Merc 496 engine alarm has me stumped

LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
edited September 2013 in Engine Discussions

I'm getting an engine alarm that I can't figure out.  Anybody wanna play?

2007 Rinker 370, Mercruiser 496 with Bravo 3 and Digital Throttle & Shift (DTS)

Both engines start and run normally.  After a few minutes at cruise speed, the starboard engine suddenly decreases power and sounds a constant-tone alarm. If I throttle back to neutral for a few seconds, the alarm stops.  If I go above 1000 RPM, the alarm comes back on.  But if I wait 30-60 seconds, everything goes back to normal operation.  I can nail the throttles and the boat jumps right up on plane as if nothing happened.

It happens at random intervals, and at any cruise speed above 3200 RPM.  Sometimes, I can run for 30-40 minutes with no alarm.  Sometimes, it happens within a minute or two.

My mechanic hooked up his code reader and found no error codes.  The only error message in the SmartCraft engine monitor is a message saying the pitot tube is not working, but that should not affect the engine.  

I have ruled out the following:

* It's not overheating.  The water temperatures are normal and steady.

* It's not a faulty neutral indicator switch.  I swapped the switches port-to-stbd and it did not change anything.

* It's not worn spark plugs.  I just replaced them.

It has to be some sort of intermittent sensor fault, but I'm stumped.  Suggestions?

Post edited by LaRea on


  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    the two that come to mind are low engine oil pressure, and low pressure sea pump.. also in there is exhaust manifold temperature..

    check the connections at the sensor itself for integrity- dielectric grease probably won't hurt anything while you're doing it.. use multi-meter to follow to the next connection, and make sure there aren't any shorts or bad connections at various harnesses or at the main PCM slave..
    Constant tone alarms are for:
    1.) engine coolant temp overheat,
    2.) low engine oil pressure,
    3.) engine over speed,
    4.) exhaust manifold cooling temp overheat
    5.) sea pump PSI low - See more at:
    Constant tone alarms are for:
    1.) engine coolant temp overheat,
    2.) low engine oil pressure,
    3.) engine over speed,
    4.) exhaust manifold cooling temp overheat
    5.) sea pump PSI low - See more at:
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Not a bad idea.  The water pumps and impellers are new, and I think the oil pressures are the same on port and stbd engines, but I will start looking at harnesses and connectors. 

    What I can't figure out is why no alarm codes show up anywhere.  I figured any condition serious enough to cut power and sound the constant alarm would drop a code.
  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    edited September 2013
    you're experiencing engine knock on your starboard engine... it will sound a constant alarm while in gear, and a 20 second alarm when moved to neutral, along with a 10% reduction in power....

    it won't set a code, as it monitors and doesn't perform a vital function, but it can be pulled using a scanner.

    engine knock (aka predetonation) is when the air:fuel load ignites previous to when it's supposed to ignite (dependent on timing, w/ top dead center being 0*)... what can cause predetonation?

    - overheating which makes the volatile environment of a cylinder even more volatile, and ignite upon compression instead of when the plug fires.

    - a hot spot inside a cylinder such as a plug electrode or point, a valve edge... (using too hot of a plug selection can cause this, and 'special metal' plugs such as iridium are notorious- iridium is longer lasting than copper is the single only advantage they have, but guess what? they have to be pulled and re-gapped at the same interval copper has to be replace.. copper = $1 a plug.. iridium = $6+ a plug.... you figure that one out.. :-) )

    - improper air to fuel ratios, lean mixes are more volatile than rich (seems counter intuitive, huh?)... gas is actually pretty stable, until it is ignited- the heavier the mix of fuel to air, the more predictable is its ignition point.. the lower the fuel, the less predictable... less predictable causes it to detonate before expected... octane or RAND counts are inert (doesn't burn) additives in the form of a chemical cocktail that makes gas even more predictable (by making it more stable............ low quality gas goes BOOM when it wants to go BOOM, much to the engines chagrin and often resulting in it's demise.. low RAND count (octane) is less controllable and therefor predictable, and can cause an engine to knock...

    - misunderstanding between actual engine temperature and registered temperature can cause pre-det, too.. the engine's trim function (via PCM) is trimming out fuel because it thinks it's hotter than it is, OR, the PCM is advancing ignition timing because it thinks the engine is cooler (and safer to advance timing with)... either can and will result in a knock...

