Loss of Power in Mercury 115hp EFI 4-Stroke

colbysmith

colbysmith

Active member
At a loss on what happened to my outboard. It had been running fine. However, the other day it started acting like it was running on only 3 of 4 cylinders. It all started after powering the boat off the trailer, and I mean powering....as in full throttle. Guess I should have backed the trailer in a little farther in the first place, but I was solo, and lazy. I don't know if anything bad happened to the motor then, or if it was just a coincidence that my loss of power started after that. I had also just ran both fuel tanks pretty low before that, completely draining one tank. Before the powering off the trailer, I filled only one tank with Premium fuel without ethanol and treated with the Marine Stabil. The motor seems to vibrate more at idle and no wake speeds, and will only get up to about 4000-4200 RPM, slowly. I can cavitate it up to 6000, but loaded 4300 is all she'll give me. The prop looks fine, and there was no known damage to it. Today I filled the other tank with premium no-ethanol fuel from another station and put stabil in it. (Getting ready for the winter storage season here in the midwest, so will run the premium fuel now until I put it away. Otherwise I run regular with 10% ethanol with no problem.) I also changed the main fuel filter, but didn't see anything in the old that would restrict fuel flow. I pulled the plugs last night and those all looked fine as well. I do still have the two inline filters to change, but doubt they are the issue, since the main filter didn't seem very dirty. It just really seems as if the motor isn't operating on all 4 cylinders. But it still runs relatively fine, except for the loss of power. I'll be calling some shops tomorrow to find the first one to get it into, but any suggestions for now? I also think I'll look into purchasing the Engine Diagnostic software as well, as it seems like in the long run that would save me from having to pay the shops to hook up their computers... Has anyone else bought their own diagnostic software, and how do you like it? Colby
 
Sometimes you can get a carbon whisker on a spark plug. With the motor running pull one spark wire plug at a time. If you pull one and nothing changes that's the problem cylinder. Remove the plug and check for carbon whisker or replace the plug. I would also change all your fuel filters.
D.D.
 
Make sure the wires are tight on the plugs. I just had a similar problem on my Volvo tow vehicle after towing home over a rough road. The wires looked tight but one was just sitting loose on top of the plug. Good luck.
 
This engine doesn't have "regular" plug wires. But rather you have "pencil coils" that sit on top of each plug, with a wiring harness into each coil. When I removed the plugs to inspect them, I also looked at the pencil coil springs to see if everything looked kosher. I will be replacing the inline fuel filters next, but really don't think that is the problem. I just replaced those earlier this season, and the fuel is already filtered by the Racor Fuel Filter/Water separator before it enters the engine cowling. That is the fuel filter I changed today and didn't see any thing unusual. I may try pulling the coil harness one at a time to see if that affects the motor.
 
Colby
Awhile back I was having a problem with my 115 Yamaha. It would start right up, idle fine, and run great. But it would not go higher than 4000 rpms. It would not cut out or anything, just stop reving at 4 grand on the nose. It turned out to be an ethnol related fuel problem. I use Stabil on every fill up, but was still hit by the ethnol bug. Was not a cheap fix.
 
Another possibility to look at is some water in the fuel. I don't know about the Merc. 115, but most of the newer EFI motors have both a filter on the engine and a separate water separator. I have seen boats which have picked up some water in the engine separator act like yours. It has happened after a full power run, and apparently some water got past the primary filter in the line. Just one more thing to check, which is easily remedied.
 
Well, hopefully changing out the two inline fuel filters will then fix the problem.... But again I have my doubts. I am looking at purchasing the Rinda Software program, or one of their scanners, to have available to diagnose engine problems now and in the future. (Seems like all the local shops want $100 just to hook the computer up....) The engine will rev up to 6000 RPM when I cavitate it. However, under load, it just accelerates the boat very slowly, and then tops out at 4100 rpm, at about 19mph. Now that is what I typically will cruise at, but I know the motor is not running at full power. It's almost like as if the boat was at twice it's weight and the prop was pitched much higher. But that is not the case! So...anyway I'll get those inline filters changed out and then try to get to the lake again in a few days and see what happens....before paying a shop. Maybe I need to put ethanol back in it! (Since it was running just fine before I refilled with pure gas. ;-( Again this all happened after:

- running both tanks quite low, one empty.
- Filling tanks with premium, no ethanol fuel, and treating with Stabil. (But each tank from different sources.)
- Power unloading the boat, at max power on a cold engine.

So, my first thought was bad gas, or gunk from running tanks low....however, first fuel filter/water seperator changed out and two separate fuel sources used, although same brand, just in different communities.
 
You might be able to isolate the cylinder by unclipping the fuel injector wire on each cylinder, similar to disconnecting the plugs. If there is one bad cylinder, then you will note a radical idle rpm change in all but the bad one.
 
