Logo
Get instant online access to repair manuals for your Mercury / Mariner outboard. Online Manuals
MercuryMariner
Order printed Mercury / Mariner service manuals — shipped to your door! Printed Manuals
MercuryMariner
Use engine diagrams to find & order parts for your Mercury / Mariner engine. Engine Diagrams
MercuryMariner
Find parts quickly with Mercury application guides. Quick Parts Guide
MercurySierraCDI
Results 1 to 16 of 16
  1. #1

    Default 2008 Mercury 15hp 4 stroke Slow at WOT

    Hey everyone first post and new to the boating world. I purchased a 2008 lund 12ft boat with a 2008 Mercury 15hp 4 stroke engine. When I purchased the boat the motor starts pretty quickly and runs perfect at idle even while revving. When it is put into forward the boat doesn't have the speed at WOT that it should have, 5mph tops. I had an uncle clean the carburetor and I changed the oil which was slightly over full. Some fresh 91 octane non ethanol fuel was put into the tank for the next ride. Took it for a spin and same issue happened. Strangely though, after an hour or so of riding around the boat magically came to life for 10-15 seconds. The RPMS seemed to get higher and the boat was rapidly speeding up, then unfortunately slowed down and back to 5mph. I loosened the gas tank cap thinking it may have been vapor lock and it didn't help. The primer bulb stays pretty much inflated the whole time with very little softness. Pinching the fuel line trying to starve it of fuel didn't seem to have much of an affect either. Any ideas?

  2. #2

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    I am having a similar issue with my 30 hp efi. I have double checked all the fuel system components and it all looks mint now. Still bogs under load. 4200 rpms max.

    At first I thought it might be safe mode, but I previously got it to run full speed after a fresh gear lube change and weighing the bow down at wot

    Think I have it narrowed to off timing, failing coil/s, or possibly something within the lower unit (least likely) since it seems to hold compression.

    I'd check plugs and coils, then move to timing. I would think electrical for sure if it suddenly ramped up out of nowhere.

    There are quite alot of these issues posted up on here but no solid answers.

  3. #3

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    How do you check for safe mode? How do you take the boat out of it of this is the case?

  4. #4

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    From what I've read, fix the issue to reset the error code - computer diagnostics needed or take shots in the dark like me.

    I've spent a bunch o bucks on gen maintenance and questionable parts. But haven't quite nailed it down.

    Upside- I have really gotten to know my motor which will probably save my butt later on - downside is it still doesn't fly.

    I would email or call around your local merc shops to see if they can do on water diagnostics- since it probably runs like a top in the driveway like mine and you would never know it had a problem.

    My Local mechanic said check timing, but I dont have a timing light. Also need to test my coils. Doing that tomorrow...

    If I figure mine out I will lyk

  5. #5

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    I borrowed my friend's multimeter. Checked my 3 coils and my #1 coil is reading 0.0 ohms resistance, should be 0.8 ohms or so. #2 & #3 coils read .7 -.8 ohms.

    No guarantee, because it is a boat, but I think I found my issue. OEM coil is coming in about 5 days

  6. #6

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Just bought a cylinder compression tester. Looks like both cylinders are at 40 and there is some oil on and around the spark plugs. Guess back to youtube and google.

  7. #7

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Quote Originally Posted by sheltz32tt View Post
    Just bought a cylinder compression tester. Looks like both cylinders are at 40 and there is some oil on and around the spark plugs. Guess back to youtube and google.
    I would think 40 is low for sure. Mine are all right around 95

  8. #8

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    I'm not sure if you got it fixed or not..but here is my two cents. The 15/20 hp has a decompression set for easier starting...hence, you may not be able to check the motor with a compression test.. Buy a can of Mercury Powertune spray....about $12...start the engine (while in water) and spray almost the whole can directly into the carburetor while keeping the motor running...should produce a fair amount of exhaust ...might only be able to spray spurts of it at a time, at first,,,,,as the can of spray is getting toward the end of it, spray the carb hard til the motor dies...pull both plugs and spray whats left, down into each cylinder......let sit overnight... Next day, run the heck out of it at WOT, preferably on your favorite lake....for a good couple of hours. I am trusting this helps...I dealt with this issue for months (on my 20hp 4 stroke) two years ago...was just carboned up...I hope this helps,,,,let us know. The Powertune solution came directly to me from one on Mercurys lead support engineers. He also said carbon build up is a problem for these four strokes.. One of the first signs of carbon is that you notice the motor doesn't quite seem to be moving you along as fast as it once did.
    Last edited by charlie_D; 08-09-2020 at 12:05 PM.

