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Discussion Starter #1
Hi all,

OK, so recently I changed the oil in my '99 OBS to Mobil 1 Synthetic 5w30. Ever since then I tend to have a strange knocking sound between 1400 and 1900 or so RPM....at least, that's when I first remember hearing the noise. The sound is definitely engine speed related...it gets faster with increased RPMs, but then disappears after a certain RPM...like I said, somewhere around 1900 RPM.

My parents were in town the other day, so I had my dad listen to the noise and he can't tell what it is either. His guesses were that it's either an exhaust leak, or that switching oils has caused some slop in the pistons/valves/etc due to the decreased viscosity of the synthetic oil. I have no idea what it is. :rolleyes:

I hardly ever drive very far (I live about a half mile from work, which is where I drive most often), but the noise doesn't seem to go away after the engine has warmed up after driving to friends' houses, etc, but it does get quieter. At least, I think it gets quieter...maybe I just get used to it, and I don't notice it as much..hehe. Some days it's worse than others, but I couldn't tell you if it's a difference in the weather temperature, or the phase of the moon, or what.

Anyone have any ideas? I plan on taking the car in to a dealership and have the service department take a look at it. I'm heading out to Utah for a ski trip next weekend and don't want to have any problems along the way.

Thanks in advance,
Matt
 

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How many miles did your OBS have when you switched to synthetic? Your father may be right in that switching oils may have slicked up your engine and could be a contributor to the problem although I don't think it is the main factor.
 

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That's funny. My car is making a strange knocking noise as well, and I don't remember it doing that when I picked it up last month. I've also changed over to Mobil 1 5w-30 just shortly after getting the car. I think it may be worse when it is colder out, but I still hear it clearly when the engine is warmed up. Only at lower RPM's, especially when accelerating. Kinda reminds me of a diseal engine in those big pick-up trucks. I have a 2000 2.5L with 70k miles, so it's a different engine I believe.

Please be sure to post back here on what your service department thinks it might be.
 

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My car has some piston slap when the engine is cold. I think it is prettty common among 2.5 Subarus. Sounds like you guys may be experiencing something different though. Mine definately goes away after the engine gets warm (within about 5 minutes of startup).
 

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Discussion Starter #6
It had about 38000 miles on it when I swapped oils. As for what kind of oil it used to have...I have no idea. I bought it the end of December, and did my first oil change about 500 miles before the sticker on the windshield said to change it, and that's why I switched to synthetic. The sticker from some car service center (Jiffy Lube, Grease Monkey, etc), so I would *assume* that it was just normal 5w30 dino oil, but have nothing to back that up.

And, as for piston slap in the 2.5L...I have no idea. But I only have the 2.2L anyway. ;)

Matt
 

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Well to both Ski4ever and karasek, I think your problem lies within the fact that you both switched to synthetic as higher mileage than recommended. Most people say that you shouldn't put synthetic in your car after 30,000 miles although I've never heard anyone having any problems unless they switch after 90,000 miles. The thing is that your engine is already so used to operating for so long with dino/petro oil that it is bound to have changes in operation due to the slickeryness (copyright Kevin Miller ;)) of synthetic oil. The only problems that you can expect if they do happen to happen is possible oil leaks in the future when you get to higher mileage ~100k. Nothing to get too worried about for now for both of you or anyone else who has or will do this.

To all, I recommend to anyone who is going to switch to synthetic at any mileage to use a semi-synthetic oil one or more changes before you switch to full synthetic for a better transition for your engine. For more tips check our Favorite Oil poll in the Polls forum. I'll provide a link:

Favorite Oil
 

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Discussion Starter #8
So would switching back to a synthetic blend cure my problem? I mean, of course you have no idea if it will, but is it worth a shot? That's definitely an easy "fix" if that ends up fixing it.

Whaddaya think?

Matt
 

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I think it would be a waste of money, but if you think its that bad of a problem I'll leave it up to you of course. If you would want to do something like that I'd recommend that you go and get a container and drain your oilpan of all the synthetic oil you can and then save that stuff for the next change. Do not replace your filter for now though, as there's no reason to. FYI, I recommend using Castrol Semi-Syn as your oil of choice, but that up to you as well. Hope what ever you do works out :)
 

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Most likely piston slap-even with the 2.2 Remember, you have a boxer engine, pistons are horizontal. I would not worry about the oiling part, synthetic is better, the molecules are smaller. I'm thinking you could gradually switch over by doing a Durablend or something, but having only 38k on it seems okay to make the full synthetic switch.
Remember, synthetic is a better lubricant, so you may/will hear things you have not in the past. I would not worry too much about the noises, you may have to retrain your ears for the new oil. Brian
 

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Discussion Starter #11
I guess I just don't see how having piston slap is ok because I switched to synthetic. Seems to me that the noise that I'm hearing is NOT normal, and probably shouldn't be there. It never made this noise before with regular oil. Now when I start it up and friends hear it, they're like "is it supposed to be running that rough?". It's kinda embarrassing, honestly. :(

Maybe this is a dumb question, but what about a different viscosity oil? Would that be bad?

Matt
 

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Being in Denver, I'd stick to 5w30. A lot of Subaru H4's have piston slap noise, just not all that loud I guess. My car has it, especially when cold. But it goes away after a minute or so under no load or maybe a couple two or three if I drive it right away.
There are many Q & A's on piston slap and belt tensioner noise over on NASIOC. You can go do a search and see. I can even hear slap in my wife's Baja with only 2,600 miles.
"is it supposed to be running that rough?".
Now, is it running rough or does it just sound rough? Brian
 

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Discussion Starter #14
OK, so I decided I was going to head to a dealership today after work (would have gone earlier this week, but worked 24 hrs in 2 days instead of my normal 16hrs and just didn't have it in me at the end of the day)...so, I go out, and hop in the car...guess what? No strange knocking sound. :rolleyes: I mean, none at all. I'm not sure if it's because we had a pretty warm day here in Denver, or if my car heard me talking bad about it and decided to fix itself, or what. hehe ;) But, for now I have no noise, and figured I would just look stupid if I went to the dealer and tried to explain to them that normally there's a sound, and now there isn't, but can they take a look at it anyway? So, if it comes back I'll take it in...otherwise I'll just count my lucky stars that it has gone away! :D

Matt
 

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I've paid a little more attention to my knocking noise. It definately goes away after the car warms up. I can still hear it about 2000rpm, but eventually it goes away or gets drowned out from ambient noise as the speed increases.

My car does run a little more ruff at idle than when I first got it. I've had the sparkplugs changed about the same time I changed the oil. Would something with the sparkplugs be causing the engine to idle a little ruff, or is that normal in this engine.
 

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Matt, warmer temps + no noise = piston slap(more than likely)
colder temps + noise = piston slap
And if you have to, the dealer may want it overnight to start it cold. Just maybe......Brian
 
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