My mechanic said my 1998 Legacy's tapping sound (which goes away warmed up)
may be piston slap. It has 136,000 miles on the 2.2 liter engine. No
troubles with car except for the light tapping noise. Is this a common issue
for Subaru's? I read on one website that the piston slap issue usually does
not effect engine wear that much. Is there any kind of engine treatment
fluid that helps piston slap? Any information appreciated.
Excessive “piston slap” occurs because an automobile manufacturer
designs and/or manufactures a defective engine in which the clearance
between the piston and cylinder bore is too great. Essentially, the
piston moves sideways and “slaps” or “knocks” hard against the cylinder
bore and causes damage to the engine pistons and cylinders, excessive
smoke emissions, excessive oil consumption, carbon buildup on piston
heads, decreased mileage, and a loud and obnoxious “slapping” or
“knocking” noise, all of which diminishes vehicle resale value in the
The above is from an internet search for piston slap.
On the 2.2l motor, lifter noise is quite common; a tic-tic-tic that
generally lessens as the car warms up.
You could try any of the various oil additives, but in my experience
they won't help much, if any.
As long as the noise mostly goes away when the engine is warm, I
wouldn't worry about it. Once it gets to the point where it's more of
a "clack-clack-clack" that _doesn't_ really go away, you'll probably
want to look into servicing the hydraulic lifter(s).
This isn't a difficult job on the 2.2; the hydraulic lifter is located
in the cam follower (rocker arm if you will), and this is readily
accessible without removing the head from the engine. A pretty simple
DIY project as it turns out.
I've found that swapping in a good lifter for the collapsed one
generally solves the problem, at least for a while. Get an entire head
(a slightly warped one is probably fine, and cheap), or even just the
cam follower assembly from a salvage yard. If the problem persists,
it's an indication that the oil pump is getting tired. (Or that there
is contamination present in the engine oiling circuit; did someone use
silicone sealant when replacing an oil pan or oil pump recently?)
You can readily find instructions for this procedure on the web, or
email me and I'll send you what I have. Takes about an hour, more or
less, with no special tools; 10 and 12mm wrenches and sockets, a set
of needle-nosed pliers, and a cup or so of fresh oil.
Another possibility is the timing belt system. I'll assume that at
130+K you have had the belt replaced. Did they also replace the
tensioner, idlers, and waterpump? Sometimes a failing idler bearing
will start off by making a clicking or tapping noise which may or may
not lessen as the engine warms. If this is the case, it will get
rapidly worse with time, and should be attended to ASAP to prevent
possible engine damage. If you use a mechanics stethoscope to listen
to the running engine, the lifter noise will obviously be associated
with one side or the other's (or both) cylinder head; the timing belt
idlers will sound more front and center. Piston slap, I couldn't say.
If I'm way off the mark, and it _is_ piston slap, there's not too much
you can do about it anyway. Drive the car until it fails outright
(rare), or gets obnoxiously loud, and then either replace the engine
or upgrade to a newer Soobie.
Hope this helps.
Steve Jernigan KG0MB
University of Colorado
You may try different oil filters as some are more obstructive
than others. This seems to make matters worse for piston
slap- high flow filter is preferable. I'd use some Marvel Mystery
oil in the fuel, 8-10 ounces per tank, and a similar amount in the
oil, for 500 miles, or so, to clean up carbon deposits, and then
change oil. At you're milage you may use something like 15w-40
diesel type oil to control carbon, and put a NAPA GOLD oil filter
(this one has good flow specs, and favourable experience) and
see if that makes any difference. Chances are it will.
Carbon deposits form on pistons, they contribute to wear
and increased space between rings and cylinder which makes
for increased piston slap. Also thicker diesel oil cusions better.
Works to provide an added coat of film that is good at cold starts.
Thin oils drain to the pan fast making piston slap worse- someone
mentioned putting some STP, this is essentially thickening the oil
for that same purpose.
I think taht my approach of installing a higher flow oil filter,
plus controlling for deposits better, plus switching to a thicker
diesel type oil, has good chance to help, both immediately,
and longer term.
I didn't think carbon would form or the sides of pistons, and then
removing it would help slap. I think different weight oil might help,
but heavier oil wouldn't flow as fast when cold, but thought thicker
or thinner might be worth a try. Don't know the flat motors, and was
wondering how they lube the cyl bore? Squirt oil to the top/upper
side, and let it run to the bottom/lower side?
I have a new 07' Impreza, and am concerned about developing piston
slap, since soon after I start driving, I often have to accelerate
harder if I get into traffic, so trying to take it easy, especially in
wrote in message>> > What do you figure carbon deposits would do to cause piston slap ?
Of course it forms there too, but doesn't have much chance to
accumulate on the extreme sides of pistons because it is constantly
ground there, wearing both rings and cylindeer walls. Synthetic oil
is great in preventing carbon build-up as it does not evaporate at
high temps. Very little to no carbon forms in the lower engine,
most forms in the cylinder area where temps are high.
This is a very difficult topic. There is a point of equilibrium for
thickness. If you start your car just once a month, every start will
most likely be a complete 'cold start.' I mean oil will have almost
completely drained into the pan, leaving little to no layer of protection.
For some aircraft that get used infrequently to protect engine parts from
rusting a single weight thick oil is recommended. It sits on parts much
If you start your car daily a slightly thicker oil may also stick to parts
longer, at least long enough to cusion piston slap at your next start up.
Experiment, experiment. Take into account the ambient temperatures.
Thin oil in hot climates, say summer in Phoenix Arizona, drains into pan
within an hour or two, so almost every start is a "cold-start."
When using thicker oil try to get a higher flow filter like the NAPA
GOLD I've mentioned.
This is really interesting, I think flats are generally better oiled
than other engines.
Don't worry, you have a newer Subbie probably less
likely to develop slap. I have a 2000 Impreza (original owner)
and am located about a quarter to a half mile away from an
on-highway ramp, where I also have to step on it, everyday.
I always warm up the car, for at least 5 minutes, and have always
since new used Marvel Mystery Oil in gas, this is a relatively cheap
upper cylinder lubricant. A few ounces like 4-5oz to tankfull.
The engine at 49k miles is in near perfect new condition, by that
I mean it is very tight, more like a 15-20k miles old engine
While I'm sure any newer soob could develop a true problem resulting in
something like piston slap, FHI/Subaru had engineered piston skirts
which were too short in the 90s, maybe early 2000s. (?) pretty sure that
design problem is fixed in newer engines.
Cold "tap-tap-tap" is very common for 1998 - 2000 Subie 2.2 engines. Could
be caused by slap due to short piston skirt, or could be timing belt
tensioner. Both can make the noise when cold. I had 1998 Outback, 1999
Impreza, and 2000 Forester, each tapped when cold. Irritating, but not
Try a can of STP or something similar. It keeps a coat of lubricant on all
parts while the engine isn't running. So, when you start a cold engine, it
should already have a good coat of lube on the piston and you likely won't
hear any noise.
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