MGB: Excessive oil use 1980 MGB

Hello, I am the new owner of a 1980 MGB so after 6 weeks I am still getting to know it and learning it's problems. The main issue at the moment is I am using a lot of oil, 500ml in about 25 miles. The engine is dripping a little, but only a little. The exhaust is clean on startup and doesn't leave an oil cloud sufficient to be vissible while driving except when pushed hard 2nd to 3rd change and occasional big clouds left at starts from junctions. With someone standing watching me drive away a blue cast is visible from the exhaust but not what you would call a cloud.

The car has 47,000 on the clock which we are sure is genuine. Has been run lightly during the summers and stored through the winters here in Wisconsin.

What I have done so far...

Compressions test; looks pretty good (cylinder 1)158, (2)165, (3)154, (4)169
book states 170 lb/in That doesn't mean that the oil rings are good though I suppose.

New spark plugs have carbon deposit after 50 miles, not oily though.

While replacing any loose fitting vacuum pipes I noticed that the breather from the front crankcase to the carb (Zenith) was pretty oily inside. Pulling this pipe from the carb you can see an obvious blue oil mist blowing from the pipe with the engine running. So I removed that pipe for a test up and down the road and the blue cast from the exhaust disappeared, not suprisingly.

It seems obvious that if I feed that breather to the carb then I will be burning oil, it's designed that way presumably. So do I assume that there is simply too much oil vapour coming through the breather, any ideas why? Or maybe I am completely on the wrong track. From a bottle left with the car it appears that the previous owner was running it on 5w/30 so I have changed it to 20w/50. The engine sounds much better on that. Incidentally there were no obvious knocks or rattles from the engine, it runs very nicely.

I looks forward to your thoughts,
Graham

Could be blow-by past the rings forcing it out. Could be the valve stem seals are shot - most likely, I'd think if the mileage is actually true. Is all the emissions stuff still in tact and working?

You're on the right track. My 80 never had a stock carburator, so I'm not too sure on the correct hook-up of the side vent cover exhaust. It certainly doesn't need manifold suction. When I had a Weber, the exhaust was plummed into the filter housing. It should not suck lots of oil if its working properly. Make sure your valve cover is vented, either a vented filler cap or a vent pipe to the canisters. Also, make sure you have not over-filled the motor oil. Maybe post some pictures of your engine bay and some of the stock 80 owners can see if theings are hooked up properly.

I would go to my local parts store and get some additive to free sticky rings and see if that helps. The compression seems good but you could still have a second ring sticking.
Doesn't cost much and might help.

Steve

I'm no expert, but I think a wet compression test, and a leak down test would tell you if it's rings. I'm sure the experts will chime in. You may want to do a search and see what comes up.

Late cars had a tendency to run hotter, sometimes MUCH hotter, than early cars. I won't go into the reasons but they exist.

What you most likely have is a situation where a period of high engine temp has caused the oil control rings to relax and not do their job. The compression rings will stand the heat better so they will still hold the compression pressures back well after the oil control rings have ducked out for an early lunch.

The fix: Replace all of the rings with Deve's after breaking up the glaze on the cylinder walls with a dingle-berry hone. That's the short version. For a longer version research past threads on this same subject. I suggest Deve's because of their superior oil control ring assemblies.

A check of the head while it is off would be in order too. Such heat MAY have affected the vale spring tension and/or the sealing of the stock guide seals. If in doubt, replace all of the springs, they are cheap, and install Chevy seals.

BTW: If you still have the converter in place you are much less likely to see any oil smoke from the exhaust simply because the converter is burning the stuff up before it makes an exit.

EDIT: As Matt points out, the crankcase breathing system is critical. Post some pictures of the driver's side of the engine and let us take a look.

Jack

I would strongly suspect that the engine breather system is hooked up incorrectly or is not functioning properly. There are lots of posts describing excessive oil consumption from incorrectly hooked up breather systems (including mine).
You need to determine exactly what parts of the breather system are still intact and functioning. Do you have a vented valve cover or a non vented aftermarket cover? If it is vented is it attached to the carbon canister?
With a Non vented cover and a non vented oil filler cap, you should run the hose from the side cover directly into the air filter. However with a vented oil filler cap that same arrangement will cause a lot of oil consumption.

Firstly welcome - you've arrived at a great place for help!