I just changed out the inline fuel filters, so I'll also try unplugging the wiring harnesses one at a time next. I also just got off the phone with Rinda Technologies, Inc. I am ordering one of their scan tools. (I think it's sort of like the OBDC readers one can get for their car.) Hopefully this will be a nice tool to keep in my bag of tricks! ;-) Colby
 
Colby
Your problem sounds more like mine the more I read about it. Mine would also rev to red line when not under load. I would have sworn it was electrical and not fuel related. I also changed my fuel filters, to no avail. All it took was the good fellows at Kitsap Marine and a fist full of dollars to fix it. That ethnol can really do a guy up. Not trying to scare you or anything, just a lot of similarities.
 
Yeah, I keep thinking about your first post Spike..... so in the end, what did they have to replace or do? Just looking for ideas right now. I did go ahead and order the Mercruiser Scanner tool by Rinda Technologies Inc. Something I'll find useful now and in the future. I still find it hard to believe it would have anything to do with Ethanol...but who knows. I've been running on that for the past two seasons I've had the boat, with no issues. About this time of the year however, I start running premium without ethanol getting ready for the end of the season. Colby
 
Looking thru some of the iboats forums, and looks like others have had this same problem. And those posts lead to VST cleaning and fuel injectors. Hope I can get to the bottom of my problems a lot faster and cheaper than a few there did... ;-(
 
Colby
I had to go get the invoice on the repairs, and yes it was the VST, some gaskets, filters, grommets and also had the injectors serviced. Now mine is a Yamaha but I think there is a lot of similarities with a Merc. In fact for awhile Yamaha was the maker of Merc power heads in that power range. But anyway I hope it works out for you. Let us know what you come up with.
 
I believe my Merc is the Yamah power head...but I could be wrong. I'll be reviewing my Mercury Service Manual and see if I can do much of this myself. Sounds like at the very least I need to pull the VST screen. It's possible by running the fuel tanks so low, I managed to get some crud even thru all 3 filters. Some other things in the other forums were also mentioned...that I'll look into as well. I think I can do much of the stuff my self to save some cash.....it's just I hate to take the time to do it! One other thing that kind of sticks in the back of my mind..... electrical issues. A while back I was having my electronics and trim tabs (basically everything on electrical at the time) flash off then back on. The first time this happen, the engine died. But it restarted right away. After that I had my electrical stuff "flash". By this, I mean go off then back on. I had just replaced some screws in the gunnel tray and thought maybe I put one thru a wire. So I removed it and checked all the wiring, with no issue. A few days later, I had it happen again. None of the later times had the motor just stop working. That only happened once. Anyway, I thought maybe then it was in the thermal CB, so I "worked it" several times to work thru any corrosion that might have been on it. (Located in one of the original transom hatches that seal so well....NOT). That seemed to fix the problem, until it happened again several trips later. Worked the cb again, and haven't had any more problems with that, so I'm still thinking that issue is one of a Thermal CB going bad or needing cleaned. The motor had been working just fine, up till the other day after I ran the tanks down, refilled with pure premium gas and then power offloaded it. Well, except on rare occasion it was hard to start up right after shutting it down. (Needed to throttle it up, which is odd, since you don't need to do that in an EFI car!) I bought the boat with 50 hours on the engine at the beginning of last year, and now have just shy of 200 hours on it. The guy I bought it from, I believe just used gas without ethanol (much more available in his area....Omaha). I've been using gas from various sources, and probably running it a lot harder. So, I should probably look into the VST regardless....
 
Spike, can I assume that you have had no other problems with your motor since that repair, and that fixed it? BTW, I think it's unfair to blame ethanol for this issue. While ethanol does attract moisture, our boats live in a very moist environment and water can get into even straight gas. I don' think there is any harm in running gas with 10% ethanol, if it's used up routinely. I think the problem lies in gas with ethanol sitting for long periods of time...that and some of the older hose materials that don't stand up to Ethanol. I think most boaters end up using the 10% since in many parts of the country that is all that is available. There are very few stations here in my area that sale gas without ethanol. Kwik Trip happens to be one of the few that will sell Premium as what they call their "recreational" fuel. But at $.50 more a gallon. I'm not even sure that many of our marina's have fuel without Ethanol. I have stored a boat over the winters with the 10% without issue also. (But full tanks, stabilized, and taped over the vents.) Colby
 
Colby
My understanding on the subject with ethanol you are correct. The harm comes from letting it sit in the fuel system unused. Not the actual use of it in gas. I have heard that people with cars like the Chevy Volt, and even the Prius that get real good milage and the gas sits longer are having issues. The tec at Kitsap Marine was telling me his neighbor who is in the navy had some troubles with his car when it sat while he was on deployment. Just got to get out as much as we can.
 
Well, I just pulled each pencil coil wiring harness, and this is what I got.

1, 3 & 4, RPM change. 2, no change. Hm........

Changed coils around on 1 and 2. 2, 3 & 4, RPM change. 1 No change.

I'm betting I have a bad coil. I'll replace it, keep my fingers crossed and see what happens. Anyone ever have problems with the pencil coils on the Merc EFI 4-strokes? If this is a common enuf problem, perhaps I should keep a spare.... Also, FWIW, I've looked thru my Merc Service Manual, and could find nothing about a VST.... ??? Colby
 
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