2.5 engine knock/rattle

[mrfeh]

Hi folks.

 

I've got a '99 Legacy Outback, and over the last few months, I've noticed a knocking/rattling noise after the car is started, until the engine warms up.

 

I've done some searching, and read about piston slap in the 2.5 engine. Sounds like what I've got.

 

I just wanted to verify that this noise is something that comes along with age; the engine has a little over 100K miles on it, and I've never heard this noise until the last few months.

 

The engine seems to run fine; it's just noisy at startup. I'd rather not take it in to a shop if it's not necessary.

 

Thanks!

[OB99W]

[...]I just wanted to verify that this noise is something that comes along with age; the engine has a little over 100K miles on it, and I've never heard this noise until the last few months.

It certainly could be piston slap, since the noise abates as the engine warms. However, you may find that engine oil initial viscosity, and the change in it as the oil "ages" (including any dilution with fuel) have an effect. Has the oil (and filter) been changed recently? Do you always use the same brand/type of oil and filter?

[NoEyeSquareGuy]

Fairly normal, though it certainly sounds concerning in person. I have the same piston slap in my '01 Forester(88k miles), which goes away after 2-5 minutes of runtime, depending on ambient temperature. It's level of severity varies with engine oil, I've found... it's at it's loudest now, with Mobil 1 5W-30, lightly thinned out with Marvel's Mystery oil. The lifters are at their loudest now as well. When using this combo in the future, I'll probably switch to a 10W-30 when mixing and matching. Sure runs strong this way, though.

[mrfeh]

Thanks for the replies.

 

I do always use the same filter, and changed the oil in April. The first 6 years I owned the car, I always used 10w-30, but over the last couple years I have been using 5w-30 in the winter.

 

I honestly don't know what viscosity is in there at the moment. It's possible I'm using 5w-30 during the summer for the first time.

 

First thing I'm gonna do is put in some 10w-30 to see if that makes a difference!

[jon38iowa]

Thanks for the replies.

 

I do always use the same filter, and changed the oil in April. The first 6 years I owned the car, I always used 10w-30, but over the last couple years I have been using 5w-30 in the winter.

 

I honestly don't know what viscosity is in there at the moment. It's possible I'm using 5w-30 during the summer for the first time.

 

First thing I'm gonna do is put in some 10w-30 to see if that makes a difference!

 

If it is piston slap, it might get substantially worse over time. At least mine('99 Forester) did, though it would always quiet down some after warm up. I tried it all! But, the only solution was a complete rebuild. My mechanic claimed the car would have gone a lot longer like this, however the noise was giving me a headache.

[crazyhorse001]

My '97 Legacy makes the same noises. Yeah it's scary at first, but from everything I've read here & elsewhere, it's not really a huge concern. I gathered from my reading, that you only need to worry if it doesn't go away once the engine comes up to temp.

[nipper]

My '97 Legacy makes the same noises. Yeah it's scary at first, but from everything I've read here & elsewhere, it's not really a huge concern. I gathered from my reading, that you only need to worry if it doesn't go away once the engine comes up to temp.

 

correct. The car will run another 100,000 miles sounding like that.

 

Some cars have it far worse, where it actually effects engine life. Its a function of a short piston skirt (reduced engine mass, etc etc). you can experiment with differnt oils to see what quiets it down.

 

 

nipper

[hankosolder2]

correct. The car will run another 100,000 miles sounding like that.

nipper

 

Well, I'd say the car COULD run another 100,000 miles like that. Do you have the upgraded headgaskets?

 

Nathan

[nipper]

at 191,000 miles, yes in a way i do, and the new pistons, and a new crank, and i can go on ....

 

 

and no Blu never needed HG's technically. It was what was attached to the HG's that went bad. :-p

 

nipper

[WoodsWagon]

We've got a 98 outback with piston slap. #4 cyl only, but it clacks away quite loudly. It lessens a bit when warmed up, but is still audible inside the car with the windows up.

 

I bought the car with blown headgaskets, so couldn't run it enough to warm it up.

 

Replaced the heagaskets, wished I had replaced the pistons too while I was in there.

 

POS design detail in my opinion. Hate the 2.5's for the crappy pistons and headgaskets and difficult head design, love them for the power boost.

[LegAC]

Anyone living up north can comfortably run Mobil 1 10w30 in lieu of Conventional 5w30 and not worry about cold starts.

 

 

----------------Conv Synth

-----Oil Weight 5w30 10w30

--Pour Point, °C (-27) (-45)

-Flash Point, °C (200) (224)

 

------Viscosity

----cSt at 40°C (62.2) (62)

---cSt at 100°C (10.6) (10.0)

 

 

As you can see, the 10w30 synthetic enjoys a lower pour point at -45 celcius, self combusts at a higher temp of 224 degrees, and is thinner at both room temperature and vehicle operating temperature.

 

Actually the pour point for a synthetic 5w30 usually only lowers the pour point by 3 or so degrees on average.

 

For the record, Mobil 1 has historically been called a "noisy oil". However, I've never seen an engine be anything other than spotless after constant usage when the engine needs to be torn down.

 

But these days, the gaps are so small between all the different brands, that you could easily just pick any "full synthetic" that's on sale and sleep good at night knowing your engine will start well in the morning.

 

I personally use Castrol Syntec 0w30 in all of my NEW cars, and Mobil 1 10w30 High Mileage in my Soob. The M1 HM stopped much of my leaking.