I'll second that I am no expert but try hitting the search with some key words and see what turns up. I believe I have read that you might also have too much pressure in the crank case and that's forcing the oil out. If you have all the smog and other stuff connected as well then maybe there's an issue with some other venting (valve cover) and or clogged charcoal cannisters?

Unfortaunetly I can only relate to what I have read/remebered said rather than *know* so hopefully some of the engine gurus will see this and post some ideas in due course.

I can confirm that on a stock setup (like my 77) that front crank breather hose is meant to go to a port on the firewall/back side of the carb.

Simon

Edit: I need to type faster :) You got 2 replies between me looking and typing but it seems I may not be out in left field :spinning:

Photos may help.

I'm leaning in Matt's direction and think that maybe you have too much suction on the crankcase. As Jack and Chris suggest, some pictures of the plumbing and parts that are there will be of great help to those of us trying to advise you. :) Welcome to this crazy bunch, it is the absolute best resource on the internet for MGs!

You may also consider doing a leak down test to see if there is excessive blow by on the rings.
Frank

Crikey, thanks for the welcome and excellent suggestions... I can't keep up! I agree photos will help so I will do that later today and post them this evening. Working through the Haynes manual all of the pipes do appear to be plumbed correctly with the exception that one of the two carbon canisters has been bypassed. The plumbing is correct though in the way that the one included canister is connected. That makes me wonder why was that one was disconnected.

Matt, The rocker cover is the version with a vent pipe towards the rear connected to the centre top pipe on the canister. The oil filler cap should be sealed but was a little loose so I have made an extra ring so that it locks down tighter.

Graham

Here are some photos which are rather busy but I hope clear enough.

...and another. This is the set up where the crankcase breather pipe goes onto the rear side of the carb itself.

Graham, see that hose that loops around from the front tappet cover breather and connects to the rear side of the carb? Disconnect it at the carb and blow through it. If it is fairly easy to get some pressure through you can rule out a stopped up side cover vent. Now go to the hose from the rocker cover and disconnect it at the canister and then blow through it back into the rocker cover. You should get some resistance there as the elbow has a very small orifice on its end as a restrictor. Now re-attach that hose and then detach it over at the rocker cover. Blow through it back toward the canister. The should be little or no resistance that way.

If all of these are clear the crankcase is venting well enough you have to go looking for other reasons for the consumption. It is pretty rare to see OEM guides go bad until LOTS of miles are on an engine, but the seals, which are marginal anyway, CAN be affected by heat and fail. See my previous post concerning them and the higher probability that the oil control rings are bad.

Jack

Who is John Galt?

twigworker Wrote:

Doesn't this model have a PCV? That controls the vacumm/pressure in the valve cover and venting. This could be bad given the age of the car.

Another thought which could impact things.

Do you know for sure you are running the correct engine oil...20w-50?

If not, this could be contributing to a small degree as well. Food for thought.

I'm not an expert, but I suspect you have a problem with venting....keep tracking....I bet you'll get her sorted out soon!

Nice looking car, atleast under the hood. You have found a good site for help!

Hello William... I don't see a PCV in the diagrams or on the car so probably not. The 20w/50 is the heaviest oil recommended in the drivers handbook. When I discovered it had been running on 5w/30 I wanted to get a much thicker oil in there thinking it just might throw around less. The engine does sound quieter on good old 20/50. Back in the 80's I remember that was the standard oil on the shelves.

Here is the rest of the car...

I'm thinking you have sufficient "blow-by" to pressurise the crankcase and blow the oil out the vent. It isn't oil rings - if it was you would still get blue smoke when you disconnect the pipe.

Could be all sorts of things - I have seen broken rings on an engine with fewer miles! Or it could be corrosion on the bores. A leak-down test may help identify the problem, but I suspect it is going to need a tear-down to solved - sorry! :(

Well over the weekend I ran the car with the vent pipe leading into an improvised collection bottle and the carb connection plugged. Having a friend watch carefully we saw no signs of oil smoke from the exhaust. Also the spark plugs we checked looked much better which all points to not burning oil in the mormal way I think.

So yes it will be a big strip down over the winter which was planned anyway so I'm not too :( about that Eric. I have a leaky rear seal to see to anyway. This has been very useful though because if the rings looked good I may well have put it back together and still had the same problem without this discussion. So I need to replace anything which could allow 'blowpast'... both up and down? Care to suggest a list?

Thanks for the great help,
Graham