  9. #9

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Quote Originally Posted by charlie_D View Post
    I'm not sure if you got it fixed or not..but here is my two cents. The 15/20 hp has a decompression set for easier starting...hence, you may not be able to check the motor with a compression test.. Buy a can of Mercury Powertune spray....about $12...start the engine (while in water) and spray almost the whole can directly into the carburetor while keeping the motor running...should produce a fair amount of exhaust ...might only be able to spray spurts of it at a time, at first,,,,,as the can of spray is getting toward the end of it, spray the carb hard til the motor dies...pull both plugs and spray whats left, down into each cylinder......let sit overnight... Next day, run the heck out of it at WOT, preferably on your favorite lake....for a good couple of hours. I am trusting this helps...I dealt with this issue for months (on my 20hp 4 stroke) two years ago...was just carboned up...I hope this helps,,,,let us know. The Powertune solution came directly to me from one on Mercurys lead support engineers. He also said carbon build up is a problem for these four strokes.. One of the first signs of carbon is that you notice the motor doesn't quite seem to be moving you along as fast as it once did.
    Thanks for the insight! Just a quick update, I poured a small amount, ~1oz, Marvel mystery oil into each cylinder and let it sit overnight. Drained some oil this morning to make sure it wasn't overfull and rechecked the compression. Went up slightly to 50. Do you think a good carb cleaning may resolve the issue? I purchased a carb rebuild kit a few days ago for $65 but have yet to dissect after finding the low compression. I'm running to a parts store now and will see if I can find a can of powertune spray and grabbing new plugs.

  10. #10

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Carb cleaning??? It may help you, but that sure didn't help me with what was happening with my motor....I rebuilt the carb,..didn't work....took it to a mercury service center,,,,they reworked the carb, said everything was great.....I go to the lake..start it up...can't get the boat to plane.

    Powertune may have to be purchased from a Merc dealer or on Ebay



    Also, go to the "forum home" button and search my login ID charlie_d

    there you can read EVERYthing that I did and checked with regards to my motor....let us know what you find as you go through your journey

  11. #11

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    To get the Powertune well into the throat of the carb, I drilled a hole into the side of the plastic cover (which is over the intake of the carb) in order to get that plastic straw in to the carb a good distance. There is a tiny holed screen over in face on the intake of that carb...I used a small screwdriver to poke a small hole in it to pass the spray straw through

  12. #12

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Quote Originally Posted by charlie_D View Post
    To get the Powertune well into the throat of the carb, I drilled a hole into the side of the plastic cover (which is over the intake of the carb) in order to get that plastic straw in to the carb a good distance. There is a tiny holed screen over in face on the intake of that carb...I used a small screwdriver to poke a small hole in it to pass the spray straw through
    Read all your posts and seems very similar to mine. Just ordered 2 cans of powertune from Amazon Ill let you know by next weekend how it worked out.

  13. #13

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Charlie_d do you recommend just spraying at idle or at all rpm ranges?

  14. #14

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    You will have to work the throttle to keep the motor going once you start spraying.....if I remember correctly, I worked the throttle just enough to keep it running. I know I never went past half throttle...I had my motor in a big garbage can of water when I was spraying the powertune into the carb....if it dies while you're spraying, just start it back up and keep on shooting it in.....it is more like heavy doses of intermittent spraying than just trying to keep a steady stream of spray. When and if you get it to a lake to run it after spraying.....you must run it hard for quite a little while. Mercury staff told me to run a whole tank of gas through it at wide open throttle....the reason to run it for so long the first time out, was in order to get the motor hot enough and maintain a high motor temperature to reset the rings properly. I ran mine hard for about an hour and a half...that worked for me Sorry it took so long to get back to you, I was out of town all day. Best of luck to you! Would be interested in knowing what you find out. charlie_d

