496 HO Problems - Help Please
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496 HO Problems - Help Please

  1. #1
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    Default 496 HO Problems - Help Please

    Have a 2001 Baja with a 496HO. It idles fine, runs great at mid-range rpm (3000-3800), runs great at higher rpm (3800+) occasionally if engine is cool. Once the engine warms up, if you try to maintain an rpm higher than 4000 it starts cutting out. It doesn't seem to backfire, it doesn't drop down in rpm more than a couple hundred rpm and usually doesn't drop at all. If it's already warm it just won't go past 4000 rpm. It varies though, sometimes it won't hold 3800 rpm, other times you can hold it at 4000 rpm. I was having this problem the last time I took it out, only the problem was at a higher rpm (4600). Since then I switched to a higher pitch prop (for other reasons, unrelated to problem) and now the problem is happening at a lower rpm range.

    I have changed the fuel filter, used fuel from 3 different stations since the problem started. Have talked to several mechanical boat friends, some are sure it must be bad fuel others are thinking bad plugs. Existing plugs are around 100h old. I do not have a scanner to put on it (a purchase I need to make, I know).

    Any ideas on which direction to go here? Any info is much appreciated.

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    It's what we do BDMarine's Avatar
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    Purchasing a scanner will most likely not give you the answers you are looking for when dealing with a 496. The only program that has any chance of giving real time information is the Mercruiser CDS. It is extremely expensive and if you're not an authorized Merc Dealer you won't be able to web sync and it won't allow you to use it. Take it to a Mercruiser Premier Dealer and pay them to scan it. If the answer is stored in the PCM, that is the only way you'll retrieve it.
    Your problem sounds like the available power is being reduced due to information the PCM is receiving, probably from a sensor. If that's the case, it will tell you why.
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    Senior Member HypNautic's Avatar
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    I believe there are two fuel filters. There should be a disk type filter on the bottom. If there is, that rarely gets changed since it's a Merc item only and runs $45+.
    Maybe a fuel pump issue?

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    New here Beer:30's Avatar
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    Any of that is viable - especially getting it scanned. HOWEVER, it should be beeping like crazy. It sounds as if it's in guardian mode. There is a problem and a code has been set. See what the code is and it should point you in the right direction.

    When was the sea-pump serviced last? Most commonly, this problem is low water pressure. There's a sensor on the sea-pump and if there's not enough pressure (worn impeller), Guardian kicks in and limits RPM and power until it has sufficient water.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Thank you all for the input. As for the scanner, I guess I had a wrong impression of what they could do. Sounds like step 1 is going to be towing it 2 hours to a Merc dealer. Was hoping it would be something simple, and could avoid that.

    I was not able to find 2 separate fuel filters Hyp, just the one canister type filter. I emptied the fuel from the filter into a jar to examine, allowed time for separation and all looks perfect there.

    With the alarms Beer, that was one thing that was throwing me, there has been no alarms. The last time I wore out the sea water pump (less than 100h ago), my engine overheated and alarms went off. My engine temp stays perfect at 160 regardless of rpm. I wasn't sure if the alarm would sound if the knock sensor was bad or if the low water pressure sensor was being activated, causing it to go into guardian mode. Is it possible for it to go into guardian mode without sounding the alarm?

    Overall this does not sound like bad spark plugs to you guys?

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    New here Beer:30's Avatar
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    Quote Originally Posted by RollOn View Post
    Thank you all for the input. As for the scanner, I guess I had a wrong impression of what they could do. Sounds like step 1 is going to be towing it 2 hours to a Merc dealer. Was hoping it would be something simple, and could avoid that.

    I was not able to find 2 separate fuel filters Hyp, just the one canister type filter. I emptied the fuel from the filter into a jar to examine, allowed time for separation and all looks perfect there.

