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View Full Version : Cold Engine Knock (piston slap?) HELP!?!



Blue Meanie
02-17-2011, 05:04 PM
I've gone through many of the NASIOC forums trying to find an answer and found so many possibilities it was daunting but still nothing definitive:

I have an '02 WRX Wagon (bone stock engine) that gets this knocking noise when its cold outside (50 or below?) that goes away as she warms up. It does not happen in warmer weather at all. This has happened for the last couple winters and seems to be worse this winter. There is no real loss in performance and it quiets down at higher RPMs like above 2000-2500.

I've heard everything from piston slap to rings to timing to fluids. I tried running some Seafoam through the last tank of gas and added some expensive cleaning oil and it seemed like she actually quieted down some but it's hard to tell with the weather getting better.

I get the feeling this is not a unique situation for the older Subis so any help would be so greatly appreciated. I've asked the dealer but couldn't reproduce the sound because she'd warmed up. They tried to make it out like I needed an upper engine rebuild but I don't trust those places to not rob me blind. I'm really worried that this could lead to a premature engine replacement (which will happen before I pay to rebuild it).

Thanks much!

TunedAlready02
02-17-2011, 05:14 PM
Most likely what you have is a case of piston slap. Subaru boxers are notorious for it.

Also, what grade of fuel are you running?

Blue Meanie
02-17-2011, 05:19 PM
Most likely what you have is a case of piston slap. Subaru boxers are notorious for it.

Also, what grade of fuel are you running?

Thanks Tuned. Premium always plus an octane booster sometimes, maybe once a month. So what's a piston slap really and what am I looking at?

TunedAlready02
02-17-2011, 06:15 PM
Oh man this is hard to explain lol...

Just imagine that your piston is too small for the cylinder so that there is play in it (just a couple thousands). When the piston is being forced out, its only making contact with one side of the cylinder, we'll say the bottom. When the piston reaches the end of the stroke, the momentum causes it to "rock over" and make contact with the top. Thats the "knock" youre probably hearing. The reason it goes away is because the piston warms up and expands to meet the wall of the entire cylinder within spec.

Blue Meanie
02-17-2011, 06:25 PM
Oh man this is hard to explain lol...

Just imagine that your piston is too small for the cylinder so that there is play in it (just a couple thousands). When the piston is being forced out, its only making contact with one side of the cylinder, we'll say the bottom. When the piston reaches the end of the stroke, the momentum causes it to "rock over" and make contact with the top. Thats the "knock" youre probably hearing. The reason it goes away is because the piston warms up and expands to meet the wall of the entire cylinder within spec.

Ugh! This is what I was most afraid of... So I'm looking at a rebuild to fix it? (F#@<!) How long before this gets critical? Am I right in assuming a (replacement) crate motor will be about as much as what this wold cost me to get fixed?

Thanks man, I appreciate all the help I can get.

TunedAlready02
02-17-2011, 06:38 PM
I honestly wouldnt worry about it. The least you could do for it is just drive it normal without romping into it until it reaches operating temperature. Both of the WRXs that I have owned had piston slap, its kind of annoying at times but its not a critical situation. When it was cold out, I would always let them warm up for at least 5 minutes before I took off to go anywhere. I live in a really small town though, it seems like there is an ordinance in the city against leaving your car idle in the driveway, but I could be wrong.

Blue Meanie
02-17-2011, 06:43 PM
Cool, thanks again... that helps my stress levels greatly. I'll just pay more attention to warming her up (because I drive her like I stole her :lol: )

Jagular1785
02-22-2011, 02:26 AM
I have it too, but it goes away within a few seconds of starting. But I have forged pistons going in my build engine, and I can see there is a lot of clearance from piston to wall clearance at the top. Forged pistons in subarus are even worse for slapping because they need more clearance due to thermal expansion. It doesn't do much damage to your motor, maybe a bit more than usual piston skirt wear but nothing really more than that.