The Knock on Knock Sensors (and recent Kawasaki Ultra Issues)

Most modern four-stroke engines utilize a knock sensor to warn operators of possibly impending engine damage due to detonation.

Detonation, sometimes referred to as “pre-ignition,” occurs when fuel (and air) ignite before the spark plug in defiance of ignition timing. This artificially creates an entirely unwanted explosion which is then exacerbated by the actual ignition caused by the spark plug firing. And this, in turn, increases pressure within the combustion chamber to the extent that engine damage – sometimes catastrophic – can occur.

Detonation and pre-ignition are often used as synonymous terms. That is factually incorrect. Although similar, they are not the same thing. More on that later.

The knock sensor’s job is to alert the owner – and the watercraft’s engine control unit – to knock it off. This is not something to ignore. Rather, it’s time to throttle down and get back to shore to diagnose and make any necessary repairs.

Contents

1 Sensor Failure Run Rampant
2 Listen In and Learn
3 A Short History of Engine Knock
4 Detonation is Not Pre-Ignition
5 The Knock on Known Issues
6 Old Men Shaking Their Fist at a Cloud Dept.

Sensor Failure Run Rampant

In a great many cases, there isn’t actually a problem within the engine, but instead with the sensor itself. Whether it’s your personal watercraft, your two-wheeled vehicle, or your daily grocery getter, sensors often send false faults that throw your machine into “limp mode.”

When they work, sensors perform various important functions. All too often in my experience, however, sensors simply go buggy and begin sending false information to your ECU.

Listen In and Learn

It’s important for internal combustion enthusiasts to learn to discern engine problems by sound alone. You should develop an ear for how your engine sounds when properly tuned – and in turn, keep both ears open for unusual noises that develop.

If you’ve ever experienced putting a batch of bad fuel into your vehicle – or maybe started up your PWC after a long hiatus without changing out the old fuel – you have probably heard a bit of a knock or ping emitting from the cylinders when accelerating.

It’s not good for your engine. But hey, if you think it’s a bad thing out on the water, just imagine how bad it would be were you way up in the sky.

A Short History of Engine Knock

In the nascent days of aviation, changes in fuel to air mixture cost many pilots their lives. Air becomes less dense at greater altitudes as the number of gas molecules in the air decreases – literally, “thin air” results as one climbs a mountain.

This was deadly to pilots in an era when the science of carburetion was still finding its way.

A once celebrated inventor, Thomas Midgely Jr., a graduate of Cornell University with a degree in mechanical engineering, was working in the research department at Dayton Engineering in 1916 when he began to develop a way to address engine knock. (This development would lead to his being reviled by environmentalists in current times, but let’s not get that far afield…)

It was during the First World War that Midgely began researching how aviation fuel could be made to burn more consistently by hydrogenating benzene. Later, in the post-war years, he concentrated on solving the then common problem of knocking in automotive engines. He discovered that the knocking was caused by fuel mixtures burning unevenly.

With a number of then knocking test engines made available by General Motors for his research, Midgely came to the realization that the addition of tetraethyl leads to gasoline virtually eliminated engine detonation.

While this would give pilots at the stick of Allied warplanes a significant advantage during the Second World War, arguably tipping the scales of the outcome of the war and doubtless saving many pilots’ lives, it was soon revealed that tetraethyl lead was a very serious poison.

The production and handling of this chemical had actually caused the death of several lab employees in the late 1920s, and at one point Midgely himself had to take a long “vacation” to recover from lead poisoning.

Ten years later, however, the war took precedence. And for that matter, automotive fuels continued to utilize the additive until the early 1970s. In the interim, lead additives were variously blamed for blood and brain disorders, lowered IQ levels, antisocial behavior, and more.

Good for engines, but bad for people. When non-lead alternatives were mandated in 1972-73, horsepower numbers in automobiles dropped precipitously. Leaded additives nonetheless weren’t phased out of gasolines intended strictly for closed course racing until after the most recent turn of the century (and are still used surreptitiously in certain circles.)

Detonation is Not Pre-Ignition

Internal combustion engines operate by first compressing a fuel air mixture, and then igniting it at the appropriate point in the motor’s rotation.

It has long been routine for high performance engines – be that on land, in the air, or on the water – to operate on the threshold of detonation. This is because high compression develops high levels of torque, and thus horsepower.

Pre-ignition is similar, but different. With pre-ignition, some factor – it could be a carbon deposit, or even a spark plug that operates in too hot a heat range – ignites the fuel-air mixture before the actual planned ignition spark occurs.

Properly tuned, the spark of ignition ignites the fuel mixture at the proper point in crankcase rotation and combustion chamber compression, and the resulting flame expands and compresses any unburned fuel ahead of it during the exhaust cycle. The flame front burns the charge all the way to the cylinder wall, clearing the way for the next planned “explosion” following the intake cycle.

