Discussion
You would normally hear piston slap when the engine is cold. Once it starts warming up the slap usually goes.
If you are sure it's piston slap and it's still there when the engine is up to normal operating temps i'd say it was needing mechanical attention and would just get progressively worse over a short period of time. You would experience more fuel and oil usage not to mention engine compression will be right down and the engine will become harder and harder to start over time.
It's not uncommon for a subaru with piston slap to run without any problems whatsoever for years on the proviso the slap keeps going when the engine has warmed up. If you suspect piston slap don't ever flush the engine with a flushing oil additive or flushing oil when doing an oil change because you could be washing away the carbon deposits that build up in the cylinders over time helping to cushion the blow of piston slap keeping it quiet. Just use a very cheap 10W40 mineral oil to do the flushing.
The other thing it might be is bottom end rattle which you will definitely hear on light engine loading whether the engine is hot or cold, and the sound is quite different from piston slap. Equally you probably won't see any loss of compression as it's the bottom end that's affected not the pistons. You need to see a Subaru specialist that will tell you exactly what the problem is, better than guesswork or long distance diagnosis.
If you are sure it's piston slap and it's still there when the engine is up to normal operating temps i'd say it was needing mechanical attention and would just get progressively worse over a short period of time. You would experience more fuel and oil usage not to mention engine compression will be right down and the engine will become harder and harder to start over time.
It's not uncommon for a subaru with piston slap to run without any problems whatsoever for years on the proviso the slap keeps going when the engine has warmed up. If you suspect piston slap don't ever flush the engine with a flushing oil additive or flushing oil when doing an oil change because you could be washing away the carbon deposits that build up in the cylinders over time helping to cushion the blow of piston slap keeping it quiet. Just use a very cheap 10W40 mineral oil to do the flushing.
The other thing it might be is bottom end rattle which you will definitely hear on light engine loading whether the engine is hot or cold, and the sound is quite different from piston slap. Equally you probably won't see any loss of compression as it's the bottom end that's affected not the pistons. You need to see a Subaru specialist that will tell you exactly what the problem is, better than guesswork or long distance diagnosis.
...I remapped my old WRX Bugeye engine to 280bhp when it was also slapping when cold and that showed no ill effects whatsoever due to the extra power output.
Forged engines also have piston slap because of the extra clearance they run between the piston and cylinder wall. Doesn't often go away either when warmed up but obviously there for a valid reason and is expected to be there.
I suspect it might actually be bottom end rattle that is your problem. Again could run for years like that but more likely to get much worse and become catastrophic if power outputs are raised or the engine is heavily worked regularly.
Ultimately i think you will be looking at a rebuild. You could rebuild to standard spec for less than £2000, forged spec for £5000-£7000 or just go used engine route from a subaru breaker.
The only thing with the used engine route is you don;t know the real mileage or condition of the engine until you fit and run it, so a rebuild is often the only sure fire way of getting a guaranteed engine with no faults if it's done properly.
Forged engines also have piston slap because of the extra clearance they run between the piston and cylinder wall. Doesn't often go away either when warmed up but obviously there for a valid reason and is expected to be there.
I suspect it might actually be bottom end rattle that is your problem. Again could run for years like that but more likely to get much worse and become catastrophic if power outputs are raised or the engine is heavily worked regularly.
Ultimately i think you will be looking at a rebuild. You could rebuild to standard spec for less than £2000, forged spec for £5000-£7000 or just go used engine route from a subaru breaker.
The only thing with the used engine route is you don;t know the real mileage or condition of the engine until you fit and run it, so a rebuild is often the only sure fire way of getting a guaranteed engine with no faults if it's done properly.
Gassing Station | Subaru | Top of Page | What's New | My Stuff