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  1. #1
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    Default Mercruiser 8.1 smelly exhaust

    My buddy has a cruisers 340 with 8.1 mercruisers. The port engine only turns 3600-3700 rpm but the starboard runs to 4300. The exhaust smells like unburned fuel on port side as well. We see no gas sheen on water however. Any ideas on what the problem could be. We are thinking maybe one cylinder is down but I would think we would see a gas sheen in exhaust. As well we definitely have an issue with debris clogging filters in starboard side(the side that runs up to 4300). The problem is it appears there is no way to remove pickup tube to see if the pickup tube screen is clogged(if there even is a screen). That engine will run fine for 15-20 min then start to die out. It will idle but won't run up. If we let it sit for 5-15 min. It will run fine then do the same thing. Are we missing something about removing pickup tube? It can't come straight out because the deck is right above it.

  2. #2
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    Spotswood, NJ, USA
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    Default Re: Mercruiser 8.1 smelly exhaust

    clogged tank vent might do this

  3. #3
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    Default Re: Mercruiser 8.1 smelly exhaust

    Post a serial number.

    Does your 8.1 have a gen3 fuel cooler?
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  4. #4
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    Default Re: Mercruiser 8.1 smelly exhaust

    My buddy removed the fuel shutoff solenoid on top of the starboard tank. It was packed almost full of redish crud. He ran for 1/2 hour and we are cautiously optimistic the stumbling is fixed. Now to move to port engine to find out the smell and the rpm out of spec. Not sure serial number, Chris. I think he said boat is a 2006.

  5. #5
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    Default Re: Mercruiser 8.1 smelly exhaust

    Ok, back with some more info. Starboard engine appears fixed. Now, port engine has the smelly exhaust and it runs rough. To the point where after 20 min it just would barely run. We removed all spark plugs which were brand new last fall. They are all carbon fouled. Completely black after no more than 5 hours run time. We put new plugs in and started it up. We felt the exhaust manifold right where it bolts to the head. The starboard bank front cylinder was hot but the last three on that side were cold. We also think the second cylinder back on the port side of that engine isn't firing. So 4 out of eight not working. We pulled a coil from the good motor but it still didn't change. We changed ecm from good motor but no change. What would be causing the motor to foul plugs so fast and why are so many cylinders not firing? Compression test was ok.

  6. #6
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    DC Burbs/Maryland Eastern Shore
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    Default Re: Mercruiser 8.1 smelly exhaust

    I would hazard a guess that the same crud has fouled the injectors. If the nozzles are dribbling and not atomizing, you would get a lot of unburned fuel. I would pull all the injectors and have them cleaned and tested.
    Practicing the ancient art of ren-ching

  7. #7
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    Default Re: Mercruiser 8.1 smelly exhaust

    Quote Originally Posted by sammi View Post
    Ok, back with some more info. Starboard engine appears fixed. Now, port engine has the smelly exhaust and it runs rough. To the point where after 20 min it just would barely run. We removed all spark plugs which were brand new last fall. They are all carbon fouled. Completely black after no more than 5 hours run time. We put new plugs in and started it up. We felt the exhaust manifold right where it bolts to the head. The starboard bank front cylinder was hot but the last three on that side were cold. We also think the second cylinder back on the port side of that engine isn't firing. So 4 out of eight not working. We pulled a coil from the good motor but it still didn't change. We changed ecm from good motor but no change. What would be causing the motor to foul plugs so fast and why are so many cylinders not firing? Compression test was ok.
    Put a fuel pressure gauge on the rail and report your findings.

    Find the serial number.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  8. #8
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    Default Re: Mercruiser 8.1 smelly exhaust

    Chris, We were looking for that fitting but didn't see it. Where would we find it?

  9. #9
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    Default Re: Mercruiser 8.1 smelly exhaust

    Quote Originally Posted by sammi View Post
    Chris, We were looking for that fitting but didn't see it. Where would we find it?
    Right behind the coolant reservoir. You have to loosen the reservoir and pull it forward.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  10. #10
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    Default Re: Mercruiser 8.1 smelly exhaust

    Awesome. Thanks.

  11. #11
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    Default Re: Mercruiser 8.1 smelly exhaust

    Ok, we removed the fuel rail completely and found injectors clogged up with crap. We cleaned rail, installed new plugs and injectors. Now they all fire up but when we start it it runs for a few seconds and dies, start it up again and it runs a little longer and dies, then runs a little longer still, then dies, till finally it runs. We put a gauge on the rail and it read 40ish but the gauge was leaking badly so we quit until we can get a better gauge. Any idea what would cause this. It seems like it losing prime somehow. Sorry Chris, I still don't have a serial number but pretty sure it's after the cool fuel paint problem if that is what your getting at.
    Last edited by sammi; 09-14-2017 at 12:43 PM.

  12. #12
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    Default Re: Mercruiser 8.1 smelly exhaust

    Unless your cooler is around 2-3 years old, you have a paint problem. Pull the regulator out of the cooler and see what comes out with it.

    It is important that you get a good fuel pressure gauge on this engine and take readings at idle and while you are running under a load.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  13. #13
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    Default Re: Mercruiser 8.1 smelly exhaust

    Will do Chris. Good to know. I thought the problem was fixed by 2006.

  14. #14
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    Default Re: Mercruiser 8.1 smelly exhaust

    Ok, got a good fuel pressure gauge and at when we turn key to on to engage fuel pump but don't start it, it reads 38. When we start engine at idle it rises to 75!

  15. #15
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    Default Re: Mercruiser 8.1 smelly exhaust

    75 psi is way too high. On the back side of the Module is a brass plug(High Pressure relief valve). Inside that plug is a spring and a check ball. Pull the plug and the spring and don't loose the ball. Look at the plug, you will see lines cut in it from the spring. Sand the brass plug smooth. Reassemble the plug, ball goes in first, the spring goes on the plug and then screw it in with some thread sealer.

    On top of the module is a regulator held in with 8mm bolt, remove the regulator and clean out the paint.

    You should replace the top of the module with the updated one and have the injectors cleaned.

    Once you get this all cleaned, retest and report what you find. Fuel pressure should rise to around 43psi with the line off the regulator and run around 38-39psi at idle.


    Anything over 43psi is a clogged regulator. Anything over 60psi is the pressure relief on the back side of the module. While the engine is at idle.

    The fuel pressure will go over 100psi while running down the bay when the relief valve is stuck.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  16. #16
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    Default Re: Mercruiser 8.1 smelly exhaust

    Thank you Chris!

  17. #17
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    Default Re: Mercruiser 8.1 smelly exhaust

    Ok, talked to my buddy who owns the boat and he says it doesn't have the Gen 3 cool fuel module. It's a 2003 and he has the kind that looks like a oil cooler. Long and thin. Does that have a similar relief valve somewhere?

  18. #18
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    Default Re: Mercruiser 8.1 smelly exhaust

    This is why we need a serial number.

    So, It has a gen 2 cooler. There is a regulator on that cooler and inside the regulator is a small screen that will clog and shoot the pressure up.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

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