Mercury sport jet 175 intermittent issues - PLEASE HELP!

[schnur07]

I have a 175 HP mercury sport jet in my 1998 sugar sand mirage. For the last three years I have this intermittent issue where the motor cuts to about half power. The engine still runs fun - no noticeable vibration or anything - just less power - top speed in the 20s - usually top speed is about 50. When it happens, it's usually after it's been running for 5 to 10 minutes. Although this year, I've used the boat for almost two months and the issue just flared up again yesterday - thought the winter had fixed it!.

I took it to a guy last year and he replaced the stator and one of the control boxes (hopefully I have that terminology correct). So $1000 buck later, same issue.

I'll be driving along and then all of a sudden it will just cut to about half power. Once in a while it will kick back in full power, but typically once it goes half power it stays that way until it has sat for a long time. It's doesn't feel like a fuel issue since it's just such a quick response - feels more electrical, but who knows. I don't know that a mechanic can even test to see whats happening sitting in a boat shop - especially if it's intermittent.

I am not a mechanic but any advice that I could pass on to a boat mechanic would be appreciated. Super frustrating! Love the boat when it runs at full speed! I can still pull tubers when it's acting up but not as much fun.

 

[Chuck Buck]

...it goes half power it stays that way until it has sat for a long time.

It doesn't sound like fuel delivery. I'd go with an electrical component breaking down from the heat. This can be darn near impossible to track down.

One trick is to take a can of refrigerant, say R-134A, and spray some directly on different parts of a circuit board (or in this case, different components) until a change is noted. THAT'S where your problem is.

 

[schnur07]

Last night when I took it out it was at half power right at the start. If the engine still runs smoothly, does that mean all of the pistons are firing but for some reason they are only producing about half the power?

 

[Chuck Buck]

Last night when I took it out it was at half power right at the start. If the engine still runs smoothly, does that mean all of the pistons are firing but for some reason they are only producing about half the power?

Good news! Your problem went from the dreaded "intermittent" state to the much easier to diagnose "constant" state. Your mechanic should now have less trouble tracking down the actual root cause. What he did last year, we technicians would say is "threw parts at it" in hopes he got it right. Sometimes you get lucky, and sometimes you get what we call "a comeback." While he will probably now say "hey, that was then, this is now, I need more $$$$ for diagnosis." I would respond with "no, I'll pay for parts and the labor to install them, but no more diagnosis time. You already got paid for your incorrect diagnosis." This is totally up to you, but the fact is, he did not "fix it right the first time." In all fairness, these ignition systems are rather failure-prone, so he probably wasn't trying to "screw" you, but for the record, he should know that the "control boxes" (technically, the switchboxes) are to be replaced in pairs. One more thing - at least in the automotive world, there are "labor time guides" that spell out exactly how much labor time should be charged to diagnose and replace each part or group of parts. If I were to get an estimate (or a bill) from a mechanic, I'd want to see how much time he wants to charge me for. Doing the math, labor time x labor charge/hour (labor rate) should equal total labor charge. E.g., if the job pays 1.5 hours (per the time guide) and his stated labor rate is $90/hour, the total labor charge should be $135. No more.

Anyway, according to the OEM Mercury Service Manual helpful tips; one coil not firing is likely a failed coil, two coils (1 each bank) not firing is likely a failed trigger, and one entire bank not firing is likely a failed switchbox. Fortunately, you already have a new stator and one new switchbox. I would financially prep myself for buying another switchbox and/or a trigger. Good luck!

Lastly, when you said "does that mean all of the pistons are firing but for some reason they are only producing about half the power?" that totally aligned with your earlier statement "I am not a mechanic ." "Truer words" and all that...

 

[Cobra Jet Steering LLC]

Could be trash or water in your fuel tank I had a similar issues once was caused by contaminated fuel tank and once was caused by water in the fuel.
If you can get to the engine with it running , when it starts to bog down spray some starting fluid in the air intake if it immediately picks up speed it is starving for fuel or has contaminated fuel if it bogs down immediately it is probably ignition or ground wire issues. Just do this a few seconds to see how it reacts.
I always removed the old style push pull fuel pumps and installed a 3.5 lb electric fuel pump on all my 2 stroke engines be it a boat or a ski, also fuel selector switches for -on- off & reserve cause a lot of issues when they get old, so bypass that and see if the problem goes away if it is starvation .
Also look for air being sucked into the fuel lines from loose hose fittings or clamps I have found that quite often, I would install a clear fuel line and watch for air bubbles in the line. And look for any obstruction inside the engine compartment that could be blocking the air flow into the carbs like some loose insulation or sound deadening material

 

[Blkrvrbart]

I just discovered my stator was bad on my 210. Pretty close cousin to the 175. From the diagnostics and what has been replaced on your 175, I would test the coils and trigger wheel. The good thing is that now it’s not intermittent. Doesn’t take as much effort as I thought it would to run through the whole set of ignition diagnostics. Also check for little easy things. Spark plugs, wires, fuel filters. Keep us posted.

 

[schnur07]

Thanks for the advice. Just for my learning, the mechanic thought that one of the switch boxes was bad and could have been running on three cylinders? Wouldn't it run rough if only running on three cylinders?

Sorry for my ignorance, but when running with this half power issue is it an electrical component that is limiting the amount of gas being fed to the carbs?

 

[Chuck Buck]

...when running with this half power issue is it an electrical component that is limiting the amount of gas being fed to the carbs?

The only electrical component related to fuel delivery is the priming pump which operates while the stater is engaged. The mechanical fuel pump takes over while the engine is running. To answer your question above, no.

 

[Diplomasta]

The weak point on these motors is the trigger. Everytime the throttle moves, the trigger wires move. Wires don't like to move. They break and corrode internally. Trigger Job Requires a flywheel puller. A harmonic balancer puller will NOT work. You will destroy it. Mercury has their own puller, which frankly works much better than the 3 bolt type. Other than that, try unplug the correct temp sensor (top of cylinder heads) when motor cuts to half to see if it is causing the issue. Has One for computer (maybe white with black stripe), One for gauge (tan), and sometimes also a third one for a buzzer (tan with blue stripe). You want to disconnect the first one. If it fixes it, then replace that sensor. Also oil level sensor will cut it to half if it thinks you are out of oil.......you're not out of oil are ya? Pull and check thermostats to make sure you are not actually overheating (Thermal gun helps). If exhaust pipes carry outside of powerhead (over 200 hp usually), then keep in mind that running engine on the water hose (on trailer) will NOT cool the engine, only the exhaust, So do these checks with boat in the water. Don't run on hose for too long.