Cos_man99
New Member
Hello All,
I am essentially at my wit's end and am ready to call in the (expensive) professionals. Recently bought the boat. Twin 7.4L MPI Mercs L29 engines with MEFI3 electronics.
This engine is serial number 0L300761
From day one it has started and run fine at idle. In the off season I did cap, rotor, plugs, and wires (as well as impeller). First run of the year it ran fine the entire day. Running up over 4,000 on the tach to get on plane then cruising around 3600 at 27 MPH. Then on the second trip out (and every trip since) it has run great until it gets around 4,000 while coming up on plane then it stumbles and the tach drops to 3200. Pushing the throttle forward gets no more out of her. Some times coming up on plane it will get to 4100, 4200, 4300, and be running perfectly even for up to 15 seconds (some trips it runs fine long enough to pull back on the throttles to get to a nice 3600 on both tachs for cruising). Then it bogs. I don't want to put ideas in people's heads but in the interest of describing it, I will say it just doesn't seem fuel related. It seems spark related. When it stumbles it happens so suddenly. It is not like a starved for fuel situation I don't think. Once the throttle is pulled back to reduce RPM's under 3,000 it runs smoothly again. It really feels like it is dropping a cylinder.
Here is what I have done in an attempt to fix it:
Compression test performed by the marina staff in the off season. All cylinders good. I don't remember the numbers but they were all right where they should be.
Swapped ECM's between engines. No change. Problem continues on the Starboard.
Swapped Distributor modules. Same. Problem continues on the starboard.
Swapped Knock sensors. Same. Problem continues on the starboard.
Swapped MAP sensor. Same. Problem continues on starboard.
Sent injectors out to be serviced and flow tested. Came back ready to go and reinstalled them.
Tested fuel pressure through schrader valve on fuel rail. It is just under 40 at idle and at 43 under load. Even during the fuel pressure test run the engine ran up to 4,000 on the tach then started bogging. No change on the fuel pressure during this time.
I have put the ECM into service mode with the paperclip technique. It gives 2 beeps and 4 pulses. Some charts online show that there is no code 24. Other charts show that 2 beeps and 4 pulses is actually code 25 which is low voltage for Manifold Air Temp Circuit (which means temp is high). I swapped MAT sensors and checked continuity and resistance on that circuit following Merc. Manual 23 recommendations. All checks out. Clearing this code does nothing for performance and after each run the code is back.
Pulled all the plug wires today (one at a time) and found them to have what I think is correct resistance ranging from 10,000-13,000 depending on length. Pulled the plugs and there is no evidence of one cylinder behaving any differently than the others.
For the 5 to 30 seconds that it gets up to and over 4,000 on the tach it is healthy and strong. Performs exactly as the port engine performs. So I do not think it is internal engine components like cam lobes or bad valves. Also it ran great this spring on the first run of the year.
Thinking about renting a diagnostic computer from a guy on ebay for $75.00/week but I am not sure if I would even know what to look for and if I can learn anything from it.
Anybody with insights they could offer, I would really appreciate it.
I am essentially at my wit's end and am ready to call in the (expensive) professionals. Recently bought the boat. Twin 7.4L MPI Mercs L29 engines with MEFI3 electronics.
This engine is serial number 0L300761
From day one it has started and run fine at idle. In the off season I did cap, rotor, plugs, and wires (as well as impeller). First run of the year it ran fine the entire day. Running up over 4,000 on the tach to get on plane then cruising around 3600 at 27 MPH. Then on the second trip out (and every trip since) it has run great until it gets around 4,000 while coming up on plane then it stumbles and the tach drops to 3200. Pushing the throttle forward gets no more out of her. Some times coming up on plane it will get to 4100, 4200, 4300, and be running perfectly even for up to 15 seconds (some trips it runs fine long enough to pull back on the throttles to get to a nice 3600 on both tachs for cruising). Then it bogs. I don't want to put ideas in people's heads but in the interest of describing it, I will say it just doesn't seem fuel related. It seems spark related. When it stumbles it happens so suddenly. It is not like a starved for fuel situation I don't think. Once the throttle is pulled back to reduce RPM's under 3,000 it runs smoothly again. It really feels like it is dropping a cylinder.
Here is what I have done in an attempt to fix it:
Compression test performed by the marina staff in the off season. All cylinders good. I don't remember the numbers but they were all right where they should be.
Swapped ECM's between engines. No change. Problem continues on the Starboard.
Swapped Distributor modules. Same. Problem continues on the starboard.
Swapped Knock sensors. Same. Problem continues on the starboard.
Swapped MAP sensor. Same. Problem continues on starboard.
Sent injectors out to be serviced and flow tested. Came back ready to go and reinstalled them.
Tested fuel pressure through schrader valve on fuel rail. It is just under 40 at idle and at 43 under load. Even during the fuel pressure test run the engine ran up to 4,000 on the tach then started bogging. No change on the fuel pressure during this time.
I have put the ECM into service mode with the paperclip technique. It gives 2 beeps and 4 pulses. Some charts online show that there is no code 24. Other charts show that 2 beeps and 4 pulses is actually code 25 which is low voltage for Manifold Air Temp Circuit (which means temp is high). I swapped MAT sensors and checked continuity and resistance on that circuit following Merc. Manual 23 recommendations. All checks out. Clearing this code does nothing for performance and after each run the code is back.
Pulled all the plug wires today (one at a time) and found them to have what I think is correct resistance ranging from 10,000-13,000 depending on length. Pulled the plugs and there is no evidence of one cylinder behaving any differently than the others.
For the 5 to 30 seconds that it gets up to and over 4,000 on the tach it is healthy and strong. Performs exactly as the port engine performs. So I do not think it is internal engine components like cam lobes or bad valves. Also it ran great this spring on the first run of the year.
Thinking about renting a diagnostic computer from a guy on ebay for $75.00/week but I am not sure if I would even know what to look for and if I can learn anything from it.
Anybody with insights they could offer, I would really appreciate it.
