2000 MAG 454 MPI, loss of power

Thanks for closing the loop. There are so many threads here (and everywhere) that get dropped when the problem gets solved. I too had suffered from an overzealous mechanic who had changed out the block 3 times within 4 years (the old Gen VI ingestion before they switched out the exhaust). I think he was so fed up with the motor that he threw it together the last time, and broke a number of injectors. Intermittent, but got worse over time. I now do all of my own mechanical work. I can afford my rate, and I know more about 1998 Magnum MPI big blocks than I ever wanted to.
 
Those injectors break pretty easy. When I was changing the manifolds and elbows I let the ECM and harness just hang when I disconnected elbow and that pulled on the injector wires breaking plastic connector. I had changed plugs, wires and cap when i did the manifolds. I was going nuts thinking i messed up putting wires back on.
 
Hi Folks,
This is an old thread but I have(had) the same power issue. I hope my nonsense can help someone. I realize I am not the smartest guy, but trying to learn.

It all started with a starboard engine failure at sea. The whole circuit to the fuel system circuit was dead. All Fuses were Good - no ECM codes.... No power to anything in the fuel circuit - fuel pump not turning on. After Lots of pulling connectors and plugs and testing to find this issue It turned out I had a problem with the connectors on the harness to the fuel pump cutoff relay(relay itself was fine), this loops from the engine harness to the fuel cutoff relay on the rear of engine bay back to the fuel pump. The bush fix was bypassing the cutoff relay using the plugs (good design in a way - allows one to bypass the relay and turn the fuel cutoff switch permanently on) ... corrosion in the plug connector was the root cause and I replaced a 4 plug connectors (with aftermarket connector plugs as I could not find the OEM style plugs - help on this appreciated).

After this fix I took the boat and have I started having this power issue in this thread (same exact thing - will not go over 3000 underway like there was rope around prop), but at dock engine revs great- I could not figure it out - bad gas? No codes .. I changed the MAP sensor - no change - issue remained. I changed the fuel filters .. pump was running pressure seems OK ... very confused - no mechanics want to touch it where I am. Boat sat all summer < 10 hours on boat.

So I tried it the other day - with renewed energy to solve issue ..

I took it out to test - first batteries were low - had to use all 4 to start the suspect engine. Did this affect anything? Did disconnecting the power to the ECU reset anything?
Took it out past no wake zone ... I started to give it throttle but saw the problem engine was getting hot (20 minutes to open water from doc) - (never goes over 170 normally) Stopped and let it cool - while cooling checked the sea traps on the raw water intakes, yep - grass found in trap - hot engine. To be truthful the water flow in the exhaust was weak, Lots of smoke but i was going to "clear it out" - " it has not been run" I think I need to change the raw water pump impellors. I should have noticed that sooner.

Cleaned them and waited to cool. The engine got to 210 deg while it sat cooling. No leaks anywhere. But after cooling for an hour. Started up and got moving and the temp was holding low and it had full power again!. Got up on plane and it stayed cool. Ran another 1/2 hour and it was great. YaY!

At this point - no issue - what was it? bad gas? ongoing intermittent wiring issue, I don't know.
But could it have been the heat and basically resetting ECU by not having power to it, and or getting past the bad gas that fixed it?

Well, I have a set of plenum gaskets and I will take it off and inspect all vacuum lines as indicated here and hopefully extend the fuel rail test( Schrader valve) port to somewhere I can test it while under operating load. (stupid design under intake). I will test injectors and wires as well and watch this thread.

The Saga will continue.. but boat comes out this Friday so no updates till the spring. No good having a boat you can't trust.

Harley - Niantic, CT
34' Sea Ray Amberjack - 2001 flavor
T-454 Magnum® MPI Horizon MerCruiser® (T-380 hp - 283 kW) 7.4L
 
Hi Folks,
This is an old thread but I have(had) the same power issue issue. I hope my nonsense can help someone. I realize I am not the smartest guy, but trying to learn.

It all started with a starboard failure at sea. The whole circuit to the fuel system was dead. Fuses Good - no ECM codes.... No power to anything - fuel pump not turning on. After Lots of pulling connectors and plugs and testing to find this issue.
It turned out I had a problem with the connectors on the harness to the fuel pump cutoff relay(relay itself was fine) , this loops from the engine harness to the fuel cutoff relay on the rear of engine bay back to the fuel pump. The bush fix was bypassing the cutoff relay using the plugs (good design in a way - allows one to bypass the relay and turn the fuel cutoff switch permanently on) ... corrosion in the plug connector was the root cause and I replaced a 4 plug connectors (with aftermarket connector plugs as I could not find the OEM style plugs - help on this appreciated).

After this fix I took the boat and have I started having this power issue in this thread (same exact thing - will not go over 3000 underway like there was rope around prop), but at dock engine revs great- I could not figure it out - bad gas? No codes .. I changed the MAP sensor - no change - issue remained. I changed the fuel filters .. pump was running pressure seems OK ... very confused - no mechanics want to touch it where I am. Boat sat all summer < 10 hours on boat.

So I tried it the other day - with renewed energy to solve issue ..

I took it out to test - first batteries were low - had to use all 4 to start the suspect engine. Did this affect anything? Did disconnecting the power to the ECU reset anything?
Took it out past no wake zone ... I started to give it throttle but saw the problem engine was getting hot (20 minutes to open water from doc) - (never goes over 170 normally) Stopped and let it cool - while cooling checked the sea traps on the raw water intakes, yep - grass found in trap - hot engine. To be truthful the water flow in the exhaust was weak, Lots of smoke but i was going to "clear it out" - " it has not been run" I think I need to change the raw water pump impellors. I should have noticed that sooner.

