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Old 11-22-2009, 02:22 PM   #1 (permalink)
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Default Too much oil = piston slap?

Hello all - I have a 2004 Forester with 51K miles. About 5 months ago I was changing the oil in my car which I have done about half the time since I bought it 5 years ago. I accidentally drained the AT fluid, but then filled the oil up with 4 quarts. So the engine had 4 extra quarts in it. Don't know what I was thinking (obviously I wasn't). I started it up and didn't notice anything wrong, but as I pulled around the block I noticed the smoke coming out the exhaust pipe and some slipping in the transmission. I turned around right away and only traveled less than a mile. I immediately realized what I had done, so I drained the oil completely and refilled to the proper level, and I filled the AT back to the right level as well. After I ran it for a couple of miles the smoke went away and everything seemed to be fine.

A few months later I noticed that the engine was louder and it has a slight knocking or slapping sound upon start up, but it usually goes away after the car is warm (15 min). I took the car to the dealer and they told me it is a piston problem and would cost $2800 to fix. My warranty ran out in March of this year (based on years not miles). The mechanic did say that it should be repaired soon, but per many forums it sounds like piston slap is common in Subarus. I also have noticed that the car does not have the acceleration that it used to.

I have two questions for the forum:

1) Could the piston slap have been cause by my blunder of putting too much oil in the engine? I called around to several dealers and they have all said probably not.

2) Should I do something about the piston noise, i.e. try to get the dealer to honor the warranty since I am only at 51K miles? Should I bring it in for an early 60K standard tune up?

Any advice would be greatly appreciated. KH
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Old 11-22-2009, 03:22 PM   #2 (permalink)
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1) Could the piston slap have been cause by my blunder of putting too much oil in the engine? I called around to several dealers and they have all said probably not.
I tend to agree. The extra oil would be more pron to blow seals(crank/cam/valve) or dump oil into the intake via the PCV/breather.

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2) Should I do something about the piston noise, i.e. try to get the dealer to honor the warranty since I am only at 51K miles? Should I bring it in for an early 60K standard tune up?

Any advice would be greatly appreciated. KH
As one who has built a fair number of engines, I say yes. Piston slap is caused by an excessive piston to cylinder wall clearance. It typically will not be fatal but it is not right either. Subaru as you have found out had a big issue several years ago with piston slap, their fix was simply to put a .06mm over sized piston in that stock bore.

Depending on what you have to work with it could be done with just those over sized pistons or you may be looking at boring the cylinders .010"/.020" over and using matching pistons.

For me it would be some thing I would have to fix, but I have the ability/tools to do it on my own eliminating about $2000 +/-300 in that cost. At $2800 I would have a hard time doing it, but it would eat me up until I did.

Welcome fellow MN member, check out MNSubaru.com
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Old 11-22-2009, 03:25 PM   #3 (permalink)
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This is a GM related site about piston slap, however much of the info it has still applies. Worth a read.
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Old 11-23-2009, 01:55 AM   #4 (permalink)
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I thought one of the symptoms of piston slap was excessive use of engine oil (i.e. it being able to get passed the seals due to slimmer pistons etc etc)???
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Old 11-23-2009, 03:44 AM   #5 (permalink)
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I thought one of the symptoms of piston slap was excessive use of engine oil (i.e. it being able to get passed the seals due to slimmer pistons etc etc)???
Not sure about Subaru,but the GM engines affected by piston slap rarely exhibit abnormal oil consumption. The slap problem relates to the skirtless piston design used. My '02 Chevy 4.8 V8 has a nasty case of the "slap knock" at start-up which goes away after 10 to 15 seconds.

May be a more serious problem in a Subaru engine, as they are aluminum block vs. the GM V8 cast iron.
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Old 11-23-2009, 05:44 AM   #6 (permalink)
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May be a more serious problem in a Subaru engine, as they are aluminum block vs. the GM V8 cast iron.
Nah. We have an aluminum block, but cast iron cylinder bores.
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Old 11-23-2009, 09:15 AM   #7 (permalink)
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Nah. We have an aluminum block, but cast iron cylinder bores.
Ah-forgot about the cast iron liners
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