Plug Knock

Also on my 2005 Dodge Caravan, I have been getting a little spark plug knock
going up hills when it kicks into overdrive while on Cruise Control - only
with the AC on.

Suggestions?

TIA,

-Tom

 

Try a tank of premium or a few tanks of whatever gas your buying plus
octane booster and see if the problem goes away. If so then
congratulations -
you got an engine that has a hair's breath higher compression ratio than
it's brethern - maybe the milling machine that mills the heads on the
assembly line was out of adjustment that day - so if you are willing to
buy the gas for it you will get a bit more power out of it.

Otherwise it could simply be a bad knock sensor.

Ted

 

Carbon build up on the pistons will cause the exact thing

 

Many years ago, when I worked on knock dection systems, I was told by
engine designers that ngines running under light knock conditions produce
MORE power

I believe your statement that if the OP buys higher octane gas he will get
higher power is false.

Higher octane gas can allow engines to produce more power, but only
because adaptive ignitions systems can advance the timing further when
running on high octane gas. From what I was told by those engine
designers, if the OP eliminates the knock by increasing the octane
rating, the power output will actually drop! Remember there is no more
energy stored in a gallon of high octane gas than is in a gallon of low
octane gas.

Now, I have also heard knock from my Voyager's 3.3L engine. It could be
that I also have a faulty knock sensor, although there are no fault codes
to indicate this. I suspect an alternative, which is that the programming
of the engine management only allows gradual changes in timing due to
knock detection, and thus a change in driving style (for example from
light town driving to going up a steep incline at speed) causes a
transient case of knock.

 

You are so right, acually running a high octane fuel in this car will do
more damage then good, It will leave deposits that can cause higher
compression , causing pinging and even stalling problems

 

Just how much knock is "light knock"? I'd expect that by the time
the knock was audible, it wouldn't be what those guys would call
"light" anymore.

Well... once again, this depends on just how much knock we're talking
about. If it's too heavy, then a higher octane gas will be able to
convert the burn to mechanical energy more efficiently, giving more
power.

 

This claim is, of course, completely absurd.

 

To each his own, But you are wrong. using a high octane fuel on this
vehicle will cause exactly what I posted

 

Umm, yeah. Let's see, how are the extra deposits left by a
high-octane fuel going to cause knocking?

Well, first, of course a high-octane fuel has to be more prone to
leaving deposits than a low-octane fuel. I never even heard that
story back when we raised octane by adding lead.

And these extra deposits... what? Hold heat, like carbon? Have so
much volume they raise the compression ratio so high they cause more
knocking than using a low-octane fuel in the first place?

There are many, many arguments that can be made about high-octane
vs. low-octane fuels. But any argument that starts by claiming
high-octane will cause *more* knocking than low-octane be *extremely*
hard to sell.

 

On a 2005? He must be using oil sludge for gasoline, then.

Ted

 

Maybe you read my post wrong or I just didnt explain waht I was saying.
I was merely stating that if you use a high octane fuel on a vehicle that
is designed to use a low octane fuel, this will cause deposits and YES it
will cause drivability problems such as pinging, stalling and some
knocking.

Glenn Beasley
Chrysler Tech

 

esp. in a flex fuel vehicle

 

I remember reading in the Ford Motor Co magazine several years ago (last or
2nd the last issue before they abandoned it) that the use of higher octane
gas was actually detrimental to a modern car engine. They remarked that the
knock sensor actually needed to sense a slight knock (which they stated was
not-harmful) and the use of high octane prevented the sensor from detecting
a knock, sending a signal to the computer that the engine was running too
rich. (Sorry for the non-technical terms as I'm writing this from memory and
don't know exactly what control the anti-knock sensor has over the PCM.)
Their summary was to use ONLY the gas octane recommend in the owners manual.

 

I won't totally disagree with you, but rather make some comments. Knock
can be caused by over-advanced timing and by other factors that can
increase temperature: such as a lean mixture or over-retarded timing.
Also, to remark on anther poster's question: knock takes place when there
is spontaneous combustion. Hot spots can help this to happen -- and hot
spots can be created by isolated pieces of carbon (by isoloated, I mean a
spur of carbon which may not conduct heat into the block efficiently).

In the days before catalytic converters became mandatory (later in
Europe), engine designers were working on the "lean burn engine". However,
this never yielded the necessary emissions performance and so the fuel/air
mixtures in modern engines are largely determined by the chemistry of the
cat.

Thus, I doubt that knock sensors were ever used to control mixtures. Their
first use was, I think, to control turbo boost and later, to control
timing.

 

I'd love to read what was actually said, and to find out when it was
said. I'm having a really hard time following why somebody doing an
engine management computer would look at something other than the O2
sensor to adjust the mixture.

Not quite. "Lean burn" was leaner than earlier carbs, but still not
especially lean (I put an O2 sensor on my '78 Newport, which has both
lean-burn and a catalytic convertor. There are conditions under which
it runs leaner than the ideal mixture, but not often). The mix in a
modern car is set as close as the computer can get to the ideal
stoichiometric ratio, which is what produces the least emissions.
It's tru that this gives the cat the least amount of emissions to
clean up (the curves are interesting to see: as the mixture gets
leaner, the HC output goes down nice and smoothly. When you get just
a smidge lean, the NOx suddenly skyrockets).

My recollection is that it all happened pretty close to
simultaneously -- and roughly a decade after Volvo started using O2
sensors.

 

The "lean burn" engine that was the subject of much research has never
been widely deplyed -- because it was incompatible with
catalytic converters, although there seems to be work on new cats to
resolve this:
http://www.lanl.gov/source/orgs/tt/...dex.php?fuseaction=home.viewTechnology&id=549

You may have had an engine that burned slightly lean, but I doubt that it
ran as lean as the target for the "lean burn" engine (generally in excess
of 20:1). In fact, I think that most cars were tuned to run slightly lean
before the introduction of catalytic converters.

 

Because combustion temperatures are strongly related to the mixture. A
lean mixture can cause very hot temperatures up to a point and then if
you keep leaning the temps come back down again.

This is car related directly, but the physics are the same - airplanes
often use a rich mixture for engine temperature management during high
power operations. I can certainly see a car engine management computer
having the capability to do the same.

Matt

 

Ah, I was talking about what Chrysler called a lean-burn engine when
they were building and selling them in the mid- to late-1970s. Note
that the catalyst patents you referenced don't actually say *how* lean
a mixture they're talking about (though they definitely are talking
about having excess oxygen), they just talk about effectively
catalyzing NOx.