Niagara Gazette

October 4, 2013

TOM'S CORNER: Touching on oil consumption and other questions

By Tom Torbjornsen
Niagara Gazette

Niagara Gazette — SPENCER FROM AUSTRALIA: I own a 2008 Ford Mustang with 91,000 miles. At about 85,000 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?

TOM: 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. If the valve seals are intact and ok, then it’s time to delve deeper into engine diagnostics with a cylinder leak-down and compression test.

Good luck, mate! (I wonder, do you knows the Geico lizard?)

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. It’s 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, then through the breather system, then 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 guides.

• In-depth testing: If nothing is found after checking for a clogged PCV system and valve stem seals & guides, then run a cylinder leak-down test followed by a compression test. These two tests will determine where the wayward oil is going.

•••

IRENE FROM BROOKLYN: My car calls for regular gasoline, but I want to use premium grade. Will this harm the engine? I’ve heard you can’t do it the other way around. However, I always thought using premium gasoline would help my car run better, even though the manual says to use regular. Is this true?

TOM: Fuel octane has nothing to do with the quality of gasoline or how the car will perform. It has everything to do with how the gas acts inside the engine. If your carmaker suggests you use regular octane fuel (87) then use it. Using a higher octane will NOT make your car run better.

Let’s look into octane ratings deeper:

What is octane? Fuel octane is a measurement of a fuel’s volatility factor in the combustion chamber. In simpler terms, octane is a measurement of how the fuel combusts in the hot combustion chamber environment.

Can I use regular gas (low octane) in my high-performance car? Different octane gas was developed to accommodate different grades of performance engines. The higher the performance engine, the hotter the combustion chamber environment, and the higher the octane used. If low octane (regular) gas is introduced into a hot, high compression combustion chamber found in high performance vehicles, it pre-ignites, creating a condition called “engine knock.” Why? When low octane fuel enters the combustion chamber, it ignites prematurely. The result is the knocking you hear. Over time, continued engine knock results in damage to the tops of pistons, cylinder head faces, and valves.

Can I use regular gas in my high performance car in a pinch? Sometimes you go to fuel up and the gas station is simply out of high-octane fuel. So what do you do? Go ahead and use regular to get to where you’re going, but don’t make a practice of it. Remember, “continued use” of low octane fuel in a high performance engine will damage it over time. Built into the performance system of today’s engine is a device called a “Knock Sensor.” This sensor constantly measures combustion chamber pressure. When it senses excessive pressure from pre-ignition, it backs off on ignition timing through the ECM. This feedback system minimizes the effects of using low octane fuel. Note I said minimizes, not eliminates. Continued use of low octane fuel in a high performance engine will damage it over time.

Will using high-octane fuel in my car that calls for regular gas make it run better? NO. High-octane gas was developed for high performance engines that run higher compression ratios and thus hotter combustion chamber temps. It makes no difference whatsoever in an engine that runs on regular gas. Premium gasoline will not enhance performance or increase fuel mileage if your car’s engine does not require it. Use the grade of gasoline recommended by the manufacturer.

‘Til next time ... Keep Rollin’

"America's Car Show" with Tom Torbjornsen airs 7:30 p.m. Wednesday, 9 a.m. Thursday and 11 a.m. Saturday on WBBZ-TV.