7.4L Mercruiser breaks down @ 3000 rpm's Please Help

[bajaman350]

Just bought a scarab with a 7.4 liter MPI, it idles great, and runs good up until about 3000 rpm's. When the boat goes to get up on plain, and reaches 3000 rpm's, you hear a bang,and the boat pretty much shuts down, you can back out of it and keep it running, and bring it back up, but when it gets to 3000 it does the same thing again. The boat sat up for around a year, so we drained the fuel, changed the filter, and put new fuel in it. Still does the same. It almost sounds like timing, but I have no idea. Please help.

 

[paulie0735]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Originally posted by bajaman350:<br />you hear a bang,and the boat pretty much shuts down,

Welcome to iboats mate, Sounds like a sensor out or not connected properly, have a look at all the sensors and check the fitting are not lose, corroded or rusty. Truth is you may have a lot of issues that are combining to cause your problems but there is some things like the connections that you can check, if they all seem ok and dry than get it to a 'authorised' merc dealer for a diagnostic check. <br /><br />On a side issue, I read a lot of posts like this and I can't believe that you can buy a boat in America from a dealer, take it out on the water and find a problem like this than have to fix it yourself?????? We may be paying more here for the same boat but at least when we buy it, it works!! or if it doesn’t you have a claim against the dealer to get it right!

 

[newport dave]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

What year is your boat/engine?<br /><br />Dave

 

[bajaman350]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Both are '96

 

[rodbolt]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

paulie<br /> your assuming it was purchased new from a dealer or used with a warrenty. I get 2 or 3 a week purchased by newsprint ads that ran on the muffs in the driveway. most have fuel issues. the MPI motors have screens in the high pressure pumps that like to clog as well as filters in the injectors that like to clog. both can cause a backfire and shut down when the air volume exceeds the fuel volume requirements in the combustion chamber.

 

[bajaman350]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Where are these screens located, and are they easy to get to?

 

[rodbolt]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

the screens are exactly where I posted, easy is a relative term.<br /> I see some hulls that it only takes an act of congress to access them and others that require engine removal. a quick test with a fuel rail pressure guage can eliminate some guess work.

 

[newport dave]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Baja,<br />Like rodbolt said, fuel delivery should be the first thing you check. Before you start pulling it apart, get a fuel pressure gauge and begin a systematic troubleshooting process. Do you have a service manual?<br /><br />Dave

 

[paulie0735]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

rodbolt, I guess when i read, "rang the dealer" i assume they rang the dealer they purchased it from, even second hand boats purchased at a dealer here are guaranteed to actually work! Other wise why would you buy from them??

 

[bajaman350]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Does Anyone know what the pressure should be on the fuel rail pressure gauge? I guess the service manual would tell me, but any general ideas would help. Trying to avoid purchasing a $75 manual from mercruiser, but sometimes I gotta do what I gotta do. P.S. I appreciate all the tips, the fuel delivery thing seems like it is exactly the problem, so I am gonna look into it.

 

[rodbolt]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

that 75 dollar manual and some test equipment is a lot cheaper than tossing parts at it.

 

[newport dave]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

You can download the service manual at no cost here;<br /><br /> http://www.boatfix.com/merc/Servmanl/srvcovr.pdf <br /><br />It is service manual #16.<br /><br />Dave

 

[bajaman350]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

Thanks everyone for all of your help. I have decided to just bring it to the boat shop this week, and let them go through and fix it. I figure I better let a professional handle it! Thanks again!

 

[bigbrownbuku]

Re: 7.4L Mercruiser breaks down @ 3000 rpm's Please Help

the rail pressure will be ~36psi. it may well make great pressure at idle but drop off at speed. it shouldnt. an engine not put into service mode then timed will also run poorly at speed.