  15. #15

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Quote Originally Posted by charlie_D View Post
    You will have to work the throttle to keep the motor going once you start spraying.....if I remember correctly, I worked the throttle just enough to keep it running. I know I never went past half throttle...I had my motor in a big garbage can of water when I was spraying the powertune into the carb....if it dies while you're spraying, just start it back up and keep on shooting it in.....it is more like heavy doses of intermittent spraying than just trying to keep a steady stream of spray. When and if you get it to a lake to run it after spraying.....you must run it hard for quite a little while. Mercury staff told me to run a whole tank of gas through it at wide open throttle....the reason to run it for so long the first time out, was in order to get the motor hot enough and maintain a high motor temperature to reset the rings properly. I ran mine hard for about an hour and a half...that worked for me Sorry it took so long to get back to you, I was out of town all day. Best of luck to you! Would be interested in knowing what you find out. charlie_d
    This is what was replaced prior to the run today. All fuel lines, fuel filter, marvel mystery oil added to oil, new spark plugs, then 2/3 can of quicksilver Powertune was sprayed through the carb while under light load. Once the can had 1/3 left i took out the newer plugs, tilted the engine up and heavily sprayed multiple times into each cylinder. The plugs were then placed back in and it soaked overnight. This morning I started the boat in a bucket of water and let it run until the smoking stopped ~3 min. I turned off the boat and once again replaced the plugs. Took the kids and drove to the lake for the true test. Once in the water I instantly felt more power from the engine. Nothing changed on the throttle response but the rpms were going much higher and the boat lifted and was on plane in no time. I drove around the lake basically WOT for about an hour and the boat had no issues. Prior to the repair the 15hp Mercury engine would only get me 3ish mph with one adult, and post repair the boat with one adult and 2 kids hit 23mph! Charlie_D thanks for the recommendation and I have an extra can of Powertune on standby! I owe you a beer do you have Venmo?

  16. #16

    Default Re: 2008 Mercury 15hp 4 stroke Slow at WOT

    Quote Originally Posted by sheltz32tt View Post
    This is what was replaced prior to the run today. All fuel lines, fuel filter, marvel mystery oil added to oil, new spark plugs, then 2/3 can of quicksilver Powertune was sprayed through the carb while under light load. Once the can had 1/3 left i took out the newer plugs, tilted the engine up and heavily sprayed multiple times into each cylinder. The plugs were then placed back in and it soaked overnight. This morning I started the boat in a bucket of water and let it run until the smoking stopped ~3 min. I turned off the boat and once again replaced the plugs. Took the kids and drove to the lake for the true test. Once in the water I instantly felt more power from the engine. Nothing changed on the throttle response but the rpms were going much higher and the boat lifted and was on plane in no time. I drove around the lake basically WOT for about an hour and the boat had no issues. Prior to the repair the 15hp Mercury engine would only get me 3ish mph with one adult, and post repair the boat with one adult and 2 kids hit 23mph! Charlie_D thanks for the recommendation and I have an extra can of Powertune on standby! I owe you a beer do you have Venmo?
    I am so glad that took care of your problem! I can imagine the smiles that must have taken place at 23mph!! That Powertune works quite well in 2 and 4 stroke motors! It's time once again to be making memories with your family on the waters. Best wishes to you and yours! P.S.-Maybe a lemonade would be great someday, should we meet....also, no Venmo....imagining the smiles is more than enough

Similar Threads

  1. 2008 mercury 50 ELPTO 2 stroke
    By Totalonenxg in forum Mercury Mariner Outboard Forum
    Replies: 11
    Last Post: 07-06-2019, 08:33 PM
  2. 2008 Mercury 90 hp EFI four stroke
    By Jim Gallagher in forum Mercury Mariner Outboard Forum
    Replies: 7
    Last Post: 05-06-2016, 09:00 PM
  3. 2008 15hp 4 stroke won't fully turn
    By Mallard1 in forum Honda Outboard Forum
    Replies: 4
    Last Post: 11-26-2015, 10:49 PM
  4. 2008 Mercury 4 Hp 4 stroke Problem
    By ricky_bailey in forum Mercury Mariner Outboard Archive
    Replies: 11
    Last Post: 10-05-2008, 10:52 PM
  5. 2008 Mercury 4 Hp 4 stroke Problem
    By Ricky bailey in forum Mercury Mariner Outboard Archive
    Replies: 0
    Last Post: 10-02-2008, 03:39 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •