2002 Mercury 125hp shakes violently, struggles to accelerate, please help

[jwferrari09]

Hello this is my first post and I thank you all for reading this. I have a 2002 Mercury 125hp saltwater that I have been having trouble with. The motor started acting up a couple weeks back so I took it to a performance shop and had them "tune" it and replace a couple parts. From what I can remember they told me they replaced the spark plugs and "tuned" the motor, whatever that means. Since then the motor has been acting up again. It starts fine and idles fine, and it sounded fine with the muffs on out of the water. I put it in the water and ran it hard for about 10 minutes until the problem arised again. It sounds as if not all the cyclinders are firing and the motor shakes violently as soon as I put it in gear. I can tell that it doesnt sound right at all, maybe the timing is way off. But it get worse as i put the throttle down and it really struggles to accelerate and the shaking gets worse and worse. So i took it out of the water, replaced the impeller, the lower unit oil, and the spark plugs. Turns out the mechanic shop had put in the plugs for the 135hp and not the 125hp, so i bought the right plugs and installed them corretly. I put the boat in and again after about 10-20 mins it started again and it just gets worse and worse to the point where i cant even get the boat on plain and have to just creep home. The weirdest part is that when it first starts to happen it, the motor shakes and then it seems to correct itself and runs fine for a couple minutes and then shakes again and maybe corrects itself one or two more times until it barely even moves. Thank you all for your help, a coulpe people have said that it may need to be retuned with the correct plugs, or that there is an injector or fuel pump problem and not all cylinders are getting enough gas, someone also said that the motor is going into s.l.o.w. mode?? thank you

 

[Texasmark]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

I happen to have the service manual for that engine in front of me. I have the '02 3 cyl 90 hp little brother and I am a retired life long boating individual not a professional dealer/mechanic, but I have maintained all my vehicles from a 1.7 hp Mighty Might OB engine to a 800 cu in Cummins powered 18 wheeler I had for a time, for the past 50 years.
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I don't know where to start with your problem as you mention several things that relate to different/different types of problems. I doubt your water pump nor your spark plug type are part of your problem. Both would exhibit a continuous problem if guilty, not what you are experiencing. When you have intermittent shaking you have lost a cylinder (probably 2 for the reasons stated below) and they are trying to recover. Sometimes they can and sometimes they can't all depends on the degradation and where it is.

With today's electronically controlled engines, I'd like to know what it means to "tune" the engine; don't have points and plugs and variable timing to fool with which used to be what was involved in an engine "tune up"....besides the plugs of course. Now if adding some sort of decarbonization is part of the process (like "Decarbing" mention within this forum's archives) then there could be some "meat" to the term "tune up". (My opinion of course)

For a time (within the late '90-early '02 building cycle at least), on 4 cyl 115/125 Mercs, they shut down 2 cylinders for low speed operation , by design. All they had to do was in a place like the Switch Box, sample rpm's and below a certain level, kill one of the (2) signals coming from the trigger coils under the flywheel which would stop the firing of plugs on 2 of the 4 cylinders. Piece of cake to accomplish, but what it does to the engine as a result is the real question.

It normally shouldn't , BUT if this continues into the higher rpm's (ignition coil or pulse module fault, switch box fault, dirty plugs, gummed up rings, varnish in carbs) this could cause your shaking and loss of power. I personally thought this was a crock of dung design by Merc because it could easily cause these problems where 4 cylinders that ran all the time wouldn't but that was not my decision.

Looking in my service manual it depends upon which section you are in (ignition or fuel) as to what the recommended troubleshooting solutions are. Obviously, each direct you to potential areas in that respective area.

You can't tell anything really about running on the muffs as there is no load on the engine and misfiring cylinders can be easily missed. BTDT You have to have it under load to really tell which means on the boat and in the water.

It's obvious you have misfiring cylinders. Question is which ones and why. Ha! Key to the city.

I have been working on this problem of yours for most of today (currently 3 pm) and I am ready for a conclusion with subsequent feedback from you after following some suggestions.

1. Fresh fuel, clean fuel filter, proper oil level with a good quality TCW-3 oil with the oil pump working...you can see the oil in the clear lines associated with the oil pump, or you can see a slight blue-white haze to the engine exhaust.

2. Get into the search engine on this site and pull up the instructions for, and do, a "Decarb process". Decarb stands for decarbonization and it is designed to get carbon out of your engine which accumulates over time and causes all sorts of problems.

3. I was going to say: "After you decarb, run the engine on the muffs for 20-30 minutes at 1000-1100 rpm's. I would recommend that the water hose pressure is adequate to force some water to spew from beneath the muffs which will run down the lower unit, across the gearbox bulge and provide cooling for lower unit gears."

But I can't say that because your engine only runs on 2 cylinders at that low rpm (if it's one of those) and the non-operational cylinders, which are probably your problem in the first place, are again non operational and thereby the process is futile. Geez I hate that design.

So I don't know what to tell you now. Thinking about it, you can't even do the decarb unless you run the engine above the 2 cyl cutout rpm's (which I think is around 2500) and some say (I don't) that you shouldn't get over 1100 rpms on muffs (quoting an owners manual for a different Merc per a poster herein). Well I do (run them over that for a short period) and I don't worry about it, but that's my engine and my decision. I would say do the decarb in the water with the boat floating, but that may be environmentally hazardous and I don't want to get into that.

