Mercruiser 7.4MPI wont go past 2600 rpm and rpm jumping

B

Bigdog98

New Member
May 8, 2016
25
NY
Boat Info
1999 270 Sundancer , 7.4MPI Bravo III
Engines
7.4 MPI
Hello all. Looking for some help as I am baffled to resolving my issue. I have a mercruiser 7.4MPI serial # 0L085399- MEFI3. I am having issues that when I go past 2000 rpm the needle on the gauge rapidly starts to jump from 2000 - 2200 -2500 and so forth its not a smooth increase in rpm like it should be. The engine at WOT wont go past 2600 rpm. I have already replaced the fuel pump, ran an auxiliary tank to see if it was gas issue, all the plugs and did cap n roter. I checked fuel pressure at the fuel rail via shrader valve and I was getting 40psi of pressure at 2600rpm and about 34psi anything below that steady. I checked my compression and that is all good, oil pressure is at about 45-50 psi, no alarms however I did have a code 44 not too long ago with no knock detected but that problem has been resolved, scanned engine and no codes come up. Anyone have any recommendations on what to check next? Thank You
 
Coil could starting to go or ignition wire could have small cracks in them.
 
Did you clean the filter on the high pressure pump in the fuel sump on top of the engine?
 
from your initial description of the problem it sounds like you may have a problem with the tach and not the engine....an engine will not be able to 'rapidly' jump in rpm's as you described unless a prop hub or engine coupler is slipping....

did you notice any difference in the performance of the boat?...does it still get on plane as always?....

cliff
 
Bigdog98 said:
Hello all. Looking for some help as I am baffled to resolving my issue. I have a mercruiser 7.4MPI serial # 0L085399- MEFI3. I am having issues that when I go past 2000 rpm the needle on the gauge rapidly starts to jump from 2000 - 2200 -2500 and so forth its not a smooth increase in rpm like it should be. The engine at WOT wont go past 2600 rpm. I have already replaced the fuel pump, ran an auxiliary tank to see if it was gas issue, all the plugs and did cap n roter. I checked fuel pressure at the fuel rail via shrader valve and I was getting 40psi of pressure at 2600rpm and about 34psi anything below that steady. I checked my compression and that is all good, oil pressure is at about 45-50 psi, no alarms however I did have a code 44 not too long ago with no knock detected but that problem has been resolved, scanned engine and no codes come up. Anyone have any recommendations on what to check next? Thank You
Click to expand...


Can you get to higher RPM’s when you are out of gear?
 
He has no rubber hub and if the coupler was failing it would rev sky high. The injectors are bank fired with that mefi 3, so if you lose 1 it could drag down the other 3 with it, making it a 4 cylinder motor. Look at your plugs. 1,7,4,6 all inject together, so if you have 4 black plugs that could be your problem or could be ignition as well.
 
CliffA said:
from your initial description of the problem it sounds like you may have a problem with the tach and not the engine....an engine will not be able to 'rapidly' jump in rpm's as you described unless a prop hub or engine coupler is slipping....

did you notice any difference in the performance of the boat?...does it still get on plane as always?....

cliff
Click to expand...
the engine will not go over 2600rpm it wont get on plane in gear but in idle the gauge works fine and i can rev it up to 4600
 
jonsm114 said:
Did you clean the filter on the high pressure pump in the fuel sump on top of the engine?
Click to expand...
my engine is a 1998 and it has a cool fuel pump on the side of the engine under the manifold which i had replaced. I dont think it has a high pressure pump ontop of the engine. But if it does then i would like to know
 
Bigdog98 said:
Yes out of gear i can get the engine to rev up to 4600 rpm as should be. In gear it wont go over 2600rpm
Click to expand...

Okay, now we are getting somewhere.

So the engine can rev to normal redline.

Next, tell me about your boats hull. Is it white, clean, kept on a trailer, or does it resemble a woodstock female hippies armpit with unidentifiable vegetation growth and its own bio-system? If kept in a slip, when was the last time you pulled the boat and pressure washed the hull from the waterline and below? I see you are in New York State. Similar to my home state of Wisconsin, this is the time of year the hull bottoms are at their worst. I want to make sure resistance from bottom growth is not your issue.

PS: I’m impressed with how much engine diagnostics you have completed. You did not mention cleaning the flame arrestor, so I’m assuming based on the rest of your post that you keep your flame arrestor reasonably clean.
 
Last edited:
I had the same problem. Replace the module attached to the underside of the distributor.
 
Here is a bit of an update. Had a bit of time to kill today and decided to play around with the boat today. I decided to replace the spark plug wires with extra ones I had laying around and also replaced the ignition coil with a spare that I had as well. Both are not in new condition! Anyhow I changed out those 2 things and what do you know, boat ran good as new. About an hour later I call my dad over to give him the good news and we decided to take the boat out again (so he can see it for himself) and this time as we were leaving the no wake zone the rpm started to jump a little and I could hear the tone of the engine change when it did this. It went from 1200rpm and dropped down to 1000 rpm and then when getting on plain it started doing that rpm jump again and gradually would get worse going from 3400 rpm being the max rpm I could push and then gradually dropping again to 3000rpm and back to 2600 rpm where it all started from. Also i notice when the engine starts to go that rpm jump the engine and boat starts to rattle and shake a lot almost like some cylinders are not firing. And then an alarm came on 2 beep and pause about every minute. This alarm is code 44 no knock detected which I thought I had resolved already. So this is my conclusion, this issue is definitely not a fuel pump issue it seems to me like it is a ignition spark issue. Looks like overtime or maybe as the engine gets hot it starts to either A) loose spark in some cylinders or B) starts to get a weak spark. Its looks like its a electrical problem but I do not know where to even start. I think this problem is also causing my alarm to go on with no knock detected. Maybe the spark advance module is not working properly. So tomorrow I will purchaser a brand new ignition coil and see what that does for me. Does anyone have any ideas as to is my theory sounds correct or what to check? Thank You
 
iBoat Skipper Doug said:
Okay, now we are getting somewhere.

So the engine can rev to normal redline.

Next, tell me about your boats hull. Is it white, clean, kept on a trailer, or does it resemble a woodstock female hippies armpit with unidentifiable vegetation growth and its own bio-system? If kept in a slip, when was the last time you pulled the boat and pressure washed the hull from the waterline and below? I see you are in New York State. Similar to my home state of Wisconsin, this is the time of year the hull bottoms are at their worst. I want to make sure resistance from bottom growth is not your issue.

PS: I’m impressed with how much engine diagnostics you have completed. You did not mention cleaning the flame arrestor, so I’m assuming based on the rest of your post that you keep your flame arrestor reasonably clean.
Click to expand...

Bottom is as smooth as a babies ass. Growth is not the issue, but read my latest post. Believe it or not I am a sort of marine mechanic work for a full service boat yard. SO you would think I know a thing or 2 except when it comes to these efi and mpi engines i am clueless.
 
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