    With the alarms Beer, that was one thing that was throwing me, there has been no alarms. The last time I wore out the sea water pump (less than 100h ago), my engine overheated and alarms went off. My engine temp stays perfect at 160 regardless of rpm. I wasn't sure if the alarm would sound if the knock sensor was bad or if the low water pressure sensor was being activated, causing it to go into guardian mode. Is it possible for it to go into guardian mode without sounding the alarm?

    Overall this does not sound like bad spark plugs to you guys?
    On 2001-2002 (and early 2003), there is only one filter. There is a SCREEN at the inlet of the first pump, which you can find easily: Follow the hose from your switch valve (if you have dual tanks) or the hose from the tank (if just one) to the inlet at the motor. That inlet to the pump has a small stainless screen. Mine had nothing in it at 250 hours.

    Mine was surging at idle, but no alarm. Scanned it and it had still set a code of "TPS out of limit". Ends up the TPS (throttle position sensor) had stuck at 3/4 throttle position and didn't follow the throttle blade back down each time. So, the sensor was telling the ECM to fuel and time the motor for 3/4 throttle, but the stick/throttle was at idle. It gets real interesting when you have blower-surge with no blower!

    Anyway, replaced the $90 TPS and it was all good.

    It doesn't sound like fouled plugs and I have dealt with fouled plugs on an HO before. A friend's boat (they actually just sold it) had a 2003 HO and it started fumbling during transition from idle to plane and the idle itself was very rough - shaking the boat. Pulled original plugs - fouled. Put new Autolites in, fouled again in 5 hours. Next question was - WHY?

    Well, my THEORY is: HOs are set up on the rich side of the fuel curve to keep it alive with all of us throttle-jockeys. They had muffler-tips (full-time) on the boat, still had the turbulators in, AND long S-pipes to get the exhaust down below the swim-step. All of that restriction caused an even richer mixture. SO:

    I had just seen a dyno-test on the (new at the time) E3 spark plugs. I told the owners it was worth a try to see if the larger spark kernel would help avoid said fouling. Well, THEY DID! When they sold the boat, they had 100 hours + on those plugs with ZERO fouling problems. NOW, some on here will scream and jump up and down that E3s WILL NOT WORK in a 496HO. I'm here to tell you that they do / did / and if you don't believe me, I will give you the phone number of the owner of that boat and they can tell you themselves.

    So, feel free to try that in the mean time. They are about $4 a plug. Maybe $6 at the most. They are SUPER EASY to change, as long as you have ample access to the sides of the motor. Bottom line - - THEY WON'T HURT A THING and you can rule-out fouled plugs altogether. You got nothing to lose.

    The other thing that this SOUNDS like is water in fuel or just old / bad fuel. I always have remnant fuel leftover from previous season. The FIRST TRIP OUT the following season - I dump the largest allowed load of LUCAS injector cleaner in both tanks. Usually two of the tiny bottles, or about 12-15 oz per tank. This helps the old fuel and keeps the fuel system clean at the same time.

    Make sure you are only running 87 octane in the boat. The timing will never optimize if you run higher octane. This will also cause slightly rich condition that could add to plug fouling.

    So, there's a few things to try before having it scanned.

    This has some more info as well; http://www.performanceboats.com/outd...er-needed.html
    And this one: http://www.performanceboats.com/bob-...tarting-2.html
    Last edited by Beer:30; 06-03-2014 at 06:12 PM.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Thanks Beer. I'll likely try the plugs next weekend when I have it out again.

    I have ran through and replaced 30 gallons of fuel 3 times now since the problem started. I always add 2 bottles of the Lucas treatment when I fill up too. So, I am thinking that should have cleared any old fuel issues..? Is there an easy way to check fuel pressure? Or is that a likely cause based on the symptoms?

    I do usually use high octane fuel(93-96), as recommended by the previous owner, but I'll try some lower octane next time I fill up.

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    Quote Originally Posted by RollOn View Post
    Thanks Beer. I'll likely try the plugs next weekend when I have it out again.

