300xs losing power

[hotpony891]

I have an '07 300xs with 180 hours. When I run @ 4k rpms the motor runs great. But when the hammer is down and the boat is aired out @ 5,600 the motor runs great for 15-20 seconds but loses 1,000 rpms and boggs down. Is this a coil problem? I heard they are notorious.....

[Revolution141]

Oh man, I chased a gremlin like this for months!
I would start with changing the plugs. If that's no good probably a coil. The ****ter is you won't know which one it is because it's only breaking down at higher rpm. 2 options, buy 1 coil and just keep swapping it to another cyl each time out until problem goes away (can take forever) or buy a full set, swap the lot and probably fix your issues straight away.
only other things to look for are dodgy tps (I had one of these start going bad and would cause a similar problem but only at 100%throttle) or fuel supply issue.

good luck!

[DonG]

Had the same thing happen to me and it turned out to be a crank sensor.......

[Euroski]

Start with something easy and cheap. Run the fuel from a portable can with new hose to engine.

[F14]

check coils and TPS.

[R.grover]

Pull the plugs and look at them. Check your fuel system and run a remote fuel supply. check fuel psi I have had pumps loss fuel psi on these after a good hard run. is there extra soot around your prop etc?

[crankbearing]

What oil are you running in the motor? Have it scanned make sure they do a drop test on the cylinders with my CDI meds program it will give you graph per cylinder after the test and you can plainly see the map and which cylinder is dropping when compared to others I had this problem several times and in most cases I have found a couple bad air injectors, check for water in the air rail at the pressure fitting tap. All motors were having issues at high rpm actually a couple of them were down 8-15 MPH. Not saying it's not coil related but check this as well.

[Revolution141]

Head spinning yet! Could be any of the above!

[gregpro50]

I had something similar on my 250xs and it turned out to be a bad spark plug that was dropping a cyl. The plug looked fine. I have never in my life had a plug do what that one did. Of course the plugs have to be some special iridium tipped plug that no one had either.

[hotpony891]

head is spinning and no extra carbon around prop. I will invest in new plugs for $38 each! Then its on to running a separate fuel tank, then swapping coils.....and so on. I will post updates....

[gregpro50]

Where the heck are you buying your plugs !?!They should be like $12 ea. online.

[the Shocker]

How old is the boat! I bet it's the fuel line . If it's over 4 yrs old replace it with the upgraded line from a mercury dealer.They sell it by the foot. It's ethanol grade. I had exact same problem with my 225 opti. Just starved for fuel. Inside of hose was deteriorated and caused a flapper inside. My boat was an 2002.cheap fix.check the hole system right away. Make Shur no deposits are plugging filter.

[hotpony891]

that's what the local Merc dealer told me @ the beginning of the year...I almost fell backwards when he told me. I had them in my hand and gave them back....I guess I could try the fuel line too.

[Sleek Jr]

Did you ever figure this out? If so what was the problem?