|The MGA With An Attitude
SMOKING ENGINE Implications - BE-104A
People sometimes worry too much about a little smoke from an engine. Smoke is not good, but most times a little smoke is not detrimental to operation of the engine and not immediately terminal in nature. If the smoking is not particularly bad, and it does not irritate you or your neighbors too much, then you might continue to drive it that way until you figure there may be something more significant wrong with it. But it does merit checking to determine why it smokes.
Up to a quart of oil in 200 miles can be burned in place of a little fuel and maybe no one would notice any smoke. An engine can be excessively worn in all respects and even smoke like crazy, but still run fairly well. When cranking speed compression test is under 100 psi it can be hard to start in cold weather, but may still have reasonably decent power. When compression is under 75 it can be hard to start a cool engine even in warm weather, and it may miss on a couple of cylinders for a minute until it warms up a little, then run well but with noticeably less power. With compression of 60 on all cylinders it might still run decent but with power low enough it may not want to do 60 mph into a head wind. With compression of 30 on all cylinders it may still run, after a push start, but you would be hard pressed to get it up a small grade on the driveway into the garage.
On a cool humid day with relative humidity near 100% any engine might emit a little light white vapor from the tail pipe. That would be water vapor which is a natural byproduct of combustion of motor fuel. Nothing to worry about there.
Copious clouds of white vapor from the tail pipe could be water getting into a cylinder from a blown head gasket (or more rarely a cracked cylinder head or cracked cylinder wall). The same heavy white vapor with a sweet smell would be antifreeze in the coolant. When there is a leak between combustion chamber and water jacket then combustion gasses can also blow into the water jacket to pressurize the cooling system and blow water past the pressure cap, thereby causing low coolant level and overheating. If you switch off the engine when it has pressure in the cooling system, then the coolant may run into a cylinder in large quantity. The next time you try to start the engine it may have hydraulic lock due to water in the cylinder, so it may not turn over to start. This condition can often be fixed by replacing the head gasket, but you should do a thorough inspection of the cylinder head and gasket seal surfaces while it is apart.
On first startup of a newly rebuilt engine where pistons and rings have bee coated with oil you may expect a LOT of pure white smoke out the tail pipe until it warms up and finishes blowing out and burning off the excess oil in the cylinders. This is perfectly normal and should subside to a minimal little white puffing within a few minutes. You might think about forewarning your neighbors so they don't panic and call the fire department thinking your garage might be on fire. Puffing of the minimal white smoke should cease within the first few hours of driving as the honed cylinder walls smooth out and the new piston rings seat for good sealing.
A little white or light gray smoke with an older engine after warm up is from oil getting past worn valve guides. You might notice it when looking at the tail pipe while it is idling. You may not notice this very much when driving at a steady pace. When you let the accelerator pedal up for coasting down or during shifting, then reapply the throttle, you may see a large puff of white smoke out the back. This is caused by vacuum in the intake tract sucking oil down past worn valve guided into the cylinders, and the oil being burned immediately on reapplication of throttle. Badly worn valve guides can cause oil consumption as high as one quart of oil in 50 miles of driving. When it gets that bad it will be constantly blowing oil out the tail pipe as a fine mist of oil droplets that may collect in the windscreen of a car following within a few hundred feet behind. The only cure for this is to replace the worn valve guides. As an interim fix you could try installing umbrella seals on the valve guides, especially for the intake valves. That might reduce oil consumption enough to dissuade you from overhauling the engine for a while longer.
Worn piston rings can cause oil consumption in two ways. Oddly enough, you can have high oil consumption caused by bad rings and still have fairly good compression test. If the engine ingests a lot of dirt for extended time, like driving on a lot of gravel roads, it can wear out the top piston rings. This might increase oil consumption dramatically with only a small compression loss. Oil wiper rings being badly worn or stuck from being excessively dirty can also cause high oil consumption. The consumption here is from oil being drawn up the cylinder wall past the rings and into the combustion chamber to be burned. This can result in loss of oil up to one quart in 200 miles of normal driving, or one quart in 100 miles of driving with heavy throttle or high engine speed. Smoking out the tail pipe may be similar to that caused by worn valve guides, but you are less likely to notice the big puff on throttle after over run.
When all piston rings or cylinder walls become heavily worn the engine can lose compression rather dramatically, dropping well below 100 psi for cranking speed compression test. In this condition combustion gasses can blow past the rings into the crankcase in a big way pressurizing the crankcase. When the rings were bad enough to cause pressure that low, it would be blowing lots of gray smoke out of the crankcase breather pipe and oiling up the air cleaner(s) in a matter of minutes. Oil consumption from this condition can be almost unlimited, commonly one quart in less then 100 miles, sometimes as little as 20 miles. Smoke under the car from the crankcase breather might be bad enough for a cop to pull you over for a sever warning.
The only cure for high oil consumption from worn piston rings it to replace the rings. If the rings were worn quickly by ingested dirt you might get away with honing the cylinders and installing new rings. If the engine is worn out from high mileage it will more likely require a rebore and a new set of oversized pistons.
500 miles per quart of oil is border line normal for an MGA, nothing to worry too much about, as oil is still cheaper than internal engine work. 700-800 miles per quart is honorable mention, and 1000 is very good. 1200 miles per quart is very unusual, and maybe only with a new engine just run in. Anyone who claims to go 3000 miles between oil changes with stock MGA engine without adding oil is probably mistaken (or lying).
Blue smoke from the tail pipe may be from too much fuel, which is corrected with carburetor adjustment. However, blue smoke from excess fuel will be only a small amount and it is rarely seen unless you are looking directly at the tail pipe. A small overly rich adjustment of the carburetor(s) does not cause blue smoke. To have that much excess fuel in the engine you would have to encounter an overflowing float chamber or a main jet stuck full down after manual choking for cold start. Overly rich fuel mixture will cause bad running, usually manifested as a cyclical galloping of the engine at idle speed. This much excess fuel can wash oil off the cylinder walls and cause accelerated wear of the piston rings as well as diluting the oil in the crankcase.