Plug Knock

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

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:

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.