Piston Slap: More “Not Piston Slap” Questions Please
First, some business: I’m seeking more questions for this column, so I need you to email me (pistonslap@hagerty.com) with any car-related question you might have. Keep this series alive, send me a question…or three!
In the meantime, let’s answer a Piston Slap question about the mechanical problem with the same name. It appears Google serves up our friendly little Sunday column to people with questions about engine noises, so let’s get to Sue’s question about her Sue-baru (sorry):
Sue writes:
I purchased a new Subaru Forester in 2013, and I now have approximately 140K on the clock. I am on the third short block with new improved oil rings, switched from 0w20 to 5w20 synthetic oil, and still have to add two quarts between 5000 mile changes.
Upon setting overnight, I get a piston slap or light clicking upon startup which goes away upon warm-up. I am thinking the last new set of oil rings are broken in and piston skirting is worn down on all pistons causing the noise? As long as the mileage is not changing can you offer any advice or figure just keep driving until it dies? Thanks for any comments!
Sajeev answers:
In these cases, unfortunately the answer is almost always to “drive it until it dies.” That advice would change if you have a warranty on this engine, but we might have a good resolution for the oil consumption no matter what. So lets get right into it.
The piston slap noise on your FB25 engine upon startup could be either the pistons clattering or the timing chain guides rattling. Either way, tearing into a relatively new engine to address it is a pretty bad idea. You can determine if the noise is piston slap (or something worse) by reading a previous installment of our series, but that won’t change the ultimate recommendation of just driving it until it dies.
The oil consumption is another issue. Everything I see suggests you need to be running 5w-30 oil, not the 20-weight oil you mentioned. The higher weight was originally specified for turbocharged applications, but (famous last words) the internet suggests that even naturally aspirated Subarus run fine with it. The heavier weight is very likely to lower your oil consumption.
If it improves with 30-weight but isn’t eliminated, do a little research into 40-weight oils for the FB25 engine, because there’s anecdotal evidence suggesting it will perform fine and lower/eliminate oil consumption. It’s hard to come to a firm conclusion on your oil consumption issue, as opinions on the matter are about as nebulous as a vague recollection or a foggy memory.
If I were in your shoes, I would perform an oil change with 5w-30 synthetic oil immediately. Do this at a Subaru dealership where you can solicit the service manager/technician’s opinion at the same time. I have a very good feeling 5w-30 oils will lower consumption significantly, with no downsides to performance or durability.
Good luck on your next oil change! Now it’s time for readers of the Hagerty Community to chime in, and email me with their own questions!
Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com—give us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.
The oil issue is generally your rings as it was a very common issue on the Subaru cars of this era.
They actually made changes with updated fix it.
But 5w30 or 10w30 might help but don’t expect miracles.
The slap is likely the pistons. They can wear a bit and knock on cold start but the heat makes the pistons grow and it goes away. Many cars have this and generally it is not fatal.
Yes drive it till it dies or if you can afford to trade it. You may get another 100k miles even using oil. But always be prepared that it could let go at any time.
A number of engines in this time were fitted with low tension rings and often pistons that were not as tight to add more MPG. If they were a bit off oil and slap happen.
Just remember heavier weight oil will just mask the issue and it will not be a permanent fix.
Sometimes I wonder if the low tension rings you mentioned are actually better for the environment, considering the amount of oil they burn in some engine designs. Granted modern engines do get amazing fuel economy in today’s big cars with blocky faces, but burning oil can’t be good for the catalyst.
Sorry Sue but it does sound like death is at your doorstep. 2 quarts is 2 quarts too much between changes. Drive it until it dies, ignores that terrible feeling one has when your everyday is living on borrowed time. ‘Did I hear something?’ – ‘Is that smoke me?’ etc. So unless you’re in the ‘I don’t drive it all that much and I never go far ‘ category that stress is always on the back of your mind. Any moment it might. Because of that I’d be looking to find a reliable replacement a.s.a.p. Advice that is of no real help what so ever .
Going with heavier oil in any case is a hail mary move for a dying engine. While it tends to improve taps, knocks, and slaps, it also tends to result in longer ‘dry’ time after start, which tends to accelerate the wear. If you haven’t heard it enough by now… drive it till it dies then LS swap it
My advice is to sell it. I soured on Subaru’s with their oil consuming engines and other failures they started to have. My Dad’s 2009 Forester XT developed a quart a week or more habit that we sold it before it blew up.
Oil consumption isn’t always certain death. Especially with modern engines. Even the venerable Honda K24 and GM 5.3 L (before the cylinder deactivation) could burn oil, and slap, and last hundreds of thousands of miles. Mine did and both reached 300k. The Honda is still running, passes emissions testing and uses about a quart per 2k miles.
The real danger to a thirsty engine is not feeding it.
I like Sajeev’s advice and would implement it ASAP if I were you. Try a 5w-30. Track your mileage and oil consumption.