BACKYARD MECHANIC: Rattle a classic case of piston slap

Hey there, time traveller!
This article was published 08/03/2013 (4685 days ago), so information in it may no longer be current.

QUESTION: I purchased a new 2012 Nissan Altima 2.5 in spring and everything seemed normal until fall (approximately 5,000 km). At temperatures below freezing, the engine has a loud knocking/rattling noise (under load) for about five to 10 minutes and accelerating at r.p.m. from approximately 1,200 to 2,000 r.p.m. The noise will diminish very slowly after normal operating temperature and the CVT shifts to reduce r,p.m., but returns quicker at -20 or more degrees.

Synthetic oil was used in December, but with little difference. When the vehicle is plugged in, the noise is almost gone. Other owners of 2.5s I talked to do not seem to have these noises. The dealer says it is “a normal characteristic of the 2.5 engine,” but does not or will not give a good answer as to what is knocking. I do not agree. Any ideas are greatly appreciated.

ANSWER: The engine noise you are describing is a classic description of piston slap. The knocking sound is caused by the piston skirt hitting against the side of the cylinder. Before you panic and see dollar signs floating in the air, this sound is often heard in modern engines.

Back in the 1960s through 1980s, piston slap or knock was a sign of worn cylinders or cracked piston skirts. The excess clearance between the piston skirt and the cylinder wall would allow the piston to bang against the cylinder wall every time the cylinder fired. Starting in the early 1990s, reducing engine friction became a goal of many auto companies so that fuel economy could be improved. One of the ways of reducing friction was to reduce the area of the piston sliding against the cylinder.

Instead of long, full skirts on pistons to guide their movement up and down, the pistons now had short, narrow skirts. One of the drawbacks, however, was that the piston could rock in the cylinder easier and knock against the cylinder wall.

Engine piston knock will always be loudest when the engine is cold and under load. As the engine warms up, the piston expands, reducing the clearance between the cylinder and piston skirt.

Higher engine load places more force on the piston skirt to create a louder knock. Usually, the knock will disappear after the engine has been running for a couple minutes and the piston has expanded. However, some may knock for up to five minutes.

If the knock is present when the engine is warm, then there is excessive piston clearance or a damaged piston. Piston replacement is required. If there is no knock after the engine warms up, it should be fine. Drive with a light load until the engine is warm, which is recommended for all engines.

QUESTION: I drive a 1998 Chevrolet pickup with a 350 V-8 engine. I overfilled my automatic transmission by approximately 1/2 litre. Will this cause problems?

ANSWER: The oil level inside the transmission is now high enough that rotating parts can touch the oil. This beats it up, causing it to foam. The foam can’t go through the pump properly, so erratic shifting, slipping clutches and a lack of transmission lubrication can occur. I would lower the oil level.

There is no drain plug on the transmission oil pan, so some shops remove the pan to drain oil, but it is possible to suck oil out of the dipstick tube with a pump. I have seen them for sale in automotive parts retailers and department stores. Another relatively easy way to lower the oil level is to remove one of the cooler lines and operate the engine for a few seconds. Catch the oil that comes out so you know when you have enough out. Don’t operate the engine for more than a minute, because the cooler lines are the supply for the lubrication circuits inside the transmission.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net