Piston Slap: Forester’s worrying clatter from low oil-pressure?


Carla writes:

My 2006 Subaru Forester clatters each morning when I start it. After a couple of minutes, the noise goes away and I don’t hear it again until the next morning when I start the car. It has 202,000+ miles on it.

I’ve read that it might be bad spark plugs (it is almost time to change them), or the tensioner on the timing belt (recently replaced with new kit) or a bad valve-cover gasket that seals itself after the engine heats up (changed at 174,000 miles along with the head gasket). I’ve read other ideas like cheap oil filter (I use Subaru oil filters) and a few other ones.

I’d like to hear if you have other thoughts.

Sajeev answers:

Engine clatter upon startup is usually more of an oil pressure or a timing-chain guide issue. The latter is clearly not relevant on your timing belt–equipped Forester, so I am focusing on oil pressure right after you twist the key. While an oil filter that lacks an anti-drain-back valve can reduce start-up oil pressure to the point of clatter, that isn’t applicable here, either. The other items you noted (plugs, valve cover gasket, belt tensioner) are highly unlikely to be an issue.

What we have here is the classic issue of old engines misbehaving like old engines normally do. Low oil pressure upon startup is sometimes just a cost of doing business at your mileage. High-mileage oil formulations aren’t likely to help with this noise.

Perhaps replacing the oil pump would fix the clatter, but a cheap part with expensive labor just doesn’t seem worth it at this stage in the engine’s life. Not to mention the fact that the oil pump may not be enough on a motor with this many miles under its belt: Only a full teardown and rebuild can determine that.

And well, it looks pretty easy if you watch this video:

If you love your Forester—more than any replacement you could afford—then you might be tempted to spend the cash to fix this issue. Except you must not, because you should wait until something worse happens under the hood. Doing so now is throwing money at a problem that doesn’t need a solution.

Bottom line: Be okay with that startup clatter, as it isn’t appreciably hurting anything. Worry about fixing it (with an engine rebuild or replacement) when the noise becomes more frequent. Or louder.

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    Might not hurt to pour in one of the additives on the market that would “cling” to the surfaces that are clattering until oil pressure come up. In the “old days” of my youth – often spent with worn-out engines in clapped out cheap jalopies – the choice was typically STP. This is absolutely not a “fix” – but more of a band-aid approach. But there are products that will “mask” the clatter somewhat while you – as Sajeev suggests – live with it.

    I tried that in my early wrenching days with one of my worn 20R Toyotas. The problem is that stuff thickens up the oil, which tends to delay getting good oil pressure and it accelerates the wear

    Subarus use short pistons as they have space constraints in the width of the engine. As such, the piston skirt is not very long and the pistons rock back and forth in the bore, especially when they are cold. All Subarus suffer from this and it is normal. Happens on both my OEM engine, and my rebuilt one. Not a consequence of low oil pressure. Low pressure would mean the rod bearings are knocking, which would result in a blown engine in no time.

    The previous poster should have written this article! The reason the noise goes away is because the piston expands to it’s correct size when warmed up, eliminating the “piston slap” noise. Nothing to do with oil pressure, which is highest at startup due to cold vicious oil being more difficult to pump.

    While it’s running, pull off a spark plug wire, one by one and see, which one is making noise

    I refer to that as the ‘death rattle’ and it is a sign of worn engine bearings. As some of the other posters have indicated, there are some other potential causes, but in my experience with high mileage engines it is fairly likely it is worn bearings. Good news is that my ’94 Blazer has been doing it for years and it gets hiway driven regularly without ejecting a piston out the side of the block. I took a ’79 Cadillac to 290000 miles with a death rattle before it finally gave up the ghost. If you have a trustworthy mechanic and are willing to spend some money, a rebuilt (reground) crank and bearings will straighten it right out

    Oil pump shaft seal? Don’t know if the following still applies but fwiw:
    Bitd, I had trouble with the oil pump in my Loyale. The shaft seal would leak, hydraulic lash adjusters would get aerated or drained, and they would make a racket on first start of the day.

    Carla wrote the noise goes away “after a couple minutes” which indicates piston slap (meaning she wrote to the right column!) but if she’d written “a couple moments” I’d be guessing oil pressure. If there’s an oil light or gauge and the noise goes away when the light goes out or gauge perks up that’s conclusive.

    The start up clatter will be due to low oil pressure within the hydraulic lash adjusters or failure of the seals within the lash adjusters or both.
    I would install new lash adjusters as this is the most likely cause. Failed seals in the lash adjusters allow the oil to drain from the lash adjusters overnight. New ones will made a huge difference even if the oil pressure is a bit low.

    I’m also in the piston slap camp. At 200K, you can bet on it. It will be slightly worse at 300K. I would replace the engine at 400K.

    Why I remember this…I couldn’t tell you. If its like the earlier cars, you’re hearing some valvetrain/ lifter noise on start up due to bad head studs. It makes little sense but you have 3 different expansion rates with steel studs, cast rockers, and aluminum°£° heads. They have shims that fit the cylinder jugs to adjust the depth of the combustion chamber/deck height after milling or when replacing head gaskets. I thought they would have fixed this by 2006.
    The bad news is that this often happens at around this mileage and is the first indication that the head gaskets are about to fail.

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