    - using a higher than the PCM expects voltage off the coil, which can be magic if accounted for, hampers engines that don't account for it.. instead of a slower ignition of the air:fuel due to a smaller spark, which allows far advanced timing to be used as the burn can happen as much as 50* south of TDC, it provides a wider spark which more thoroughly ignites (more surface area) of the air:fuel load and creates a more powerful burn, but necessitates a later ignition (retarding ignition timing, or advancing to a lessor degree)

    - there are other things that can cause predet, too, i've just suddenly became too lazy to type them out. 

    these engines (unless catalytic engines) don't sniff exhaust.. they aren't capable of reading the o2 content of exhaust, and therefor don't 'trim on the fly' based on atmospheric conditions... it's trimmed via a formula existing on the PCM which monitors the engine temperature, the measure of mass air flow at the manifold, the temperature of air inducted, and it takes for granted the fuel pressure regulator is functioning within tolerances which provides the needed variable for injector pulse width (duty cycle, or how long the injector stays open)... 

    if you have a dead or dying injector, the engine can knock.... you'll feel a misfire...

    the knock sensor itself could be failing... that is a question, too... it would result in an alarm and a reduction in power if it is simply lying to the PCM, which it kinda sounds like it is if you haven't noticed any other symptoms.
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Great post!

    I have read about the infamous Cool Fuel unit, in which ethanol-induced paint flakes can damage the fuel pressure regulator and/or injectors.  I wonder if that's what is going on.  I actually replaced the pressure regulator in hopes of identifying the problem, but it didn't change anything.  Maybe I have some clogged injectors. 

    It doesn't feel like the engine is running rough.  Both engines feel the same, but I'm only getting alarms on the starboard side. 
  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    if it wasn't such a pain to do, due to location, I'd suggest you swap knock sensors between engines and see if it follows....

    they are located in the back of the block in the place of a drain plug... it's just above the oil pan lip...

    before you do that, though, check to see if the connection from that thing is seated and clear of corrosion.
  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    if you were leaning out an engine due to fuel pressure loss (or gain, for that matter), you would not only feel, but hear and actually see the engines running differently... or... with an IR heat gun, you could register a different temperature of the engines operating in duplicate environments...

    I'm thinking you have some wayward wiring.. or a faulted knock sensor...
  • Michael TMichael T Posts: 6,281Member ✭✭✭✭✭
    La Rea, what octane fuel are you puchasing? MT
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Generally 89 octane E10.  
  • Dream_InnDream_Inn Annapolis, MDPosts: 4,400Member, Moderator mod

    I'm a bit curious as well since I have the same engines as well.  It does not sound like the symptoms I've had when I had my fuel cooler issues. (I had no alarms, just one engine could not keep up with the other when getting on plane - but you are lucky if you hadn't had the cooler issue, both of mine are now changed)

    My bet is on a sensor going bad.

    Dream 'Inn III -- 2008 400 Express

  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    this thread prompted me to do some reading about issues with not only this engine, but all engines that use knock sensors and modules to monitor... for whatever value this has, and specific to merc engines, here you go:

    there is a programmed 10% reduction in power... that is scripted.. the purpose of the knock sensor is to determine if predetonation is present, and if so, how to manage it.. one of the things that can be done to thwart predetonation is to reduce the timing advance, which works in the case of inferior fuel... reducing the timing advance ALSO reduces power- which means, to me, that there will be MORE than a 10% reduction of power, when the two are combined (PCM restricting injector pulse, AND the reduction of timing advance lessening the power expected)....

    one item that causes a knock sensor to give bad readings is inference from other electrical items, such as (and dang near specific to) spark plug wires... so- make sure those things don't near the module, and if they do, shield them.

    on an aside, and maybe of value to folks who can't obtain recreational (no ethanol) fuel:

    other than corrosion and deposits throughout your fuel systems, ethanol can be managed, and here is how.... implement a hotter spark.. up your spark ignition voltage from 35~45k to at least 60k, AND widen your spark plug gaps to match... if @ 35~45k your plugs ask for .045 gap, bump it to .055~.060 along with the jump in voltage...

    ethanol is actually pretty stable, and has a value nearing blue fuel (108 octane) if the scales were to be translated to match.. that stability prior to ignition is negated by how much faster and hotter it burns when it's torched.. it doesn't store as much energy as gasoline as it explodes as opposed to burns really fast (like gas does)... that gives the piston face hella energy when it 'splodes, but it doesn't sustain the press like gasoline does as it continues to burn down the power stroke..

