Problem with 2017 300xs .. loss of 1000rpm

**Pachanga_rick** · 06-23-2018, 08:30 PM

Motor - Mercury 300xs , 30hrs.

First time taking the boat out this season and could only hit 5300 rpm, boat felt very sluggish but seems to idle fine.

I took it back to the house pulled the fuel filter, drained the lines to check for Ethanol damage and also replaced the plugs.

I pulled the prop, hooked it up to the hose, fired her up and kicked it into gear so I could rev it up and she sounded great and buzzed to 6450 with no problems.

Ran back to the lake and it ran strong up to 6400rpm/93mph. I drove around for maybe 35 min at various rpm ranges and she started feeling sluggish again and once again max rpm was around 5300.

Back home I pulled lines, drained the pump tank, pulled the filter and even replaced the squeeze ball.. back to the lake..

Still only 5300rpm.. it's under warranty still but if I can fix it quick myself I will..

Any suggestions?

**darkdream** · 06-24-2018, 04:26 AM

Looks like 1 off the ignition coils did the smartcraft give a alarm?

**Pachanga_rick** · 06-24-2018, 07:07 AM

No codes, no alarms.. that's the troubling part.

**2jz** · 06-24-2018, 08:11 AM

Originally Posted by darkdream

Looks like 1 off the ignition coils did the smartcraft give a alarm?

Agreed on coil. Coil dropping out when motor temperature under cowling gets hot typical of coil goin bad

**Pachanga_rick** · 06-24-2018, 10:36 AM

Would that not trip a smart craft signal, or do I need the Mercury computer to see deeper inside the motors computer?

**Davidlake** · 06-24-2018, 11:18 AM

Loosing a coil will not give a signal. Check the plugs and see if one if fouled. That will tell you which coil if it is one. Had the same thing happen on one of my 300xs's. Mercury replaced all six coils under warranty.

**Pachanga_rick** · 06-24-2018, 02:02 PM

Well I gave her a quick hillbilly tune up at the dock. With the prop off I reved up the to motor to 6500rpm with no load.

I took the boat back out and she picked up 300/400 rpm, but after a few minutes dropped back down to 5500.

I just replaced the battery on the off chance it some how caused it and will be heading out shortly.

**aggiestckl** · 06-24-2018, 02:07 PM

Check the diaphram of pump on the fuel rail. Ethanol corrodes this and I had same issue you describe

**Pachanga_rick** · 06-24-2018, 04:24 PM

Plug #6 shows a heavier carbon build up compared to the other 5.

**TraceF3** · 06-24-2018, 04:35 PM

I am having the same problem with a 2014 model. It's on a center console fishing boat. At times it won't even plane. Just coughs and bucks. Then other times it runs decent but just no power. The 225 EFI that I replaced ran as good as this 300. Been to the dealer 3x, (2 different ones). They've checked everything and still haven't figured it out. It's sooting up my prop too.

**Davidlake** · 06-25-2018, 09:21 AM

Originally Posted by Pachanga_rick

Plug #6 shows a heavier carbon build up compared to the other 5.

Buy a coil and replace the #6 one. Abut $130.

**Pachanga_rick** · 06-25-2018, 10:10 AM

Drove her over to the dealer this morning and hooked up to the computer.

No codes tripped other than the port cylinder bank is running cool (I'll clean the thermostat later tonight)

He went deeper into the computer while the engine was running and performed a neat little trick that he was able to check each coil bank individually and number 6 sounded a misfire. Still under warranty as the motor is not even a year old. He's checking some numbers and Mercury just might replace all 6 coils. But I wont know till later as I needed to get into work.

**Pachanga_rick** · 06-29-2018, 08:34 PM

Well #6 coil was replaced and new plugs installed.. Same problem.

Just for giggles I checked the compression on the motor while it was still warm. 80psi and 70psi on number 6.

Not happy.. motor has 32 hrs. Maybe my guage is bad, but maybe not.

**TraceF3** · 06-30-2018, 07:36 AM

Originally Posted by Pachanga_rick

Well #6 coil was replaced and new plugs installed.. Same problem.

Just for giggles I checked the compression on the motor while it was still warm. 80psi and 70psi on number 6.

Not happy.. motor has 32 hrs. Maybe my guage is bad, but maybe not.

I feel your pain. I'm going through the same issues. I've spent almost $1400 and they've tested everything... nest step is to pull the reeds to see if there's any defective.

If I were you I'd get a quality compression gauge and do the comp test again first.

**Euroski** · 06-30-2018, 03:09 PM

Originally Posted by Pachanga_rick

Well #6 coil was replaced and new plugs installed.. Same problem.

Just for giggles I checked the compression on the motor while it was still warm. 80psi and 70psi on number 6.

Not happy.. motor has 32 hrs. Maybe my guage is bad, but maybe not.

My 2007 300xs always had 115 +/- 3lbs even after 200hrs.