Merc 7.4 MPI engine runs rough

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  #1  
Old 05-11-09, 03:27 PM
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Merc 7.4 MPI engine runs rough

I have a 1998 Formula with a Mercruiser 7.4 MPI engine and a bravo 3 outdrive (697 hrs). My problem is that it hasn't been maintained well in the last 3 years. The boat has old gas in it and runs rough. When I took it out and ran it, the alarm went off and I had no oil pressure. In the shop they were able to start it and it ran but ran rough. When they removed the value covers they found goo that you could scrap up on your finger. My question is, can I use something like kemtool to clean out the goo and do 5 or 6 oil changes to clean this out? They advised trading up to a new boat. I need help as I'm not a mechanic and don't have a lot of money.

I had them clean out the goo and I have the boat back. It runs great at high speeds but at 10000 rpm, it runs very rough cycling the engine from 9000 to 13000 rpms. Would bad gas make it run so rough at low rpms but not affect it at high rpms?
 
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  #2  
Old 05-11-09, 07:35 PM
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Of corse the salesmen want you to buy a new boat.

Old fuel is bad stuff that can cause odd problems. Do you know how much fuel you have in it?

Does your fuel have ethonol in it. If it does change your fuel filter. The ethonal attracts water so if your fuel is 10% like ours it can turn to 10% water in your tank.

When was the last time you changed your distributer cap and rotor.
 
  #3  
Old 05-12-09, 04:47 AM
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Welcome to the forums.

Three-year-old gasoline is probably the cause of a lot of your problems. Especially in Florida, because gas goes bad faster in higher temperatures. Up north we use stabilizer for the six months of the year that the boats sit in drydock, but the cold winters slow the process. You really should drain the old gas out of the tank.

Old gasoline turns to varnish, so the injectors should be cleaned. Definitely replace the fuel filter(s). Check the water separator. Timing could be a contributing factor -- especially if it's not adjusting to advance.

Has this engine been modified? Most Mercs run at 4800 RPM wide open throttle. I've never heard of any four-stroke engine revving 13,000 RPMs. Top performance drag racing boats with superchargers top off at around 10k.

Lastly, I would find a mechanic, not a salesman who's posing as a mechanic.
 
  #4  
Old 05-12-09, 04:13 PM
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Thanks for the reply. We do have 10% ethanol. I just had the oil and filter changed before it conched out on me. The just changed the rotor and had new plugs put in.
 
  #5  
Old 05-12-09, 04:17 PM
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Thanks for the advice. I was wrong about the rpms. I meant to put 900 to 1300 instead of 9000 to 13000. This is at the low end. When at full throttle, I'm running about 4600 rpms. My fuel gauge is not working which is a subject of another thread but I'll need to check to see how much fuel I have left. Nevertheless, I'm going to drain what is in there.

Would it help to put in a fuel additive and add some premium gas?
 
  #6  
Old 05-12-09, 08:21 PM
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We use a service that clean injectors for our outboards as new ones are over $250 each. I'm not sure what the 7.4 cost but I think you can find a shop localy that can do a set for cheaper than the cost of one.

If you go this route ask for a flow chart showing fuel flow before and after. They will flow more after and they will have a better spray pattern than when dirty.

It's a good idea to change your fuel filter after you get new fuel running through your system. When I drain old fuel I put new in and run for 1/2 hour then change filter so I know the old is out and start all fresh.
 
  #7  
Old 05-13-09, 04:30 AM
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900 to 1300 is considered the low/idle range, and it could most certainly be caused by bad gas, water in the gas, clogged injectors, or timing that's not adjusting. The fact that it runs properly at 4600 WOT points to those causes even more. It's probably not the fuel filter, since a clogged filter would slow the flow at high speed.

Try running premium gas and a can or two of 104 octane boost (naptha) in the tank after you drain the old gas. Sometimes this will clean out borderline injectors. Pay attention to the fuel filters if you use that stuff -- it tends to loosen gunk from the entire fuel system. Also, don't run it full open with that stuff in the tank.
 
  #8  
Old 05-13-09, 04:59 PM
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I was thinking more the water seperator filter. The spin on one.
 
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