    if you use a hotter spark, the spark can jump a wider gap... a wider gap allows for more surface area of spark... a wider surface area of spark ignites a larger surface area of air:fuel, and allows a more complete burn.. it also allows a less advanced ignition timing to take place that nets the same initial press on the piston face, which translates to us- less likelihood of predetonation, and nearing same economy per stroke (volume of fuel burned- but this doesn't equate/translate to better economy in gph, per say, because the 'burn' is gone less than halfway down the pistons stroke with ethanol, while gas is pushing 65~75% of the way down, which means more torque per power stroke with gas than ethanol... ethanol initially punches the piston harder which is more powerful and awesome for short stroke engines, but it expends quicker than gas which is still pushing... in that extended push, gasoline makes up the energy lost in the initial punch, and then adds more to it)...

    ethanol laden gas will never give the same performance of gas in engines where torque is the prize, such as marine engines, truck or car engines that near 1:1 in piston face to stroke ratio... they'll do fine in other applications, though.. when recreational gas disappears, and all of us will have to start using e10 or God help us e15, the changes to wires, plugs, coils, ignition timing, and apparently shielding of inference will become a much more investigated subject. 
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Wow, a lot to think about. Based on these comments, the leading suspects are:
    * Intermittent fault in a sensor, e.g. knock sensor
    * Intermittent fault in wiring or connector
    * Plug wire interfering with knock sensor
  • explore7425explore7425 Newark, DEPosts: 150Member ✭✭✭
    I have the same exact boat with the same engines. I had those exact symptoms. Mechanic hooked up the computer. Not sure what codes came up but he felt it was the flow censor on the back of the motor somewhere. It senses the water flow on the raw water side. He swapped the sensors and then we knew. The other engine started to fail the same way. $90.00 part. he said we may have been able to clean it but I didn't want to take the chance. Just something to think about.
    Bella Sabrina II  370EC
  • AlswaggAlswagg Posts: 2,123Member ✭✭✭✭✭
    Wow, I am surprised at how many techs do not know how to read the scan tools.  First off,  attach the scanner and run the motor.  If an alarm develops, the scan tool will read.  Check the basics.  lower unit reservour bottle. first.  this is very common.  Next, water pressure sensor.  what does it read?  the Knock sensor will not go off until around 3K rpm.  that is where it takes a reading.   Next check your voltage, Many are still wired from the battery voltage and not the alternator voltage.  This can lead to failing fuel pumps and low voltage for the PCM.   IF you were to call me, I would bring a water pressure sensor and gauge.  I suspect this is your whole problem.  Good luck  Al
  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    for what it's worth:

    he stated an alarm, constant in gear, and that lasted a few seconds in neutral...

    that is why I went straight to knock sensor, as it appears to be the only one that does that- the chart says it will alarm in neutral for a 20 second period after alarming in gear constantly..
  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    I'd bet fairly confidently it has temperature and oil pressure... It may monitor for contribution...

    Do you have any pyrometers in the exhaust either before or after the turbo? If not, I'd tap one... The exhaust gas temperatures are likely the single most important reading on a diesel.. it determines load, but can also determine contribution issues such as a stuck injector before even the black sooty smoke appears, and hopefully before a piston cracks or a headgaskets join hoffa or harry (henderson, that is).
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Many thanks to all who have responded so far ...

    I did a quick check for loose cables or connectors, but saw nothing obvious.  Voltages are fine underway, and the lower unit reservoirs are not low.

    Next, I swapped the raw water pressure sensors (located on the power steering cooler at the back of the engine).  I'll take it for a cruise tonight to see if the problem moved with the sensor.   

    If not, I'll need to take my mechanic out for a cruise so he can have a code scanner hooked up while the alarm is going.  (Can't recreate the alarm in the slip.)  

  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Denied!  Swapping water pressure sensors didn't change anything.  The water pressure reads 1-2 psi at idle and 5-7 psi at cruise speeds.  

    I did note that the alarm does not go for 20 seconds.  If I immediately come down off plane, the alarm stops within a few seconds.  I can make it come back on by hitting the throttle again, but the alarm will stop 2-3 seconds after going back to neutral.  Once I stay in neutral or low idle for 20-30 seconds, it resets. 

  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    If it is a predet, generally an engine has to be under load to do it.. if it will truly predet without load, there is an explanation, and that explanation has strictly to do with ignition timing...

    I keep looking at the charts and trying to use the built in diagnostics to figure your issue, and it keeps pointing to knock sensors.. those built in diagnosis things may or may not be accurate, and may be misleading us..

    What do your oil pressures and temperature gauges look like when this thang happens?

    It's the reduction in power that is confounding if its done by guardian/pcm after detecting an issue.. if its not a sensor failing and the pcm isnt logging a fault, it could be we're charging up the wrong tree..