    I have ran through and replaced 30 gallons of fuel 3 times now since the problem started. I always add 2 bottles of the Lucas treatment when I fill up too. So, I am thinking that should have cleared any old fuel issues..? Is there an easy way to check fuel pressure? Or is that a likely cause based on the symptoms?

    I do usually use high octane fuel(93-96), as recommended by the previous owner, but I'll try some lower octane next time I fill up.
    If you have never ran 87 (or so) octane, you probably did kill the plugs with retarded timing / in complete fuel burn. You've also been leaving a bit of power on the table.

    If you take the cover off of the motor (mercruiser plastic cover), you'll see the Schrader valve on the fuel rail. I bought a master fuel pressure test kit, which has all kinds of adapters and large precision gauge. If you were trying to troubleshoot this on the phone with a Merc tech, they would FIRST ask you to verify fuel pressure. If the 43psi is not there, it would point to fuel pump or other fuel-related problem. Once the 43 (KOEO) pressure is there, IF IT STILL didn't run right, you have ruled out fuel supply and would carry on to checking codes in the ECM to see what has gone wrong.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Senior Member roughneck427's Avatar
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    If its a MEFI 3 controller TTSdatamaster will log it I have a free demo copy that Gary at TTS said I could share. They did give me a full registration for beta testing it I used a cable from Moates.net that I had already and made a conversion connector from a car aldl to the 10 pn Delphi I can give you the info on what to do the cable is only $80 and you get 20 free recording runs after that its just monitor. PM me your email if you want a copy of scanner software .Attached is a screen shot of some idle tuning I was doing in my driveway with the scanner.
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    New here Beer:30's Avatar
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    I wish. No, it's not a MEFI-3, it's a PCM555 (Motorola). If it were a MEFI, we could adjust parameters ourselves. Being a 555, only Mercury themselves or Whipple Industries has the software/hardware to adjust the programming.

    I checked with Whipple and their normal performance reprogramming for these units is $900~ish. For $1500, they will have you drive the boat and adjust the ECM customized to your boat, your driving, in real-time.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Senior Member roughneck427's Avatar
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    Oh ok gotcha yeah I have mefi burn for my boat and I use team viewer to remote tune as well. Heck that ecm looks just like a dodge cummins unit. or JTEC unit used in Chrysler stuff.
    Last edited by roughneck427; 06-06-2014 at 06:29 PM.

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    Drained all existing fuel, replaced with fresh 87 octane, and replaced spark plugs. Headed back to the lake today for a test run, finally.

    Spark plugs had definitely been seeing some heat. They were changed last year, less than 100h ago. But even still, insulators were blistered on a couple, and some substantial ash deposits built up on several. No obvious major faults on any of them though. Still hoping that was the problem. I'll see here shortly..

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    New here Beer:30's Avatar
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    Quote Originally Posted by RollOn View Post
    Drained all existing fuel, replaced with fresh 87 octane, and replaced spark plugs. Headed back to the lake today for a test run, finally.

    Spark plugs had definitely been seeing some heat. They were changed last year, less than 100h ago. But even still, insulators were blistered on a couple, and some substantial ash deposits built up on several. No obvious major faults on any of them though. Still hoping that was the problem. I'll see here shortly..
    Well, without knowing the condition of the fuel pump - I would guess you were killing the plugs with the high octane not letting the timing settle to optimum. It keeps trying to advance up to the edge of knock/ping. If it never finds that (91 octane) - it keeps advancing and hunting.

    Let us know how it comes out.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Well that all did a whole lot of nothing. Still cuts out at around 3800rpm. It definitely needed the plugs anyway though. With no alarms, I guess weak water pressure or weak fuel pressure are my next most likely causes.

    Not too familiar with marine fuel systems, I assume they are the same as any others though. I have a Holley high performance 12v fuel pump from an off-road racing truck of mine. Would there be any downsides to using it versus what is there now?

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