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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15633
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Looking for an expert in Large Merc outboard powerheads. I

Customer Question

Looking for an expert in Large Merc outboard powerheads. I am buying a 2000 Seadoo Challenger 20'. The boat is powered by a 240 EFI Mercury Sport Jet motor mated to a jet drive. The previous owner bought the boat with a fried motor and replaced it with a completely rebuilt one. It was done at a fairly high-volume shop. After he and a couple friends "put the new motor in" he claims the boat starts fine but has an erratic idle that goes up to around 2500 rpms and back down to idle over and over again.
He says he took it to a Merc shop who diagnosed a probable vacuum issue. They wanted to just start poking around at $125 an hour and he didn't have the money to do it. Compression is @ 130lbs on all 6 cyl.
I'm buying the boat tomorrow. I just want to know,given this limited info, is there a possible doomsday condition here or something that can be traced down. And, Obvoiusly, what would be your opinion on what is likely causing this.
Submitted: 1 month ago.
Category: Boat
Expert:  Jason replied 1 month ago.

H​ello my name is ***** ***** am I speaking with? I read what you wrote. Yes there is a possible doomsday condition. I would not personally buy this boat unless the issue is corrected. It could be a sensor issue, it could be a fuel pressure issue, it could be a short in the harness keeping the injectors open for to long. Or worst case scenario is the crankshaft is not installed correctly. Multi cylinder 2 stroke engines have seals on the crankshaft that separate each cylinder from the one next to it. If something was up with any of these internal seals, any number of running conditions including this one. Either way, if it is a vacuum leak or a crankshaft seal issue, in either event you would be looking at more or less total engine disassembly, which is going to cost thousands to fix. I would not purchase this boat unless the issue was fixed first, there are so many other good boats out there that don't have any problems.

Expert:  Jason replied 1 month ago.

I do want to make sure you are happy with my service before you go. If you had a further or more specific question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you

Expert:  Jason replied 1 month ago.

Was there anything else?

Customer: replied 1 month ago.
My name is Steve. The boat was physically examined buy a certified merc mechanic who believes it is vacuum related. So for now, let's Loosely go with that diagnosis. Assuming he's right, why would the entire motor need to be disassembled to track or repair as you indicated above?
Expert:  Jason replied 1 month ago.

Hi Steve. Well. There are 4 basic places a vacuum leak is going to come from.

1. It could come from the intake manifold to engine block gasket.

2. It could come from the upper crank seal.

The motor would not need to come out or be disassembled for these 2 areas.

3. The crankcase, the engine block itself, is 2 halves. And there is a seam on both sides of the engine block. If this seam is the source of the leak the engine block will need to come out and be disassembled to fix this.

4. There is a lower crank seal as well. If this is the cause of the leak it's the same thing. Engine removal and disassembly.

Expert:  Jason replied 1 month ago.

Was there anything else Steve?

Expert:  Jason replied 1 month ago.

H​I Steve, just checking in. No rating has been put through for my service yet, did you have any further questions on the issue?

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