    Have you swapped fuel pressure regulators from side to side? A shortage of fuel could cause a knock as it leans out, and it leans out when it runs out of fuel.. there may be sufficient flow and volume at lower rev, but not enough up top..
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    I installed a new fuel pressure regulator and filter, and it didn't change the equation.  Under load, I'm running around 50 psi oil pressure and 150-160 deg water temp.  

    Just to recap the steps I took so far:
    * Replaced worn raw water pump
    * Checked outdrive lube reservoirs
    * Replaced spark plugs
    * Replaced fuel pressure regulator and fuel filter
    * Swapped gear selector switches
    * Swapped raw water pressure sensors
    * Basic visual inspection of wiring and connectors

  • 212rowboat212rowboat Posts: 2,095Member ✭✭✭✭✭
    If i ain't mistaken, you'll be wanting to see oil pressure at least 40psi at idle, and then rpm/1k+10 underway.. so, at 3000rpm, 40psi is sufficient.. at 4000 you'll be wanting to see 50ish psi.. if you're seeing high pressure due to load at lower rpms, your engine is a candidate for oil burning..

    The 'stat is rated 160*, which is to say it should be opening around there, and wide open around 170*... It sounds like you're running cool... Slightly, but cool nonetheless... What that means to an efi engine is sustained open loop...

    Open loop is logic the pcm uses when operating temperatures aren't yet reached... It dumps excess fuel in such a situation for two reasons, both important.. first is to make the burn more certain when spark hits it.. the second is to make sure it doesn't fire until spark hits it.. a predet on a cold engine is bad news and a good way to do some damage.. rich air:fuel is harder to ignite, but burns cooler and slower.. lean air:fuel is hella more volatile and is likely to explode more than burn.. exploding is bad... Burning is good.

    The sensors your engine uses to trim fuel, after operational temperatures are achieved and because the pcm can now trust the readings, are manifold absolute pressure (map), air intake temperature (ait) and throttle position... It lacks an o2 sensor to dial in the rest or to make it silly close to stoich.. so it will err on the side of rich than lean.. the constant the pcm needs (which is unmonitored) is fuel pressure to dial in the duty cycle / pulse width of the injectors.. this is why I asked you about that..

    Fuel... Engine needs it.. fire... Engine needs that, too.. air in air out.. is there any obstruction on that engines air hat?

    The alarm, by the way I'm looking at this over the electrons of interwebby, still sounds like a knock sensor.. dunno if its truly knocking or just a bad sensor.. running too cool would actually reduce the likelihood of predet, instead of promote it.. so if its predet, it likely has more to do with spark than fuel.. a wiggy crank position sensor can cause the mapping of a pistons location, and cause predet.. a wiggy map sensor can too (if the pcm is closing loop)...

    When you changed plugs and wires, did you by chance rotate the dissy at all? The dissy does nothing for timing on a pcm controlled engine, but it does affect fuel sync, or when the injectors fire.. if it was turned even 1*, your fuel sync could be whacked and advancing squirt, or regarding it.. both will cause knock at high rpm.
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    I didn't touch the distributor cap, but I'll add that to my list of items to check.  Maybe it's loose or rotated.

    I'll take a look at the air intake too.  Man, I really appreciate all these suggestions.
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Problem solved at last!  It was a partial blockage in the raw water pickup hose inside the Bravo III.  See my summary HERE
  • Michael TMichael T Posts: 6,281Member ✭✭✭✭✭
    Great post LaRea! My 2 cents - once in awhile run a tank of 91 through those 496s, they'll love you for it. Really glad you found the problem - it was driving me nuts and it's not even my boat! :-) MT
  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Thanks MT.  I am pretty thrilled about it!  We did a lot of great boating last year at 3200 RPM, but the sweet spot for planing trim seems to be 3500.  Today was my first chance ever to run the boat at WOT.  I had a grin from ear to ear at 43 mph!
  • Michael TMichael T Posts: 6,281Member ✭✭✭✭✭
    Oh yeah! -  43mph - that's haulin' a rig like yours! I can see your grin! :-) MT
  • boatmandanboatmandan Posts: 10Member
    Having the same problem.  Looks like it is the idle indicator button on the DTS controls mounted on the engine.  The pin gets stuck and it sends false readings to the computer and sets off alarms.

  • LaReaLaRea Alexandria VirginiaPosts: 2,606Member ✭✭✭✭✭
    Interesting - I haven't heard of that one before.
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