Loss of Power in Mercury 115hp EFI 4-Stroke

Well here's to hoping that it is a pencil coil. A heck of a lot cheaper than going through your fuel system. I think that if it was mine I would change out the coil and do the spark plugs while everything is apart. Good luck
 
I just did the spark plugs last year. Figured I would leave them in this year, but since I've put 100 hours on the boat this season, and have to fog thru the plug holes anyway (according to the service manual), I'll replace them then. Hopefully I still have at least a month and a half of boating left! But the weather has been pretty weird. Fall is in the air, and somebody said we are suppose to have a cold and snowy winter..... ;-(
 
I had my Merc 115 EFI (2006 Yamaha block) serviced in late October. Complete change, fluids, plugs, seals, etc. Sat all winter waiting for annual trip to Yellowstone Lake. Didn't start the engine until early August at the Lake. It started fine but ran rough.

Did everything you did, but one thing. I hauled the Far West to the dealer (400 miles round trip) but he was reluctant to do a make an appt. until a few months went by. After some earnest exchanges, including several insults - me included, he had his mechanics to a quick analysis. Guess what? The number 3 spark plug was defective. It was brand new but defective. New plug and I returned to the Lake for the rest of the boating season. No problem after that. Engine ran fine.

Lesson for me? The Merc EFI will start on three cylinders, but just limp along and not go on plane. I have the complete service manual for the 115 Merc EFI, but it was sitting at home 200 miles away. Really careless of me, because with it I would have been able to make a quick diagnosis. I had four (4) used plugs aboard which were OK. We live and learn.

John
 
I worried about the fuel injectors, the VST...whatever that is, among other things. I know anytime the boat goes to the shop, it's going to be lots of $$$. But it just continued to seem to me that I was only running on 3 of 4 cylinders. Be it a bad fuel injector, bad plug, bad coil, or bad cylinder.... I'm still hoping it is just the coil problem. But I won't know until later this week when I get the new one and install it and run the boat. If that fixes it, then my next question is why did the coil quit working in the first place? Makes me worry a bit more about the electrical problem I was having. I suppose I should just go ahead and replace that thermal CB too...unless I find out I should just clean and lubricate the old one some how. I'm glad I'm handy mechanically....I just hate to take the time to fix stuff like this! Another thought that just popped in my head. Where did all that raw fuel go that was dumped into the cylinder with the bad coil? Or would the computer know to shut that fuel injector down?...
 
Check the oil and see if there is any dilution of the oil. Most likely it went out with the exhaust. Fortunately you did not run it a long time. If it were to get past the oil rings and wash the oil off the cyl wall, it could cause damage; not likely in your case.
 
Very similar symptoms on my Yami 150. Would not rev above 4000 under load. Changed all filters, including the $50 one on the motor, new plugs, drained the VST, check compression with no joy. Finally took it to a dealer to put it on their diagnostic computer, nothing showed up. Dealer pulled the injectors and send them out for service. I was very sceptical that it could be the injectors without it showing up on the computer diagnostics but that is what it was. Runs fine now. My wallet was $750 lighter.
In hind site it probably was a bad load of fuel I got last october. The motor started to miss and I changed the fuel filter but probably not quick enough and I probably should have changed it again shortly there after but I didn't untill I started to have the power problem in March. Now I will change the filter every 50 hours problems or not.
Ron
 
The question in Ron's boat is why the filters didn't catch any bad fuel--were they 2 mm filters? Most of us are running 10 micron Racors--most of the carburetor engines will tolerate 30 micron filters. In my diesels I run 2 mm on the engines to protect the injectors--which as Ron notes can be very expensive.

Was there some phase separation which occured with the bad fuel? A good filter should plug up and stop fuel flow--but I have observed with the ethanol problem than I have had water separators fill up on the engine down stream of Racor filters with water separators. I think that is a potential issue when the engines are stored with bad fuel.

A lot depends of if you need to change external filters as frequently as 50 hours--most outboards will go 200 or more hours with good fuel. Again I have to go back to diesels, but with the ISC 8.3 liter I change the filters every 15,000 miles. At 50 mph that is 300 hours..and somewhere about 1800 gallons of fuel.
 
From diagnostics you did, I would say there is about 95 percent probability that the coil is the problem. Good, logical process to discern the problem.
 
Replaced the coil today, and that seemed to clear up the problem, while on the muffs. I hope to take it to the lake tomorrow to find out. Also got my new Mercruiser Scan tool to hook up to the diagnostic lead. Didn't have time to do that to night, and hope to hook that up also tomorrow. Talked to my brother who knows a little more mechanics than I do, but a whole lot more electrical/electronics stuff. The bad pencil coil I pulled had a whole lot of resistance when I tested it with a meter. He said that would be enuf to kill it. (It's suppose to have 2-4 million Ohms according to the service manual, mine had about 11 million.... Don't know why the coil would just up and die, but then as my brother said, and I understand quite well, crap happens! Would have nothing to do with power unloading it off the trailer, or running into a low fuel state, etc. Just all coincidental.
 
The question in Ron's boat is why the filters didn't catch any bad fuel--were they 2 mm filters? Most of us are running 10 micron Racors--most of the carburetor engines will tolerate 30 micron filters. In my diesels I run 2 mm on the engines to protect the injectors--which as Ron notes can be very expensive.
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My thoughts that the injector problem was caused by bad fuel is just a guess. I have a 10 micron filter (not a racor) and the missing problem cleared up after I changed the filter the first time. Maybe it wasn't the filter and the injector problem is just coincidental. At any rate the injector service wasn't cheap but it solved the problem and has made me more inclined to change the filters regularly.
 
The VST tank is kind of like a float bowl for fuel injected engines. All of the injectors pull fuel from it. It's a reservoir to prevent the injectors from starving for fuel. It has filters and a screen to prevent foreign matter from clogging injectors. Cleaning those screens and filters are done at 250 or 500 hours can't remember exactly. If those filters and screens get clogged or if bad fuel accumulates and sits for a long period of time in the VST it can get pretty badly gummed up. We drain our VST tank before we put the boat up every winter. The VST tank has a brass screw like a float bowl drain and a hose so to can drain it into a container. At least on our Yamaha 150.
Smooth Sailing
D.D.
 
Just reporting back on this issue, since someone recently pm'd me having the same problem. It was the pencil coil. Changed the one out that I thought was bad with a new coil, and it has worked fine ever since! Colby
 
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