Less welcome explosions occur when an unburned charge ahead of the flame front is already abnormally heated due to a number of factors ranging from an excessive compression ratio for a given octane fuel to an engine that has overheated due to other reasons.

In most engines, the “flame front” of ignition burns the fuel-air mixture at a rate of somewhere between 50 and 200 feet per second. But when the unburned charge is already too hot prior to actual ignition, spontaneous combustion can occur – and this can happen to an extent where the strength of the explosion becomes a real explosion, traveling as fast as 3000 feet per second. The resultant shock waves can destroy an engine almost as quickly.

That’s a lot of information, perhaps, but fair warning: Don’t ignore your knock sensors.

The Knock on Known Issues

Well, actually… yeah, don’t ignore them. But feel free to accuse them.

Every PWC manufacturer has had a particular problem with certain models of certain years that had owners all up in arms about impending engine explosions – and in nearly every case, anecdotal or in total, the issue has been faulty knock sensors.

The most notorious of late is Kawasaki’s Ultra 310. Message boards are alight with owners with pitchforks and torches in hand.

But yeah, once again, it appears that the problem is not actual engine knock, but the circuitry designed to warn the owner of engine knock. Oh, joy.

Kawasaki has – after agonizing delays for Ultra owners – finally acknowledged they have an issue. The company says it’s not the sensor itself, and has instead issued a service bulletin to its dealers authoring an engine control module reprogram free to all owners of the Ultra 310. So, I’d have that, if I had a Kawasaki.

I further suspect that they did in fact have a bad run of sensors from their supplier – Mitsubishi in this case, but it happens. No finger pointing here. Anecdotally, including one of the guys I ride with sometimes, relief was sometimes found in simply replacing the old knock sensor with a new part in the many months before Kawasaki admitted the problem existed.

If you’re experiencing problems with your 310, contact your dealer and make an appointment to have the software reflashed. They should be happy to accommodate, and can do it in mere minutes while you wait.

Old Men Shaking Their Fist at a Cloud Dept.

Listen, I’m probably a bit atypical of most PWC owners. As a nearly lifelong motorcycle racer, personal watercraft rider and writer, and performance car enthusiast who grew up in his granddad’s motorcycle/automotive dealership and hanging around with greasy ill-tempered mechanics after school every day, I can be a bit of a curmudgeon when it comes to all these idiot lights.

I’m also one of the first guys most people around here turn to when they perceive a potential problem with their vehicle – or rather, are alerted by some damned infernal buzzer.

I’ve got more seat time than most, and I can easily tell when an engine isn’t performing properly without the help of a flashing light on the dashboard. And I think more riders and/or drivers should strive towards more awareness of how their machine performs from one time to the next.

Listen with your ears, and listen with the seat of your pants.

Modern automobiles and trucks have literally thousands of sensors – almost more sensors than they have moving parts, it seems. A modern personal watercraft has many hundreds, and that’s certainly many hundreds more than your dad’s car had.

Currently, I drive a 10-year-old Toyota Tundra. And yes, I’m more than a bit disgruntled that after nearly 200,000 trouble-free miles, it’s suddenly throwing an air intake code that puts it into limp mode.

There’s nothing wrong with the intake system. I know that, and I don’t need the engine light starting to flash to tell me that my 5.7 liters Tundra has suddenly gone from a really strong running truck into something that will barely peel the paint off of a Prius. What I do need – and fortunately have – is a code reader that allows me to kill the offending signal, at which point the truck runs great. Until it of course throws the code again.

It’s unfortunate that keeping my code reader in the console isn’t really a great long-term solution. It’s quite a bit more unfortunate that the damn sensor that needs to be replaced is located beneath the intake manifold. So, yeah, that’ll be an all-afternoon session of wrenching and cussing loudly as I’m not about to pay the Toyota dealer thousands of dollars to rectify the problem when I can damn well do it myself.

Happily, most knock sensors on personal watercraft are easily accessible, easily replaceable, and more affordable in all likelihood than the tank of fuel it might take to haul it to the dealership for inspection – and certainly far less expensive than the resulting service bill.

There are no money-back guarantees on the Internet, you understand. Don’t come crying to me if your motor does grenade – unless you’re the guy who lives just across the lake that I talked with the other day about his boat issues. He might have a case.

But yeah, with my two cents, if I were having a recurring fault with a knock sensor, I’d certainly give a new sensor a go before paying someone with the technical skill that they deserve to tear into the engine internals. I’m betting on a faulty sensor, every time.

And if I owned an Ultra 310, I’d simply trailer it to the dealer for an ECU reflash, and then drop it into the nearest lake for a high-speed test ride – after first canceling out the erroneous fault code on my trusty tow vehicle, of course.