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Discussion Starter #1
I have about 1100 miles on my STi now. It has been very hot and humid here lately.. 90-100 degrees farenheit. When I have the car parked and just sitting there letting the engine idle for a while.. the engine will rumble randomly.. Could this be detonation? I never notice a change in rpm when at idle.. it always stays at 500 rpm even if it rumbles (shakes the car slightly) or if I have the air conditioning on or off.

I am using 93 octane pump gas.. I notice this happening whether or not I've used a bottle of octane booster or not.

I have not noticed detonation happening under hard acceleration yet.. although I've yet to go past 6000 rpm.. and even then only during a slow pull at high speed in 5th or 6th gear.
 

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I assume you are running the A/C? My Forrester idles really rough when the A/C is on. Also, I'm not sure how to check this or not but if you are getting a bit too much fuel in your engine (running 'rich') you might get a rougher idle. Is there any black buildup around the exaust on your car? I think that's an indication of too much fuel in the mixture.
 

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You sure it's not just the fan kicking on? Also I would not get in the habit of using octane booster. Repeated use of that stuff will leave a sludge in your engine.
 

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Discussion Starter #6
The rumble is not violent.. the rpm's are locked at 500 even when it rumbles..

I don't -think- it's the fan kicking in.. it just randomly 'shakes' a bit.. no aparrent rhythm to it. It typically idles completely smooth, but periodically it will have 'the shakes' and then go back to normal.

If I turn off the A/C while it's doing it, it has no effect.

My tailpipe does have a small amount of black buildup on the bottom of the tailpipe.. I figured that was just normal.. it happened with my Civic Si.

I am definitely not going to be in the habit of using octane booster.. I just did it 'for the hell of it' this one time. :wink:

I know the boxer motor has it's own sound to it.. it sounds fine to me. The sound is not part of the 'rumble' I'm observing.. the sound stays constant.. it's just a feeling of the car erratically shaking ever so slightly.. and only periodically when idling for a long time.

It's probably just the fan turning on and off.. maybe it just shakes a good bit every time that happens and it was turning on and off very fast since it was almost 100 degrees farenheit outside.
 

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I noticed the rough idle from day #1 (now has 440 miles). I also noticed how @#$! fast this engine spools up to speed, then it all made sense. Combine the hollow valves, hollow cam, lightweight flywheel, etc. and you have a very low inertia engine. Sometimes the vibrations cancel out and it's smooth, a few revs later they superimpose and it's rough.

Again, thank you Subaru!
 

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Well said. I see the same thing and that was my thought also. Its a combination of compression, low rotational mass of the engine, flywheel, cams, etc. and there you have it.

Now, given my druthers (not necessarilty realistic, I know) I love the fast revving, high compression enignes that go from 1k to 5k back to 1k in a NY second, but for the overall package, I will give up the stop-light 'cool' appeal of the HC engine, for the price and the 'whip your a$$ when I am on boost' appeal anyday. :D
 

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Discussion Starter #10
Cool, it makes sense to me now that you explain it.

I love this motor!!! :D
 

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Re: Black on Silver - Nebraska - Took Delivery

I noticed the rough idle from day #1 (now has 440 miles). I also noticed how @#$! fast this engine spools up to speed, then it all made sense. Combine the hollow valves, hollow cam, lightweight flywheel, etc. and you have a very low inertia engine. Sometimes the vibrations cancel out and it's smooth, a few revs later they superimpose and it's rough.
Sounds like something my 4Runner does, at least this is our assumption. The front rotors are warped and sometimes you dont notice it and sometimes its really bad. I think they eventually cancell themselves out after a lot of steering. But thats just a guess from a uneducated person! :p
 

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my 1979 Buick Park Avenue idles really rough, the entire car shakes slighty... i also run very rich... when i first start up the car and warm it up you can smell the exhaust...

just one more year till i get an sti next summer, sigh...
 
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