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MG Midget and Sprite Technical - Burning a Quart of Oil per tank of petrol

on startup, or pulling away from a light while the car is still warming in the morning I see large billowing clouds of smoke out the tailpipe and the distinct smell of burnt oil. Temperature and oil pressure gauges read normal. Engine had new rings and big end bearings installed about 8,000 miles ago. This is not a new problem. Its been steadily getting to this point for the past 4 years. When the bores were measured they were out of round just barely below the tolerancs listed in the haynes manual. I suspect piston rings as the phenominn is much worse when the oil level is closer to the high mark on the dipstick. Time for a rebuild?
S.A. Jones

Inlet valve stem seals?

Nick

Whilst being no expert I had a similar situation on my hillclimb midget, when the gudgeon pins shifted and grooved the bores. While I'm not suggesting for one minute this has happened on yours, one of the discussions we had with another racer suggested that the bores might be glazed, which with the ovality in the bores might suck up a good bit of oil. I would have thought though if the rings were that bad a fit that you would get crankcase compression and oil pi**ing out of every orifice?

Cheers
Neil
Neil Williams

Well there its a lot of oil all over the engine and I had degreased it when it was out several years ago. there was a bit of a tricle down the front and the rear of the oil pan too. and yes I did isntall new inlet valve stem seals at the same time as replacing the rings and big end bearings. before I replaced the seals the spark plugs would foul up with oil and turn black. After I replaced the seals all I get on the plugs are nice tan to whitish deposits. Perhaps it is leaking into the exhaust valve guide (which has no seal). i have new teflon seals and enough for all 8 valves but I don't really have the means to replace the seals without pulling the head and I would rather like to avoid that untill I do the full rebuild or replace the head with something nicer.
S.A. Jones

unlikely to be an exhaust valve stem leak.

If you know your inlets are ok, then it probably something else. Rings are a possibility, but a rear crank leak is almost a certainty then.
Nick

Is the crankcase ventilation connected and working properly?

I had much the same problem with excessive oil consumption when we bought the 1380 engine with around 10,000 miles on it. No oil clouds, but used around 1 litre (~2 pints) per tankful, with no obvious real leaks. Eventually stripped the engine and had the bores re-honed (sizes all good) to remove the glaze, fitted new rings, shells etc., and 15,000 later the consumption is now around 1 litre per 1,000 miles.

The previous owner of the engine, who had had it built, told me that he had 'always' run it on full synthetic oil, so it had probably never 'run-in'. After the re-hone, used a basic 20w/50 for 500 miles or so, and then back to full synthetic, which seems to have worked well.
Richard Wale

This is quite a well debated subject. My newly rebuilt 1275 motor was doing this, albeit collecting in a catch tank. The reason in the end seemed to be that the carbs were sucking air through the engine..nothing to do with the bores/pistons! The dipstick hole was not sealed..so I sealed that up (machined up a 'bung' that the dipstick sits in but is a tight fit), replaced the oil filler cap and put a restricter in the breather pipe to the carbs and it seems to have worked.
Robin Cohen

"Is the crankcase ventilation connected and working properly?" As far as i know it is. The oil cap is not vented but doesnt really seem to seal properly anymore and there is a hose going to the carbon canister out of the top of the valve covert. One of the three tubes in the top of the canister is broken off instead of sealed up (as per factory installation). It has the 12CJ smog equipment installed and also a hose coming from the front of the timing chain cover and into a y and then into each of the carburettors.

Once she is warmed up and just cruising along you dont see any oil clouds but you still burn i qrt per tank. Its only on startup, and when the engine is cold and accelerating from idle that things get cloudy.

If the dipstick has anything to do with it, I should note that when I had the oil pan off I noticed the broken remains of the dipstick tube at the bottom of the pan. It was originally plastic, the engine is the original engine and as far as I know, I am the first person in its 38 year history to have opened up the engine. I bought a replacement but havent installed it yet. I dont know if this is important, but the oil pressure relief valve rattles sometimes at idle.
S.A. Jones

With a broken dipstick tube are you certain that you are not putting too much oil in it. when it goes down one guart after one tank try running it a half tank further and see if it continues to use as much oil.
To me it sounds like there is too much oil and it is seeking it's level.
Noisy relief valve? Have you put a stethoscope on it to actually hear it rattling? I do not see how it can rattle.
Sandy
Sandy Sanders

I can actually hear a rattle from that direction when I am sitting in the car. The Barney Gaylord of mgaguru.com heard it too (he lives like an hour away from me) and told me that it was teh releaf valve and that I should lap that valve when I get the chance.

As for teh dipstick tube. There is a brass one comming out the top and a plastic one that gois into the oil pan. While it is possible that it is trying to find a level, it does continue to burn oil this way after it goes down past the low end of stick, by which point I am having oil starvation problems when I gently slow to a stop. This is because the pick up tube momentarilly is out of the oil and sucks a little air. Honestly i am not sure what the inner dipstick tube does. it might seal things up better and prevent oil from splashing out the top of the tube but I haven't noticed this happening.
S.A. Jones

I have similar problems with my trials midget - ie clouds of smoke at start up and and also when descending steep hills. I have put it down to the possibility of two reasons;
a) Oil from the carburettor dash pots seeping into the carburettors when not in use; and/or
b) oil being sucked in from the front timing chain breather - this is based on the assumption that when descending hills the oil flows to the front of the engine.

My reasons for both of the above is that the smoke dissapears when warm.

I also have a high consumption but believe this is mainly from the rear "half moon" gasket. However it may be due to glazed bores as the car was "run in" - or perhaps not on semi synthetic oil.
colin weeks

... and possibly oil being sucked in through worn valve-guides, or past worn rings.

A
Anthony

This thread was discussed between 22/05/2008 and 31/05/2008

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