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Metallic Rattle Under Acceleration


RedLine

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Hey all, quick background about my car before I get into in guts of my question... its a 2001 Rev-D BE5 Legacy B4 RSK with around 187,000kms, only mods are a bigger rear muffler and tailpipe (done by previous owner), plus the Hose10 Mod (done by me, with a lot of help from members here).

For the past few months, I've started noticing something a bit odd. When the car is under significant acceleration (ie - after downshifting to overtake or if I'm pushing it from 0 to 100 quite quickly), I've noticed this metallic sort-of... graunching noise. It's a bit tricky to describe, but it sounds like metal on metal, and is somewhere between scraping and rattling. It comes right when I ease off the accelerator, and doesn't happen under normal acceleration (or I can't hear it!). I can generally only hear it with the window down, but it seems to get louder the harder I push it. At the risk of stating the obvious, it's definitely coming from under the bonnet.

My first thought was a turbo on its way out, given that it seems to be related to boost. So I've done some Googling around, and have found quite a few different suggestions, including not enough oil (seemed fine when I checked the dipstick yesterday) or a loose valve (I really hope not).

I don't have a full service history for the car, so I don't know if the turbos have had anything done to them. I had it serviced about 6 months ago, where the mechanic did the rocker cover gaskets and cambelt. It's running Fuchs 5W30.

Anyone here experienced something similar, or have any ideas what it might be? I'm a noob at mechanical stuff, so I hope this isn't a stupid question... :confused:

Cheers!

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Like... when you have foot flat, you get this erratic clack clackety tap tap clack sound? If so that's severe pinging (AKA knock, detonation, engine self immolation whatever you want to call it) which the rev D is horrific for

I re-read what you wrote and you said it's only after you button off?

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 Marky said:
Like... when you have foot flat, you get this erratic clack clackety tap tap clack sound? If so that's severe pinging (AKA knock, detonation, engine self immolation whatever you want to call it) which the rev D is horrific for

I re-read what you wrote and you said it's only after you button off?

Yep, its only when I have my foot flat, or near flat. I really haven't explained the noise very well... It is pretty erratic though.

I meant it stops when I ease off the throttle. The noise is only there when I am accelerating hard. The flatter I have my foot, the worse the noise is.

Come to think of it, I don't recall it at higher revs... I'll give it a punt through the full rev range tonight in 2nd and report back, hopefully with an audio clip.

Thanks for your help.

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 Marky said:
You won't *feel* det

A misfire yes, det not so much

Some of these legacies sound like diesels under acceleration, they genuinely get that bad. Most noticable in midrange when the secondary is waking up

If it's detonating it's not accelerating smoothly... which you'd feel... IMO. I could feel a subtle drop in power/light surging when my wrx was detonating.

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Yeah to a degree, depends on the severity I guess. I've seen plenty of cars where you could hear them det through knock cans but still not drop power - that said once it gets to being audible to the unassisted ear then yes it does make things get sad

To be fair the average stock suby is so inconsistent with performance that it's hard to pick when it's down on power due to a problem or if it's just heat soaked

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  • Admin

The imports being tuned for jap fuel is the big one. The octane number refers to the fuels resistance to knock, so the cars are tuned for 100octane over there, then come here and get run on 95 or 98 where the high compression that resisted knock on 100 no longer can on those fuels, hence the solution is to reduce ignition timing, since reducing compression is in no way a practical solution for most.

Edited by Andy_Mac
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 Marky said:

That one at 0:22 is perfect too

That's it... not as loud, but that's definitely the noise when he accelerates at 0:22.

Soo I guess it makes it pretty urgent that I re-flash the ECU now. It is a Japanese import, so probably designed to run on 100 or something. Would putting an octane booster in it for now (and not hoofing it) be a good idea until I can get the ECU done? I always run it on BP98.

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 Marky said:
Yep, it'll help, for sure. Also be aware of when you're fanging it, it takes the top mount a good couple hundred metres to get down to normal temp if you've been sitting in traffic a while

Thanks Marky, appreciate your help once again!

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One last question...

The time that this issue started was shortly after I got the cambelt done by (in hindsight) a muppet garage (link to thread).

Is it possible that they could have gotten the timing wrong on the belt when they replaced it? I mean, if they had it off by a tooth, would it advance the timing just enough to make it ping? Seems unlikely, but after my dealings with them, I wouldn't just assume they got it right. Anyone had this happen to them? Is it even possible that the engine would still run, but just with the wrong timing?

Edited by RedLine
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Cheers, loner. I've decided to try the cheaper options first and bought a bottle of this octane booster / injector cleaner and emptied it into the fuel tank and took it for a long drive. Could just be my optimism and wishful thinking, but it seems smoother already. Didn't get too many chances to hit boost, but when I did, there was no dreaded ping! There was another noise it has always made at idle as well that I assumed was normal, and its not doing that anymore.

Thanks again for everyone's suggestions and help with this. Will update again if the pinging comes back or if this fixes it longer term.

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Well, I spoke too soon. Its still pinging, but only with my foot flat. It's not doing it at 3/4 throttle anymore, and the idle is MUCH smoother than it has ever been, which is making me think that the minor octane boost from the Nulon stuff that I put through it is helping, but not enough. Definitely considering an ECU tune now to get it optimized for BP98. Speed Tech Motorsport have quoted me $1495 which includes a full dyno tune, so if Marky has a way to fix it and its less than that, I'm all ears :D

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 loner said:
A dyno tune will not fix a mechanical problem though... at least check the cambelt is on correctly first!

Yep, definitely want to get that looked at, along with pretty much everything that mechanic did to it. I'm going to see if I can get to the cambelt this weekend and match up the notches, but beyond that I don't really know what to look for. Might have to bribe someone with a 6-pack (maybe a 12) :)

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