Engine oil consumption is a problem nobody wants. Most new engines today use less than half a quart of oil in 3,000 miles. Some use almost no oil. But as the miles accumulate, wear and oil consumption naturally go up.
Using a quart of oil every 1,000 miles is not unusual for a high mileage engine. The amount of oil used is still acceptable, but by the time it reaches a quart of more in 500 miles it's using a LOT of oil. Blue smoke in the exhaust is a classic sign that an engine is burning too much oil.
Should you be concerned? It depends on your budget, the value of your vehicle, if you can afford to rebuild or replace the engine, and whether the oil consumption is causing other problems.
An engine that burns oil will usually foul the spark plugs. That, in turn will cause ignition misfire, higher emissions and likely damage the catalytic converter. Also, an engine that is burning oil usually won't pass an emissions test because of elevated hydrocarbon (HC) emissions.
If an engine is using oil because of leaks (valve cover gaskets, oil pan gasket, crankshaft end seals, etc.), the problem can be fixed by simply replacing the leaky gaskets.
WHAT CAUSES EXCESSIVE OIL CONSUMPTION
Oil consumption depends primarily on two things: the valve guides and piston rings. If the valve guides are worn, or if there's too much clearance between the valve stems and guides, or if the valve guide seals are worn, cracked, missing, broken or improperly installed, the engine will suck oil down the guides and into the cylinders. The engine may still have good compression, but will use a lot of oil.
Worn valve guides can usually be restored a number of different ways. One popular method machine shops use is to ream out the guides and install thin bronze or cast iron guide liners. Knurling is another procedure that can reduce valve guide clearances. With aluminum heads, the original guides can be driven out and replaced with new ones. With cast iron heads, the guides can be reamed out to accept new valves with oversized stems.
If the oil burning is due to worn or broken rings, or wear in the cylinders, the engine will have low compression. The only cure here is to bore or hone the cylinders and replace the worn or broken piston rings
Oil burning can also occur if the cylinders in a newly rebuilt engine are not honed properly (too rough or too smooth), or if the rings are installed upside down, twisted onto the pistons, or the end gaps are too large or are not staggered to reduce blowby.
HOW TO REDUCE OIL CONSUMPTION
There are no "miracle" engine treatments or pills that will stop oil burning. But some crankcase additives can slow oil burning. There are also "high mileage" motor oils that are specially formulated with extra additives to slow oil consumption. Switching to a slightly higher viscosity motor oil (say changing from a 5W-30 to a 10W-30 or a 10W-40) may also help reduce oil consumption.
If an engine is using oil because of a leak, the leak must be fixed to stop the loss of oil. Valve cover, timing cover and oil pan gaskets are usually not too difficult to replace, but leaky crankshaft end seals can require a lot of disassembly (particularly the rear main crankshaft oil seal). One alternative to replacing a leaky gasket or seal is to add some "seal conditioner" to the crankcase, or to switch to a "high mileage" motor oil that contains additional seal conditions. The additives soak into the seals and gaskets, causing them to swell slightly. Hopefully, this will slow or seal the leak.
If the engine is using oil because of worn valve guides or valve guide seals, it is possible to replace just the valve guide seals without having to remove the cylinder heads or overhaul the engine. New valve guide seals can drastically reduce oil consumption. I have seen engines go from using a quart of oil every 500 miles to using no oil between oil changes (3000 miles)!
Replacing the valve stem seals requires a special valve spring compressor to disassemble the valve springs on each cylinder (one at a time). Remove the valve cover and all of the spark plugs. The piston in the first cylinder must then be placed at top dead center. This can be done by rotating the engine with a wrench on the crankshaft pulley until the timing marks line up. If the engine has no timing marks, insert a plastic straw into the cylinder through the spark plug hole so you can feel the piston as it approaches top dead center.
The cylinder must then be pressurized with compressed air through the spark plug hole to prevent the valves from dropping down into the cylinder when the valve springs and retainers are removed. Another trick for holding the valves in place is to snake a piece of rope or rubber tubing into the combustion chamber through the spark plug hole when the piston is at top dead center. The rope will fill the void between the piston and valves to hold the valves in place while you change the seals.
Be careful, because if a valve accidentally drops down into the cylinder, the cylinder head will have to come off the engine.