2006 Mercruiser 496 HO Engine starts, rough idle, stalls Help!!!!
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2006 Mercruiser 496 HO Engine starts, rough idle, stalls Help!!!!

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    Default 2006 Mercruiser 496 HO Engine starts, rough idle, stalls Help!!!!

    Hello,
    My apology if this questions has been asked many times. I did a search but could not locate an answer.
    As the title indicates, my 2006 496 HO starts then stalls. I called for service. I was told they had to replace both fuel pumps (Low & high pressure). They contacted me said the boat is running. I went to start the engine -- same problem. The engine cranks then stalls within 5 seconds. The engine never runs normal then stalls, more link a rough idle. I replaced the IAC, no change. I try to advance the throttle while I'm trying to get the engine to run. It dies.
    I called the tech to come back to the boat. He connected his computer for diagnoses. It indicated both cam and crank sensors were bad. He replaced them. Now the boat will not start -- almost like its not getting a spark (where is that old distributor cap. I will see if its providing a spark!) I asked him if either one of the sensors he replaced could be causing the engine not to start? He said NO. He came back the next day. Spent 8 hours trying to figure out the problem. He disconnected all the electrical connections from the boat network to try to isolate the engine. He connected a start button to crank the engine. The engine would crank but never started.
    He said the boat needs to come out of the water (Formula 370 Super Sport). The engine needs a new wiring assembly. You have got to be kidding! The engine started before you messed with it. Now it will not crank over. So he said, I have no other option.
    Today, I decided to reconnect everything he disconnected. I removed the cam & crank sensors he installed. I looked at all the connections looking for any wires that might have been frayed/damaged/etc... I checked all the fuses, the lanyards, anything I found on the internet that provided some clues.
    The engine now starts (again) but stalls. I am back to where I started after spending $1300 for travel, labor, and parts to put me back at square 1 again. I am not a mechanic but I can read and follow directions. Can someone help me figure out what is wrong with my engine? Initially, everyone I spoke to said it was -- 100% the IAC. Now that I replaced it and the IAC did not solve the problem, what other options do I have? Here is a recap: both fuel pumps were replaced. Technician said the boat was running -- I doubt it!!. Replaced the IAC. NO change. Engine starts then stalls. Technician comes back. Replaces CAM and Crank sensor. Now the engines cranks but does not start. I removed the sensors he replaced and put back the old ones. The engine starts, rough idle, stalls. Where do I go from here?
    Thank you in advance for your recommendations.
    Jeff
    Last edited by Jeffrey262; 10-06-2013 at 05:39 PM. Reason: error

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    New here Beer:30's Avatar
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    Quote Originally Posted by Jeffrey262 View Post
    Where do I go from here?
    Thank you in advance for your recommendations.
    Jeff
    To another mechanic.

    I would check fuel pressure, KOEO. Should be 43psi or +/- 1 to 2.

    IF FUEL PRESSURE is good: See if it will start with throttle forward from idle about an inch or so. IF it does run, check the IAC WIRES. They have been known to fry, become brittle, and break.

    Have a certified mechanic with the Rinda software (on a laptop) hook up to the motor and see what codes have been set. There should be one or two. Write them down, clear them, and try to run again. See if any codes re-set.

    Check the TPS. My throttle position sensor went and caused kinda the opposite of yours. It would idle high and surge-idle. Made it hard to shift and dock. Replaced IAC, no change. Put in Irridium plugs, the surging got SMOOTHER, but still surged. Found TPS had stuck at 3/4 throttle position and was telling the motor to fuel and time accordingly. New $80 TPS sensor and all was fixed. Code said "TPS OUT OF IDLE SPEC". Told me right where to look!
    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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    Quote Originally Posted by Beer:30 View Post
    To another mechanic.

    I would check fuel pressure, KOEO. Should be 43psi or +/- 1 to 2.

    IF FUEL PRESSURE is good: See if it will start with throttle forward from idle about an inch or so. IF it does run, check the IAC WIRES. They have been known to fry, become brittle, and break.

    Have a certified mechanic with the Rinda software (on a laptop) hook up to the motor and see what codes have been set. There should be one or two. Write them down, clear them, and try to run again. See if any codes re-set.

    Check the TPS. My throttle position sensor went and caused kinda the opposite of yours. It would idle high and surge-idle. Made it hard to shift and dock. Replaced IAC, no change. Put in Irridium plugs, the surging got SMOOTHER, but still surged. Found TPS had stuck at 3/4 throttle position and was telling the motor to fuel and time accordingly. New $80 TPS sensor and all was fixed. Code said "TPS OUT OF IDLE SPEC". Told me right where to look!
    I will try advancing the throttle an 1" to see if it will start. I will check the IAC again. I looked at the wires yesterday. They were in great shape. Looked new. I will check the TPS. If I remove the IAC from the other engine, do I have to replaced it with a new gasket? I have no way of checking the oil pressure guage. My Formula 370 SuperSport does not have an oil pressure gauge. I will try to get the tech to come back out.

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    It's what we do BDMarine's Avatar
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    I agree with looking into the TPS. However find a certified mechanic with the Merc CDS. It will give way more information than the Rinda. We have both and when it is a 496 the CDS is the only choice.
    Your previous mechanic is just throwing parts at it hoping it will fix the problem. Find someone that can actually diagnose what the problem is.
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    Senior Member namba860's Avatar
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    Does you IAC motor use the little foam muffler. If so remove it from its location and then check it. I have seen dirty plugged up mufflers cause this and not let the engine idle.

