Oil Consumption
This weekly feature for Warranty Direct about car repair and maintenance is written by Tom Torbjornsen, the popular host of America’s Car Show on satellite radio. Below are some recent Q&A from the America’s Car Show email bag.
Dear Tom,
I own a ’94 Mitsubishi 3000GT-SL with 121,000 miles. At about 95 – 100,000 miles it started burning oil (no leaks obvious) with blue smoke in the exhaust on startup. In the last couple of months the oil consumption is rapidly increasing. On “general principles” I replaced the PCV valve recently to no avail. On a bulletin board for these cars, there has been talk of valve oil seal problems. Could the seals get to the point where they fail completely and oil consumption take a sudden leap?
Spencer from Australia (Down Under)
Spencer,
Yes, seals could harden and break up, causing oil consumption to accelerate greatly as oil spills down the valve stems and into the combustion chambers to be burned. Pull a valve cover and inspect the valve seals. It’s time to delve deeper into engine diagnostics with cylinder leak-down and compression testing.
Good luck, mate!
Tom
Spencer asks a question that many vehicles experience, namely oil consumption at high mileage. What are the causes? What diagnostic tests should be performed?
Causes of oil consumption
Bad valve seals: The valves are located in the cylinder head above the combustion chamber. Oil is pumped at 50 to 80 psi of pressure into the top of the head, lubricating the valve-train. The valves have seals that stop the flow of oil down into the engine when the valve is open. If the seals fail, oil flows down into the combustion chamber and is burned.
Worn valve guides: The valves are guided by a small cylindrical chamber called a valve guide. These guides wear over time causing eccentricity (slop). The excess gap allows oil to flow down the valve stem and into the combustion chamber to be burned. You might be wondering why the valve seals don’t stop the oil. Because the gap is too large for the seal to work.
Pressurized crankcase due to clogged PCV or breather system: Your car’s engine is a giant pump, therefore it must breathe. The PCV (Positive Crankcase Ventilation) system does just this; it allows the engine to exhaust the excess crankcase pressure that builds up (a natural phenomenon of the internal combustion engine). These gases are captured and fed back into the engine to be burned. Carbon (a by-product of engines) can build up in the PCV system, clogging the breathing passages. This, in turn, pressurizes the oil pan and pushes oil up into the fuel delivery system, where it is fed into the engine and burned.
Blow-by from worn piston rings: The pistons in your car’s engine have seals around them in the form of rings. These rings have two functions: (1) They seal the combustion chamber so that the precious power produced from the firing of the cylinder is not lost. (2) They provide vital lubrication to the cylinder walls. When the rings wear out the pressure from combustion reverses down into the oil pan. Pressure in the oil pan forces oil into the valve covers, through the breather system, back into the fuel delivery system, and into the engine to be burned.
Tracking down the cause of oil consumption
PCV system: Remove the PCV valve with the engine running. There should be a strong vacuum pulling on the valve. If there is no vacuum, the system is clogged with sludge and carbon. It should be cleaned and the valve replaced.
Valve stem seals and guides: Remove the valve cover and shine a strong light on each valve stem. If the seals are gone, then further inspection is warranted. Pressurize the cylinder and remove the valve spring to closely inspect for a worn valve guide evidenced by broken-up valve stem seals).
If nothing is found after checking for a clogged PCV system and valve stem seals & guides, then run a cylinder leak-down test. In this test, each cylinder is charged with 120 lbs of compressed air and the engine is monitored to determine where the air is escaping from the cylinder. If air is escaping from the crankcase, then the piston rings are worn. You can also perform a compression test. Remove the spark plugs and test each cylinder individually for the total PSI of compression. If you find that one cylinder is low, then wet test it. To wet test the cylinder, remove the gauge, squirt oil into the cylinder, and then retest it. If the compression in that cylinder comes up, then that cylinder has worn piston rings. How do we know this? Because when oil was squirted into the cylinder, it filled the gap between the worn rings and cylinder wall, sealing the ring gap and thus increasing compression. In this is the case, consider rebuilding or replacing the engine.
‘‘Til next time…Keep Rollin’
For more articles by Tom Torbjornsen, visit AMERICA’S CAR SHOW web site: www.americascarshow.com
Tom Torbjornsen is an automotive expert of 37 years. An automotive journalist in good standing with the IMPA (International Motor Press Association), Torbjornsen is the Repair and Maintenance Editor for AOL Autos, Edmunds.Com, At Home Portals and many other websites. Torbjornsen is the auto expert of record for Channel 4 WIVB TV, Western New York’s CBS affiliate. Hear his radio show AMERICA’S CAR SHOW, on the Sirius XM Satellite Radio Channel – Stars Too – Sirius 108 and XM 139; Sunday nights – 10PM – Mid EST. Send your car questions to his website at: www.americascarshow.com
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Filed under: Americas Car Show, Driving Tips, Maintenance, Vehicle Maintenance, Vehicle reliability, Vehicle repairs
Nice brief and this post helped me alot in my college assignement. Say thank you you on your information.
Checking and changing the oil is a very simple way to insure that you understand the vehicles performance. If not done on a regular basis you might have a great big surprise when you pull the dip stick or you hear the lifters tapping away.