Piston Slap or Something Else?

[Nick Bruns]

Hey everyone. Ill be quick. 2002 Subaru Legacy (2.5 dohc) with 110,xxx miles. Its been making a peculiar noise at start up. It started after drove the car to the beach for my honeymoon in July 2010 about 3 weeks after I bought it (had 70k on it). It was cool at the beach at night and I noticed the sound when I drove it at night. It hadnt made that noise before then. It sounds like a lifter or a rod bearing making noise, but it only happens on startup and goes away 15-20 minutes of driving (ever after its up to temperature). I have no idea what it is. I’ve heard it could be “piston slap” as its not uncommon to the flat 4 motor. I tried additives to no effect. I run Valvoline High Mileage oil and changed from 10w-30 to 5-w30 about 11,000 miles ago. The car rus fine. Is it piston slap or something else? If it is piston slap do I need to be worried? Thanks!

Topic

[dan]

now my 3100 SFI in my Buick Century Custom had piston slap and it was very common in those engines, when the engine just starts up it will make sort of a light ticking or knocking noise and it is caused buy excessive clearences between the cylinder and the pistons, something with the pistons being too small of a diameter and when they wore in they slapped in the cylinder bore.

if its piston slap what usually happens is the engine at cold start up is a little drier than usual particularly the cylinder walls which yet need a little oil flung on them and soaked up a little bit. until the engine warms up too operating temperature so the pistons can expand and the cylinders get some oil on them the piston slap will continue, this may explain why you are hearing a slapping noise now driving the car gets the engine up in the revs and flings some oil around and heats the parts up, that might be why it goes away after you take it for a drive….

now piston slap can be caused buy a couple of things, warn out cylinder walls, warn out pistons, or a cracked piston… some engines like the 3100 SFI will piston slap like a fool and just don’t care, they will continue too run and run and run, however some engines that have piston slap don’t like it and eventually a piston goes sideways in the cylinder bore, this quite commonly happens when you have a cracked piston skirt, however if its a common issue with your engine and its commonly jut like the cylinder bores wear out or the pistons wear out then it should be fine, just take it easy when its cold, let it warm up before you really get on it.

[spelunkerd]

Your story sounds like noisy lifters, to me — that would be a common problem in many vehicles. My old Suburban has done that for over a decade in cold weather, but since we’ve moved to a warmer climate it doesn’t happen. I suppose a switch to oil with better cold weather properties, maybe synthetic, might help. The real question is whether this might be a warning that something more serious is about to happen. It’s hard to be certain, short of going through the expense of cleaning and replacing lifters. The previous owner might be honest enough to tell you how long it was going on before he sold it to you. And it wouldn’t hurt to get a pro to have a look.

[dan]

could check oil pressure.

[Lorrin Barth]

My Subaru with do that on startup sometimes. It doesn’t last for twenty minutes though – more like twenty seconds. What it is – dunno. Everything about a Subaru is a little weird.

I’d suggest you need a trained ear. Start asking the mechanics at the dealership how long they have been working on Subarus. When someone says 10 to twenty years have then listen to your car.

[Nick Bruns]

I should have mentioned that it mainly does it in cold weather, like when it gets 40 degrees or less. I don’t notice it during the warmer months here in WA State. I had heard that this wasn’t uncommon to the EJ25 motor due to the configuration of the engine (horizontal cylinders). I just don’t know if it’s something to be concerned about. I’ve had it to a mechanic last year about this same time (August) for a service related to a CEL, and it doesn’t do it when it’s warm, so there was no way for them to hear the sound :unsure:

[Lorrin Barth]

Well, if it is something that changes with the season I say not very serious. Let it get bad enough to the point where you can demonstrate it to your mechanic.

[Tom]

I had an exhaust leak at the manifold that two different mechanics were certain was an internal engine issue. I was mighty relieved when I figured out it was just a leak at the manifold, caused by bad threads in one of the exhaust manifold stud holes.

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