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    Quote Originally Posted by Beer:30 View Post
    To another mechanic.

    I would check fuel pressure, KOEO. Should be 43psi or +/- 1 to 2.

    IF FUEL PRESSURE is good: See if it will start with throttle forward from idle about an inch or so. IF it does run, check the IAC WIRES. They have been known to fry, become brittle, and break.

    Have a certified mechanic with the Rinda software (on a laptop) hook up to the motor and see what codes have been set. There should be one or two. Write them down, clear them, and try to run again. See if any codes re-set.

    Check the TPS. My throttle position sensor went and caused kinda the opposite of yours. It would idle high and surge-idle. Made it hard to shift and dock. Replaced IAC, no change. Put in Irridium plugs, the surging got SMOOTHER, but still surged. Found TPS had stuck at 3/4 throttle position and was telling the motor to fuel and time accordingly. New $80 TPS sensor and all was fixed. Code said "TPS OUT OF IDLE SPEC". Told me right where to look!
    Quote Originally Posted by BDMarine View Post
    I agree with looking into the TPS. However find a certified mechanic with the Merc CDS. It will give way more information than the Rinda. We have both and when it is a 496 the CDS is the only choice.
    Your previous mechanic is just throwing parts at it hoping it will fix the problem. Find someone that can actually diagnose what the problem is.
    Quote Originally Posted by namba860 View Post
    Does you IAC motor use the little foam muffler. If so remove it from its location and then check it. I have seen dirty plugged up mufflers cause this and not let the engine idle.
    OK guys. I got the boat running. After swapping almost every part from the good engine -- IAC, CAM and Crank sensor, ignition switch, fuel filter, PCM, both fuel pumps,fuel rely, testing all connection for 5v connectivity to all connections, I put all the parts from the bad engine on the good engine. The engine started right up. None of the part were bad. The problem was the fuel pick up from the tank. It was cracked. All it was doing was picking up air. I don't recall which expert -- in this group, thank you very much, suggested that I put a 3 gallon tank between the boats fuel tank and the engine. After opening the fuel pump and filter and pouring some fuel into the compartment, then cycling the engine, the boat started right up.


    Here is what I can tell you. Never contact a mechanic to do work on your boat unless you have absolutely no mechanical skills. I never attempted to work on my engines because I was concerned I'd break something. I learned so much by asking all you for assistance. Your recommendations were very valuable. Even though it took me longer to figure out the issue, I know I can do a lot of the work myself now. Trust me, mechanics are very informative. Many of you might even be a service provider. I applaud your your skills. You help me figure this one out. However, the first service provider changed both fuel pumps. As I mentioned before, who knows if there were even broken before. I just received the bill $1243.35. I am waiting for the other service providers bill. They said I needed a new wiring harness, the boat would need to be taken out of the water (Formula 370 SuperSport), the engine would require the exhaust system to be remove, etc...The mechanic worked on the boat for 13 hours. Any guess how much his bill is going to be?


    Once again. I thank you all for your help.

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    Senior Member ol guy's Avatar
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    I am glad you got it running again! One thing I would check is the Dielectric grease on ALL connections. It is amazing how that electric lube will effect the function of the electronics. By changing out parts that are prelubed or old parts that still have the dielectric grease may fix a simple problem. Just a thought and good luck!! M

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    It's what we do BDMarine's Avatar
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    Quote Originally Posted by Jeffrey262 View Post
    OK guys. I got the boat running. After swapping almost every part from the good engine -- IAC, CAM and Crank sensor, ignition switch, fuel filter, PCM, both fuel pumps,fuel rely, testing all connection for 5v connectivity to all connections, I put all the parts from the bad engine on the good engine. The engine started right up. None of the part were bad. The problem was the fuel pick up from the tank. It was cracked. All it was doing was picking up air. I don't recall which expert -- in this group, thank you very much, suggested that I put a 3 gallon tank between the boats fuel tank and the engine. After opening the fuel pump and filter and pouring some fuel into the compartment, then cycling the engine, the boat started right up.


    Here is what I can tell you. Never contact a mechanic to do work on your boat unless you have absolutely no mechanical skills. I never attempted to work on my engines because I was concerned I'd break something. I learned so much by asking all you for assistance. Your recommendations were very valuable. Even though it took me longer to figure out the issue, I know I can do a lot of the work myself now. Trust me, mechanics are very informative. Many of you might even be a service provider. I applaud your your skills. You help me figure this one out. However, the first service provider changed both fuel pumps. As I mentioned before, who knows if there were even broken before. I just received the bill $1243.35. I am waiting for the other service providers bill. They said I needed a new wiring harness, the boat would need to be taken out of the water (Formula 370 SuperSport), the engine would require the exhaust system to be remove, etc...The mechanic worked on the boat for 13 hours. Any guess how much his bill is going to be?


    Once again. I thank you all for your help.
    Glad you were able to figure it out. It's a shame the first "mechanic" didn't check the fuel supply first when they replaced the fuel pumps. My guess is they checked fuel psi, as they should have, and determined the psi was low. The next step should have been to check the supply lines/pick up for an air leak. Then you would have been on your way and no wasted time.
    http://www.performanceboats.com//signaturepics/sigpic837_1.gif
    B & D Marine Performance
    El Monte, Ca.

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