Merc Sportjet 175x Hell

[8 FOOT LEADER]

For the life of me I can't figure out what the heck is wrong with my merc 175 sportjet! I've taken it to 3 different marine places and have had the same result- Boat engine mechanics scratching their heads! <br /><br />Heres the problem - With the throttle wide open the boat cavitates so bad it wants to bog down and die. As a matter of fact, if you leave it wide open it will just die on its own in no more then 10 seconds. Otherwise it seems to run fine about 1/2 throttle which doesnt make me happy when I'm on the river and need to get over a shallow gravel bar. This is an issue because as you know with a jet its your only way to steer the boat and if your power gives out at the wrong momment on a river, your dead!<br /><br />The motor has been inspected, ran up and tested. The impellar and stator is in good condition. We even swapped out fuel tanks to check to see if air was getting into the lines. Fuel filters are clean as a whistle and all the wiring was inspected for faults.<br /><br />What the heck can be causing this problem?<br /><br />Now I love my sportjet because its got some serious power, I just wish it would work right at its full capacity like its supposed to. I emailed merc and have gotten no response in several attempts. Maybe someone will see this thats experienced the same problem or knows of it.<br /><br />By the way I'm running it on a 19ft Alumaweld stryker. Good boat!<br /><br />-8 ft

 

[Intruder]

Re: Merc Sportjet 175x Hell

I have a SportJet 175 on a 20 foot Alumaweld Intruder ... about 150 hours on my motor. I've also spent quite a bit of time in two other SportJet boats. Hopefully I can provide a couple ideas that may help diagnose your problem.<br /><br />Let me first be clear, when you say "cavitation", normally that occurs when a propeller starts sucking air. On a jet I assume what you mean is that you are getting air into the impellor resulting in a loss of thrust and over-reving of the motor.<br /><br />My boat exhibits the symptom you describe when I have something like a stick or weeds stuck on the impellor plate, when weeds wrap around the impellor shaft or, intermittently in rough water where the impellor intake lifts from the water or air passes under the hull. Since you describe the situation as ongoing, I would suspect something like an unbalanced shaft or a blown shaft bearing that would cause the impellor shaft to function improperly at higher RPMs. <br /><br />It is important that you register this issue with Mercury. Call the Customer Care number listed in the back of your Owners Manual. This will preserve your warrantee coverage and open doors to more adequate service. They will then contact a service shop in your area and help them diagnose the problem.

 

[8 FOOT LEADER]

Re: Merc Sportjet 175x Hell

Well with cavitation resulting from air getting sucked into the impellar area, you'd think that if you backed off the throttle a bit in rough water that would cure the problem. With weeds and other objects getting sucked into the intake grate, clearing the debris would fix that as well. My cavitation isn't air related, as far as two dealerships worth of mechanics that I've personally taken out on the river for testing are concerned. The result like I said is the engine will die if you don't back off the throttle. I see your point when cavitation occurs when operating in rough water as sometimes the unit will come out of the water resulting in higher RPS usually.<br /><br />My problem just started happening one day probably about 3 months after I purchased my boat. I've had it for just over a year now. Before that everything was working in tip-top order. I like your idea of the Impellar shaft being out of balance since I've viewed some chaffing inside the impellar housing. Or maybe the idea of the impellar bearing being blown out. Would that make some sort of noise? I've also noticed what the dealers call impellar burns. Thats when air heats up the impellar, and as a result causes pitting on the inside of the impellar. Not the impellar shaft, but the impellar itself. They claim this isn't having any effect on the sportjets overall performance.<br /><br />Thanks for your ideas.

 

[clanton]

Re: Merc Sportjet 175x Hell

From you 1st post 2nd par. If you hold wide open throttle for over 10 seconds it dies. Is this a V6 fuel injection or carbuators? Sounds like fuel flow problem or some mechanical part binding. You also say stator is in good condition, did someone take reading with peak meter at high rpms?

 

[12Footer]

Re: Merc Sportjet 175x Hell

I know nothing about jet boats. But have had extensive experiance with high RPM turbines.<br />The principle is the same, the only difference is air/vs/water.<br />One thing you state jumps out and makes me post a reply...Balance. Your jet thruster is a high RPM turbine. Being such, the impeller (turbine blade), shaft, bearings are susceptable to the least imbalance. A gram of wieght more on one vane of a turbine blade will cause bearing failure and vibration. Some turbines are mounted with hydraulic shock absorbing mounts, to cut down on vibration. Otherwise, they should be mounted solidly.<br />The speed at which a turbine with it's associated shaft and bearings can desintigrate due to imbalance is sometimes measured in seconds (depending on RPM and blade mass).<br />I am sure the mechs have seen this before. After all, thier associate dealers sell them,and it's a common ailment with an impeller that may have injested sand or other debris. It sounds to me as if they are stalling till the warantee goes away??<br />Just a guess. But do document your corespondance with Merc's reps on this, just in case.<br /><br />It may be very rare on your particular setup, too. Who knows? But an additional theory I'll pass on is, the vibration may be causing the fuel to foam and run lean. It may also be a protective "vibration cut-off" to keep the impeller from flying apart.

 

[8 FOOT LEADER]

Re: Merc Sportjet 175x Hell

Hehe 12 ftr, sleepn at motel 6?<br /><br />Wel I think I will look into getting a manual for my sportjet and dig into it a little to see if theres a problem with the way the impellar shaft is balanced. I've inspected the impellar housing again, and I'm not sure if theres supposed to be any wear on the inside of it but it sure looks like theres been some.