I get these informative emails every so often and this one talks a little
about the knock sensor. It appears that you have a good start on the
problem. This author and one of the suggestions is there are more than one
sensor. I have no idea what you might have on your vehicle. This knock
sensor seems to be touchy and a source of real annoyance. BUT I am sure that
it is worth having in the system...
Hope some of this help.
PS: I do own this Auto Tap toll and it works great on my wife's 1995 Subaru
Legacy LSI. Yes, this '95 does have the OBDII connector...
Bill
From Autotap website email...
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Ready for another automotive topic? Today I'm stuffing your inbox with info
on your car's knock sensor and just how it helps you get the most
durability, power and economy out of your engine.
First, a couple sentences on just what "knock" is. In your engine, the fuel
mixture is injected, and then the piston begins its compression stroke.
Somewhere just before the piston reaches the top of its travel, the spark
plug fires, igniting the mixture. That's actually a pretty well-controlled
burn and it creates the pressure needed to drive the piston down through its
power stroke. "Knock", also called pinging, occurs when something causes the
fuel to ignite in an uncontrolled manner, usually before the spark plug even
fires. This is a violent event in the engine - heavy pinging can lead to all
kinds of internal engine damage.
Remember the cars of the late 70's and early 80's? It sure wasn't unusual to
here them knock. But in today's vehicles you (almost) never hear as much as
the slightest knock. Improved electronic controls play a big part in that,
but the unseen hero is our friend the knock sensor. This little guy detects
vibration - but much more than that, it can actually tell the difference
between the engine's normal shakes and rattles and knock. Pretty impressive
considering all the mechanical motion going on in an engine.
You'll find the knock sensor mounted somewhere on the engine block or intake
manifold. I'm told that some engines even use more than one.
What are the symptoms of a failed knock sensor? It can be pretty subtle and
not that noticeable in every day driving, or if you don't have a scantool.
There are two things that can happen. First, the knock sensor could fail to
detect knock leading to audible pinging when conditions are right. If you
hear pinging you definitely need to check out the knock sensor. Second, the
knock sensor could be overactive - detecting knock that's not really
present. That translates into reduced power and economy.
So how do you test a knock sensor? Pretty simple really. Plug in AutoTap or
another scantool, then take your favorite wrench and give the engine block a
couple raps next to the knock sensor (please, don't hit the sensor itself!).
Watch the ignition timing and knock sensor parameters on AutoTap's screen -
you'll see knock events register on the knock sensor parameter, and you'll
see the ignition timing retard a few degrees. If you don't see any change,
be suspicious of the knock sensor and its wiring harness.
Another possible problem with knock sensors is mis-triggering on some other
vibration. One example where this is common is on modified cars with
aftermarket headers. Space is usually pretty tight, and a header tube might
rattle against another part under acceleration, or a manifold bolt might
work loose. These subtle problems can drive knock sensors nuts. You might
also see some knock sensor activity on a badly worn engine from a noisy
valve train or worn bearings. Odds are good on those engines that the few
degrees of lost timing advance isn't the biggest issue to worry about!
Happy Repairs,
Mike Fahrion
www.autotap.com