Cleaned them and waited to cool. The engine got to 210 deg while it sat cooling. No leaks anywhere. But after cooling for an hour. Started up and got moving and the temp was holding low and it had full power again!. Got up on plane and it stayed cool. Ran another 1/2 hour and it was great. YaY!

At this point - no issue - what was it? bad gas? ongoing intermittent wiring issue, I don't know.
But could it have been the heat and basically resetting ECU by not having power to it, and or getting past the bad gas that fixed it?

Well, I have a set of plenum gaskets and I will take it off and inspect all vacuum lines as indicated here and hopefully extend the fuel rail test( Schrader valve) port to somewhere I can test it while under operating load. (stupid design under intake). I will test injectors and wires as well and watch this thread.

The Saga will continue.. but boat comes out this Friday so no updates till the spring. No good having a boat you can't trust.

Harley - Niantic, CT
34" Sea Ray Amberjack - 2001 flavor
T-454 Magnum® MPI Horizon MerCruiser® (T-380 hp - 283 kW) 7.4L
Great update
, glad it’s going your way!

please let me know the oars you use to extend the shrader valve, I need to do that on mine
 
Hi Folks,
This is an old thread but I have(had) the same power issue issue. I hope my nonsense can help someone. I realize I am not the smartest guy, but trying to learn.

It all started with a starboard failure at sea. The whole circuit to the fuel system was dead. Fuses Good - no ECM codes.... No power to anything - fuel pump not turning on. After Lots of pulling connectors and plugs and testing to find this issue.
It turned out I had a problem with the connectors on the harness to the fuel pump cutoff relay(relay itself was fine) , this loops from the engine harness to the fuel cutoff relay on the rear of engine bay back to the fuel pump. The bush fix was bypassing the cutoff relay using the plugs (good design in a way - allows one to bypass the relay and turn the fuel cutoff switch permanently on) ... corrosion in the plug connector was the root cause and I replaced a 4 plug connectors (with aftermarket connector plugs as I could not find the OEM style plugs - help on this appreciated).

After this fix I took the boat and have I started having this power issue in this thread (same exact thing - will not go over 3000 underway like there was rope around prop), but at dock engine revs great- I could not figure it out - bad gas? No codes .. I changed the MAP sensor - no change - issue remained. I changed the fuel filters .. pump was running pressure seems OK ... very confused - no mechanics want to touch it where I am. Boat sat all summer < 10 hours on boat.

So I tried it the other day - with renewed energy to solve issue ..

I took it out to test - first batteries were low - had to use all 4 to start the suspect engine. Did this affect anything? Did disconnecting the power to the ECU reset anything?
Took it out past no wake zone ... I started to give it throttle but saw the problem engine was getting hot (20 minutes to open water from doc) - (never goes over 170 normally) Stopped and let it cool - while cooling checked the sea traps on the raw water intakes, yep - grass found in trap - hot engine. To be truthful the water flow in the exhaust was weak, Lots of smoke but i was going to "clear it out" - " it has not been run" I think I need to change the raw water pump impellors. I should have noticed that sooner.

Cleaned them and waited to cool. The engine got to 210 deg while it sat cooling. No leaks anywhere. But after cooling for an hour. Started up and got moving and the temp was holding low and it had full power again!. Got up on plane and it stayed cool. Ran another 1/2 hour and it was great. YaY!

At this point - no issue - what was it? bad gas? ongoing intermittent wiring issue, I don't know.
But could it have been the heat and basically resetting ECU by not having power to it, and or getting past the bad gas that fixed it?

Well, I have a set of plenum gaskets and I will take it off and inspect all vacuum lines as indicated here and hopefully extend the fuel rail test( Schrader valve) port to somewhere I can test it while under operating load. (stupid design under intake). I will test injectors and wires as well and watch this thread.

The Saga will continue.. but boat comes out this Friday so no updates till the spring. No good having a boat you can't trust.

Harley - Niantic, CT
34" Sea Ray Amberjack - 2001 flavor
T-454 Magnum® MPI Horizon MerCruiser® (T-380 hp - 283 kW) 7.4L

When was the last time you replaced the ignition modules under the distributor? I had an issue once with a 454 Horizon that the motor would not go over 3000. I replaced the module and the problem went away.
 
When was the last time you replaced the ignition modules under the distributor? I had an issue once with a 454 Horizon that the motor would not go over 3000. I replaced the module and the problem went away.

ALSO - I'll add my $0.02 on this topic for future reference. There are two screws that hold down that ignition module and one (1) of them serves as a ground. Without a solid ground many weird things can happen (no-start, rough idle, high speed issues, etc). After experiencing issues two seasons ago and last season in different engines, I replaced the modules and it fixed the problems. The ignition module is NOT supplied with new screws. In hindsight i think it was the process of pulling the screws, cleaning them and putting them back in that solved the problem. My future maintenance plan is to pull the ignition module every year and clean up the screws, and at the same time put new heat transfer lube on the module when installing.

In a 1998 370 DA 7.4 MAG MPI Horizon, the Mercruiser part number for the screws are 10-811638 and it is no longer available (dreaded NLA). The screw thread is M3.5 by 0.35 pitch by 20 mm length, and is NOT a normal screw, and are pretty much unobtanium at places like bolt depot, graingers, etc. You don't want SS either as SS does not conduct ground well. Finally found the AC Delco part number (AC Delco 10469668) somewhere on the interweb and puchased at RockAuto. They are a galvanized or cadmium coated screw. I will be installing them next year.

Hope this helps someone else.
 
When was the last time you replaced the ignition modules under the distributor? I had an issue once with a 454 Horizon that the motor would not go over 3000. I replaced the module and the problem went away.
Never ... but i can try that too. Thank you.
 
You KNOW it will...... These are fragile beasts. Any little thing makes them squirm.
 

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