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MG Midget and Sprite Technical - Smoking problem

A friend has a 1275 midget and has a strange smoking problem. This is what he emailed to me.

Cold engine, first thing in the morning, starts OK, no smoke. Warms up, and then as (I think) the thermostat opens, huge clouds of smoke on acceleration. This lasts for about 1 minute, then stops,and won't happen again for the whole day, however long you leave the engine off. The smoke looks blue, but smells like......well not like burning oil, but like that smell you get from old fashioned steam engines - a mix of steam and oil. I would suspect head gasket, but why only for that one minute????

Any thoughts?
Mike Howlett

I would suspect a cold water leak through the head gasket or a crack in a cooling passage...leaks while cold, but metal expands and seals leak when hot....not sure why it wouldn't smoke on intial cold start up.

I've seen radiators, but not head gaskets, develop cold water leaks through very small cracks...coolant on the floor overnight, but not while running...found only by pressure testing the cooling system COLD.

I would pull the spark plugs and spin the engine to see if any coolant comes out of the holes, then do a compression test.

Yes, this is indeed a puzzling one...all of the head gasket failures I've seen have just continued to leak once it started and continued to smoke!

Maybe someone else has ideas?
DL Rhine

We experienced similar symptoms with a car that had a head gasket problem. The clouds of smoke were from the antifreeze in the coolant, and had that sniff of coolant about them.
Paul Walbran

I too have exactly the same problem and have found that if you do not put your throttle down as much the cloud is smaller. If you also let it warm up on the driveway first no problem either.

I have put it down to worn bores as I have tried replacing the head with another one putting new valve stem seals on at the same time.

I have a feeling my issue is with cylinder 4 as I seem to have issues when trying trying to tune the carb on the rear cylinders as it does not make much difference when I adjust it at idle but when moving you can feel it kick in.

My coolant level never drops but I do have to top up the oil every so often.

If you find the issue I am all ears as to the solution however I am pulling the engine and putting a k in so will then have a look at the bores for a hint of the issue.

Shaun

Shaun

I wouldn't be surprised the crank ventilation is causing this. Try to start it cold with the vent disconnected
a.o. arnold

I had this problem some years back. It went away when I rebuilt the engine including re-bore but no change to cyl head.

I don't think it's anything to do with the thermostat opening.

In my case, it occured when throttle was applied after a period of deceleration. There must be an explanation - this is all I can think of...

- oil being pulled down the valve-guides on over-run and burned next time throttle opened: but the head wasn't changed and problem was cured

- oil being sucked into the inlet manifold on trailing throttle, and being burned next time throttle opened: but the oil is still cool and (I imagine) low level of fumes at this time

- the worn piston rings allow oil to be sucked by on trailing throttle, and the bores/oil are cool, so the rings find scraping the oil harder with oil being of low viscocity; oil burned next time throttle opened. This seems most likely.

A
Anthony Cutler

From the description it isn't oil burning, but steam from coolant although with some oil in it.

I think it is down to an incipient head gasket failure. When the engine is started from cold there is no problem. Only when the coolant temperature rises and it becomes pressurised is some water forced through a weak point in the gasket (though possibly through a cracked head or cylinder) This creates the cloud of steam. As the cooling system is now no longer pressurised it is a one-shot event. I would replace the HG and see if that sorts it.

Guy
Guy Weller

Well if there is steam, I'd try torquing down the head...

A
Anthony Cutler

This thread was discussed between 26/09/2009 and 30/09/2009

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