No Power over 3000 R.P.M on 7.4 M.P.I.

Same problems here. I just changed the long overdue plugs, rotors and caps and still have the same problem. It is really pointing towards fuel. Runs like a champ up to 1,100 RPM, then bogs down. Timing is good @ 8* BTDC. Exactly how does one check the fuel pump pressure and what should it be? Do you remove the outflow tubing from the fuel pump and put some kind of gauge on it?
 
Sea Ray Mark said:
It may be difficult to pinpoint which one it is since it only happens at high RPM when everything is being stressed. In my opinion it's not worth trying to fix just one. Replace all of the plugs and all of the wires. Chances are if you have a bad wire the rest of them are probably on their way out too. If the plugs are new you can try the wires first and then do the plugs if the problem doesn't go away. Not sure where to get parts in your area but make sure you have the serial number of one of your engines with you. Almost any thing I buy for my engines they want to know the serial number to make sure they can look up the right part.. There are a lot of subtle differences in years and models so the serial number is the best way to make sure you'll get the right stuff. I hope this will solve your problem. Mark
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Just an update.
i have not been able to get to the boat until this weekend.
I added new plugs and wires on the Starboard engine.
It now runs like new.
Thank you for your input Searay Mark. Much appreciated.
i have ordered wires and plugs for Port engine. That way both engines are on the same maintenance schedule.

Chuck
 
That's great news Chuck. Glad it all worked out for you. Mark
 
I too have a smiliar problem to this. My starboard engine will oscillate/cycle up and down. At 1800 RPMs she will drop to 800 and then go back up. After awhile she will settle at 1800. This happens all the way up to 3200 and beyond.
Sometimes I can get up on plane and run for a while and then it will drop to 800 or so and then jump back up to whatever RPM I was running at. I decided to change both water separators and fuel filters on both engines. I even switched both tanks and then just ran both engines off the Port tank. I did a fuel pressure test 25 was the output. However, I am not sure what it supposed to read as I can't find it anywhere.
Here is the other rub in all of this. Two weeks ago on a Sat afternoon I thought I would take the Vapor Separator Tank apart and clean it. However, at the cost of $1600 if I screw it up I decided against it. So I started the Starboard engine and ran it from idle to different RPMs all the way up to 3200 RPM's. Not one fluctuation. Not one. Did the Port engine as well. Nothing. Smooth as glass. Now I have been putting in some StarTron b/c I was told it might be water. Sunday I took the boat out for a run. From idle coming out of the creek to 3200+ RPMs she was spot on. I ran her for about 5 miles. Even coming back into the creek and the slip it was smooth and better than ever.
Last Friday night topped off with gas and more StarTron and then decided to take her for a spin to burn off a little fuel. Couldn't get up on plane for all the fluctuation and oscillating. My mechanic says to replace the fuel pump? But it could be a hose? Plugs. Wires. So, right now I am totally lost as to what to do. Do I replace the High Pressure Fuel pump? Spark plugs and wires? Burn the boat in place? Any ideas please feel free to reply.
 
I would recommend replacing the cap, rotor, wires, and plugs on the problem motor. If it solves the problem great, if not you have replaced normal wear items that eventually need replacing anyway so you didn't throw any money down the drain. Eliminate the easy inexpensive stuff first. Also make sure to dump the fuel out of your filters into a clear container to check for water. I'm no expert on this stuff so maybe you'll get some better advise from others that chime in. Mark
 
Mark - Interesting. I am searching for wires/plugs when you wrote what you did. I do know that the wires are only three years old. My boat is a 1996 has fuel injectors and electronic ignition. As for the fuel I did do the clear container to check for water. None found.
 
OK, so wires have been the only thing I haven't tried yet. I started different thread on my 7.4 problems, so this must be the year. Three marine mechanics and untold number of backyard mechanics and I still can't get my Port engine started after changing the exhaust manifolds.

Do ignition wires go bad without warning like that?

Gene
 
I was on a friend's boat this past weekend and all of a sudden engine shut down. Went from 1500rpm to off. Turned out to b an ignition coil. Swap yours with your other motor and c what happens. Or u can take another spark plug and pull a wire atach it to the new plug ground it to the engine to c if your getting spark. Just a thought
 
RubberDuckeeToo2 said:
OK, so wires have been the only thing I haven't tried yet. I started different thread on my 7.4 problems, so this must be the year. Three marine mechanics and untold number of backyard mechanics and I still can't get my Port engine started after changing the exhaust manifolds.

Do ignition wires go bad without warning like that?

Gene
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Wow Gene, the boat still isn't running? If it won't start at all it could possibly be the coil wire but not the plug wires. They don't all go bad at once. I know you've already tested a lot of other components so I won't get into that here. If you want to send me a PM so we can connect by phone I'll be glad to talk to you about it to see if I can think of anything. Mark
 
Thanks. I'll try that this weekend or today after work. Right now I'll try anything. If it had tires I'd kick them.
 
Searaybio270 said:
Mark - Interesting. I am searching for wires/plugs when you wrote what you did. I do know that the wires are only three years old. My boat is a 1996 has fuel injectors and electronic ignition. As for the fuel I did do the clear container to check for water. None found.
Click to expand...

Joe, I wouldn't think plug wires would go bad in three years but if they were burned slightly by a manifold or something the can go bad in three minutes. When I had the problem with my boat not going over 3k RPM I thought changing the plugs was a waste of time but I wanted to be sure to eliminate the simple stuff. Sure enough the plug change fixed the problem much to my surprise. Mark
 
The issue I mentioned previously in this thread turned out to be a bad fuel line collapsing internally. Even though the line was the CG approved type that was supposed to be impervious to ethanol degradation. Since they changed the fuel line she has run like a champ! Good luck with your repairs. Please keep us posted!


Sent from my iPhone using Tapatalk
Todd
 
Mark/Todd,
thanks for the info. These are things I will ask the mechanic to check. I think tonight I am going to change the spark plugs and check the wires. My tacs both with bounce up and down no matter what RPM I am running. But the engine never feels like it is going to shut down - just the needles jumping around. I just want one summer, month, week even a day without worrying or issues. Then again, I guess it wouldn't be boating. LOL
 
To the advice from Mark.
i changed the wires on my engine and it fixed the problem.
i could not get above 3000 rpm and Mark suggested wires.
pretty cheap and pretty easy.
 
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