing the original rocker cover.

Marcel
Marcel

Marcel

I feel some of the explanations you have received don't quite add up.

As I see it there would be very little if any actual suction from the air filter. So that when a pipe is correctly connected between the air filter and the standard rocker cover, any pressure would simply be relieved through that pipe with any oil vapour carried along with it.

There would certainly not be a 'depression' within the rocker cover. How could a filter housing full of holes actually provide any suction? The filter would have to be blocked to do this!

Coming back to the hot oil smell and high oil consumption you mentioned, it is quite possible, even with the correct 'rocker' breather pipe, that you could still get this smell, if either your valve guides and/or pistons are badly worn and letting combustion gases into the crankcase which pass out through the vertical crankcase breather pipe.

It's generally reckoned with the B-series engine, that when you get down to a consumption of 100 miles/pint of oil, it's time for a rebore, etc. (Apologies for the English units, but am sure you can sort it)

Mine's down to about 150 mils/pint, with correct standard breathers and I've been told by following MG club members/friends(!) that mine stinks too!

Pete
Pete Tipping

Pete, I am being told that the compressions are OK for all 4 cylinders.
So the 4 pistons should be oK.
Would I have good compressions with bad valve guides ?

Marcel
marcel

Marcel

Yes...
Good compression means your bores, pistons, rings and valve seats are good.

It is possible the valve guides could be worn with the valves actually seating OK. In this case you could draw oil down the guides from the rocker box on over-run (vacuum in induction system), this oil gets into the cylinders and blows out as burnt oil once you accelarate again.

There you have, smoke and a smell - should clear once at a steady speed...

When applying throttle you could also get gasses passing up the valve guides which should be able to pass through the rocker box vent pipe...

Back to that again...

Pete
Pete Tipping

Oil getting past worn valve guides usually results in blue smoke, most noticable at start up. If your car smokes on start up, but then appears to clear up, it is probably worn valve guides. When the engine is off, oil can drain down the valve stems and collect in the intake chambers of the head. This is quickly flushed out by the fuel mixture when the engine is started, resulting in a cloud of blue smoke.
Ed Bell

Have heavy oil consumption with pressure in the crankcase blowing fumes out of the vent tube with some oil drips onto exhaust. Also pressure in the rocker cover although this is vented correctly to the first air cleaner. Negligible drips from rear of engine into clutch bellhousing. Have read a lot about blowby from worn pistons and valve guides but I have 175 psi compression on all cylinders and no smoke from the exhaust tailpipe. Any further ideas?
H L Davy

Draft pipe hanging down from the front tappet cover serves as a venture, drawing a slight vacuum in the crankcase when driving forward. This slight depression inside the engine draws air from the front air cleaner into the valve cover. This provides crankcase ventilation to prevent build up of water in the sump (as water is a byproduct of combustion).

If there is no crankcase through ventilation you will get scum in the valve cover that looks like dirty Mayonnaise (emulsified water in oil condensing in the valve cover).

When standing still the venturi does not work, in which case some oil vapor can move through the valve cover vent into the air cleaner, so you can get a bit of oil in the air filter (normal).

As there are two open vents, tappet cover and valve cover, it is not possible to have any significant pressure in the crankcase. If you have pressure the vents are clogged.

If you have bad rings (not properly sealing) then you can have excessive blow-by forcing a bit of oil vapor out the vents. This would be most noticeable when standing still, when some vapor may be pushed into the air cleaner.

It is possible to have good compression readings and still have significant blow-by. I had this condition after my Alaska trip, having driven 5000 miles of gravel roads with standard air filters (bug screens). The top compression ring was badly worn, but still had good compression numbers, and it was using a quart of oil in 150 miles.

You can also have excessive consumption if teh oil control rings are bad, and have good compression readings. This can use a lot of oil with not much blow by.

barneymg

As Barney says, the important point is that the suction effect is actually created by airflow passing over the draft pipe that hangs down from the tappet cover beneath the car. There is no suction effect from the carbs.

So air is drawn in through the pipe FROM the carburettor air filters which then flows through the valve rocker cover and then down through the crankcase to exit via the draft tube under the car.

I have run an MGB engine in my MGA for the last 7 years with a non vented alloy rocker cover and have no obvious issue with the engine ventilation apart from the usual drip from the oil scroll at the rear and a slight misting of oil on the top of the alloy cover that blows out of the small vent hole in the filler cap.

I am in the process of fixing up a crankcase ventilation vent in the rocker cover as I understand that the crankcase does need to be properly ventilated.

This hasn't been a straightforward process for me as my car runs with ram pipes and foam sock type air filters and so there is no air filter housing to connect the rubber pipe into. So I have just bought a K & N crankcase filter and all I have to do now is just decide where is best to fit it.

Back to your car Marcel, if you want to check to see if it is worn valve guides that is causing the increased oil consumption, you could try the following.

Once your engine is up to normal temperature, find a long hill that you can drive down, one that has a straight section once you reach the bottom.
Drive down it with your foot off the accelerator with the car in either 4th or 3rd gear.(ideally 4th)

The engine is then being turned by the momentum of the car and is creating a suction effect.

Then when you reach the bottom of the hill, floor the accelerator and watch to see if there is any significant blue smoke coming from the exhaust. (you may need someone to follow you to check this)

If the valve guides are worn, this suction effect of the engine spinning with the throttle closed will draw a significant amount of oil into the combustion chamber and this is what burns off to cause the blue smoke.

If it is the piston rings that are worn the you will tend to get blue oil smoke each time you open the throttle wide.

Colyn
c firth

This thread was discussed between 14/11/2005 and 25/08/2015

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