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-   -   spark knock, piston slap, and a po420! (

hardcoresteve 12-10-2012 07:49 AM

spark knock, piston slap, and a po420!
hi newbie to subarus here. Having trouble finding info on this site. Either a link to an existing thread or some help would be appreciated.

So I got an 02 legacy with 5spd last Wednesday. Its been a headache so far. I love the car. Its in great shape other than the motor :-(

So the second day the checl engine came on. Po420. Dealer promises to fix that.

Then I notice a screaching under load. Replaced tensioner pulley and tensioner belt. Also tightened a/c belt. Not helping at all.

Then I notice when started cold I get a piston slap. Goes away after 10-15 minutes so im guessing pistons. Also twice I heard a spark knock when pulling out from a stop sign, and seems to drop rpm kind of fast when I let off idle. This causes a delayed shift as the car heaves back.

Do I try to undo the sale? I know they wont put a new engine in it. Any advice?info? Im in love with the car but idk what to do.

rSTimpreza 12-10-2012 12:07 PM

P0420 normal fix fOR this is to install a o2 anti fouler after the cat then reset check engine light.

Piston slap is normal at cold start change your oil... I use 10w30 high mileage.

Try doing a tune up first and see what happens

octain11 12-10-2012 01:37 PM

1)P0420 is either O2 sensors or a bad cats
2)Change your oil with good quality oil such as Rotella (my favorite) or Mobile 1, before getting new pistons
4)Check out the spark plugs and wires see if they need changing

BrandonJ 2001 GT Limited 12-10-2012 02:12 PM

Piston slap is a known Subie quirk...Rotella T6 seemed to quiet down the "Subaru Tick" some. But if it's below 50 deg, it's there until the engine temp gets to the first solid mark at least.

I'd be willing to bet the PO420 is in relation to the spark knock. If your cats are'll restrict the exhaust flow and cause all kinds if issues. I'd have the dealer do their repairs and see if it remedies everything except the slap.

rSTimpreza 12-10-2012 03:02 PM

Check p0420 DIY. Has been done many times.

johnegg 12-10-2012 05:57 PM

if you just bought the car and the dealer said they would fix the p0420 let them do it. subaru dealers fix it for customers by selling them new cats. it should be interesting to see what your dealer does for you.

and the 02 extender is not the fix for the p0420. it is the solution when all else fails. it is in every ones best interest to find the actual cause and correct it. lots of time a new front o2 sensor from subaru and maybe rear o2 sensor will do the trick. the list of things that can cause this code is very long and ranges from bad cats (which is really rare) to an exhaust or intake leak.

if the engine does have an ignition ''ping'' or ''knock'' you WILL have a CEL for the knock sensor., p0325. which you may have in addition to the p0420. but you will not know unless you cheack it with a reader. the symptom is a sluggish off the line, no power and lousy MPG. get the codes read again, all of them. you can get good knock sensors from ebay for under $20. if you are in a hurry, make sure it ships from the US, not china.

screeching under load is probably one of your belts, AC or steering pump / alt. as a test you can remove them one at a time and test drive the car for a short drive. you can drive it without the alt belt as long as the battery has enough charge. do not go too far.

does the screech change when you turn the AC on / off??

good luck, and let us know what you learn.

hardcoresteve 12-10-2012 09:46 PM

ive heard the spark knock only twice. Only a po420. Nothing for knock sensor codes. It really seems to drop rpm fast when I let off the throttle. Im thinking the cats may be clogged. If they insist I pay out the wazoo ill just do what I did with my civic and have high flow cats from vibrant. I had a good experience and they are only like $100 each. Plus maybe $100 for a shop to weld a bung for o2 in them. The amount of cats on this car sucks lol

Ive replaced the tensioner pulley and the belt that goes with it. Ive adjusted both fairly tight. Still have the screaching. Its only from like 3500RPMs on up. Thinking maybe the alternator. It sounds rough when turned with the belt off. May be ceasing under load. Just a guess. Idk

hardcoresteve 12-10-2012 09:52 PM

Power steering is smooth, a/c is quiet, just the rate the RPMs drop between shifts, and, the screaching sound. Motor mounts feel flimsy but im not getting into that stuff yet. 24-25mpg averaged over the 600 miles ive put on it so far. Ran two bottles of injector cleaner/lubricant. Going to use throttle body cleaner tomorrow. Wires, plugs, some canister with lines on the driverside near strut, and brake fluid all look newly replaced.

johnegg 12-11-2012 06:54 AM


Wires, plugs, some canister (fuel filter) with lines on the driverside near strut, and brake fluid all look new
looking for the screech, a mechanics stethoscope may help track it down if you are not sure.

hardcoresteve 12-11-2012 06:57 AM

Update on this....

Noticed I hear the noise when I tromp the throttle in neutral. With air ducts removed(all but airbox) and hood open too. The strange part is revving it by opening the throttle body by hand isnt making the sound. Only by throttle pedal. Hmm. Ill have a fellow mechanic try to listen for it later.

johnegg 12-11-2012 07:05 AM

other things that can cause noise under acceleration, in no particular order, the exhaust doughnut gasket, the exhaust heat shield, the air filter box not fully seated,.......

hardcoresteve 12-14-2012 09:13 PM

the dealer replaced o2 and knock sensors. The code is gone and running a little stronger. Still screeching under load unfortunately. It does it when I tromp the pedal, but not when turning the throttle body by hand so I cant self diagnose. Will have my son rev it up for me so I can listen for it tomorrow

rSTimpreza 12-15-2012 12:26 AM

Which o2 sensor you replaced? upstream or downstream?

octain11 12-15-2012 06:55 AM

It is usually the sensor in bank 1

hardcoresteve 12-15-2012 04:49 PM

Not sure which o2. the car dealer replaced it for me. I do plan on checking it out, but not yet. Too much to get ready for Christmas this weekend. Im thinking about grabbing a dynomax muffler and replacing the stock muffler. I can weld it in and make brackets. Just wont be corrosion resistant with the welder I have to use. Decided it is the cause of the loud whistling sound. Baffles probably rotted out. Ill eventually Get a good header and upgrade the intake. Im still lugging through the forums gathering info. new to subarus. Thinking about a swap for next year. May build a motor so its ready for when this one goes.

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