So where does that leave us? Beats me. I have a 3 cyl 90 rather than a 4 cyl 115 for that very reason and I don't have those problems. I know you don't want/need to hear that but that's how I addressed the issue.

Good luck and I hope you find a service center that is equipped to adequately investigate and solve your problem.

Best I can do.

Mark

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[Texasmark]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

I slept on this and I have one final suggestion that will help to isolate the problem. First thing you must do is separate it into one of two categories: Fuel or ignition.
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If it's fuel I have an idea; may take 2 people to do it; one to steer the boat the other to do the work.

First go to an autoparts store and get a spray can of Deep Creep. WM, Napa, and AutoZone carry it around here. It's Sea Foam in a Red and white spray can. This product has alcohol, naptha, and oil in it and it lubricates while it dissolves gum and varnish. It's combustible so once in a cylinder, it will explode and produce power similar to gasoline.

Go to where you boat and launch your boat. Pop the cowling and look right at the front of the engine. There should be a large black plastic cover over the carburetors and the front of the engine. In that cover should be some rubber plugs about 1" or so in diameter. Remove all 4 of those plugs which should expose the throats of the carbs.

Get cranked up and move out where you can accelerate. Advance the throttle to say 3/4 or so and when the engine starts shaking, spray a couple seconds worth of spray into the carb air inlets (throats) one at a time and wait 15-20 seconds before you go to the next carb.

If your problem is fuel or combustion chamber related, when you hit the cylinder where the problem exists, the spray will/should cause that cylinder to surge (kick in and start pulling).

Then it's just a matter of cleaning up that carb and cylinder. Sea Foam in the can is made to be added to gas. Since you may not be able to do an adequate Decarb, you might just add some to your fuel tank and run the engine for awhile and get it to clean up that way (30 minutes or so). This stuff likes to soak so after a run, stop and fish for awhile (30 minutes or so) and make another run. Might work for you without having to remove the carbs and all. If it doesn't do the job then you will probably need a carb cleaning.

The normal ratio for Sea Foam is 1 oz per gallon of gas, but that is just for maintenance. To attack your problem, I'd add a whole 16 oz. can to 5 gallons of fuel. If things clean up then dilute the mixture in your tank by adding another 10 gallons of fuel. Subsequently every time you gas up, add the 1 oz to each gallon. I use it all the time and my engine stays clean, smoke is almost non-existent, starts faster than you can get off the key and runs like a top.
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If it's ignition, my first guess would be the Switch Box because if your engine normally cuts off 2 cylinders at low speed, in looking at the wiring harness diagram and components of it, it seems to me that it would be the critter that performs that function. It's somewhere in the engine connected to the engine's wiring harness.

Your marine dealer/mechanic can fix that problem for you.

Again, good luck and let us hear from you.

Mark

 

[Laddies]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

If the engine was in the shop for a tuneup the first thing to do is contact them and let them know that the engine is not running properly, if the shop is a reputable business they will repair it. Just make if there is a charge that they call before the work is done with a estimate.

 

[CharlieB]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

Laddies is the best advise, call the shop, let them know that you still have the problem with the motor and find out what they are willing to do for you.

Do you still have the spark plugs you removed?

How did they look? Carboned, oily? Do your new plugs look like that now also?

Trim the motor so that the carbs/motor is level. Remove the airbox cover, while watching down the carb throats, squeeze the primer bulb until firm then continue to hold the squeeze, don't smash it but just hold a firm handshake. Watch the carb throats for any fuel, if any then that carb needs cleaning/repair to stop the flooding.

Unfortunately carb cleaning/repairs are not usually part of regular service/tune ups, the shop will charge you for this.

 

[jwferrari09]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

Hey thanks for all the feedback, im currently back at college and cant look at the boat right now but i had a friend of mine take a look at it back home and hes worked on boats for most of his life.
He said that he thinks its an ignition problem and that the injectors giving ample gaS to all the cylinders. The weirdest part is that he told me he took the boat for a ride and said that the problem resolved itself for a minute or two after he squeezed the ball thing that comes out of the motor.
I dont know what this thing is called but its the thing that comes right out of the front of the motor that you squeeze prior to starting the engine to "prime" with gas, u know it gets harder as you squeeze it. He said if he squeezed it a couple times while the engine was running the problem went away for a little while but then came back. This makes me think that the cylinders arent getting all the fuel they need. Oh and to answer another question, the motor is the type that idles and cruises on 2 cylinders and then jumps to 4 when the throttle is opened up, but the problem occurs both at a steady cruise and at a fast pace. Thank you.

 

[Texasmark]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

Well he just nailed the problem. You have a fuel starvation problem caused by:

Bad fuel pump
Varnished carbs
plugged fuel filter
bad squeeze bulb

Address those problems, run it and get back with us and while you are in there put some Sea Foam in the gas.

Mark

 

[Silvertip]

Re: 2002 Mercury 125hp shakes violently, struggles to accelerate, please help

I guess we don't know if this is a carbed motor (the two cylinder killer) or an Optimax (DFI) motor. Troubleshooting is very different on each.