Mercruiser 7.4MPI wont go past 2600 rpm and rpm jumping at acceleration

B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
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
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
Sounds like you have a tach problem. Get another tach and/or a diagnostic box and see what it says your RPM's are.
Tach's jumping around used to be common on cable driven tachs but that's not normal for electronic tachs.
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
greno said:
Sounds like you have a tach problem. Get another tach and/or a diagnostic box and see what it says your RPM's are.
Tach's jumping around used to be common on cable driven tachs but that's not normal for electronic tachs.
Click to expand...

But would a bad tach cause the boat to only go 2600 rpm max at WOT?
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
BigDog98 said:
But would a bad tach cause the boat to only go 2600 rpm max at WOT?
Click to expand...
No, the tach wouldn't cause that. But maybe your tach is reading wrong. Let's find that out first. Then let's see where the RPM's are.
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
greno said:
No, the tach wouldn't cause that. But maybe your tach is reading wrong. Let's find that out first. Then let's see where the RPM's are.
Click to expand...

Well that really does not matter since there is a issue to where it wont even get on plane. I really dont think its the tach because it only starts to do this after 2000 rpm.
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
Ok, let's skip the tach for now. With all the other things you've done I didn't see where you checked the injectors. Maybe you have some dirty injectors.
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
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
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
BigDog98 said:
...boat starts to rattle and shake a lot...
Click to expand...
Ok, that could mean you have a bad harmonic balancer or even a coupler going bad. Bad ignition doesn't usually produce bad shaking just no power.
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
greno said:
Ok, that could mean you have a bad harmonic balancer or even a coupler going bad. Bad ignition doesn't usually produce bad shaking just no power.
Click to expand...

From my experiance a dead cylinder can cause a vibration in the engine or several dead cylinders. Coupler is brand new and replaced last season. Harmonic balance is straight and true according to my caliper meter.
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
If you're dropping several cylinders, yes, you'll get vibration. One cylinder, not so much. I've had people bring cars to me that had one cylinder completely inoperable for months and they never knew it. Could have a bad distributor, bad cap, bad rotor or bad ignition module. Something that would cause spark to go where it shouldn't go.
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
greno said:
.... One cylinder, not so much. I've had people bring cars to me that had one cylinder completely inoperable for months and they never knew it. ...
Click to expand...

You can't compare car engines to boat engines. Load profiles are different. Most of the time camshafts are different fuel and ignitions are different. Comparing apples and oranges..

Chris....
 
G

greno

Petty Officer 2nd Class
Joined
Nov 23, 2012
Messages
182
Sure, the load profile is different but the amount of vibration one cylinder dropping imparts is not much different. I had one plug wire drop off on my 5.0L and there was no vibration. I could tell something wasn't quite right because it wouldn't get out of the hole as quick.
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
greno said:
Sure, the load profile is different but the amount of vibration one cylinder dropping imparts is not much different. I had one plug wire drop off on my 5.0L and there was no vibration. I could tell something wasn't quite right because it wouldn't get out of the hole as quick.
Click to expand...

Tomorrow i am going to get a new module that goes under the distributor cap and see what that does for me.
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
BigDog98 said:
Tomorrow i am going to get a new module that goes under the distributor cap and see what that does for me.
Click to expand...

If the ignition sensor was faulty, it would fail on all cylinders. Same with the main ignition amplifier. If it's just one or a couple of cylinders, it's more likely a fuel or mechanical problem. Have you had the injectors cleaned?

Chris........
 
B

BigDog98

Chief Petty Officer
Joined
Nov 12, 2013
Messages
525
achris said:
If the ignition sensor was faulty, it would fail on all cylinders. Same with the main ignition amplifier. If it's just one or a couple of cylinders, it's more likely a fuel or mechanical problem. Have you had the injectors cleaned?

Chris........
Click to expand...

Here is my logic, If the injectors were clogged then it would always run like crap but since it has its up and downs then it is very unlikely to be the injectors. I was talking to a guy who has the same exact problem as i am having and said that the module was the solution for him. Seems like that module failing is very common on those engines. Cant hurt to try. The one thing that points me to the module is also that i have hard time starting the engine at times when it get hot or even cant start it sometimes.
 
tpenfield

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
18,224
I think you are going to have to hook the computer up to the engine and get some real time readings of the ECM data.

the alarm may be a sensor issues, but the computer diagnostics will help identify the particular sensor.
 
Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
50,487
its not the coil, almost never is. it was a fluke it ran better for a short period of time. probably simply removed the corrosion on the wires as you tightened the terminals.

its also not the ignition module as Chris pointed out.

low fuel pressure is one issue. however why.

will need to not only plug in a mechanical fuel gauge, however as Ted pointed out. need to plug in rhinda as well.

the shaking from the motor is either multiple cylinders firing at once, or multiple cylinders not firing. now you need to plug in to find out why.
 
You must log in or register to reply here.
Top