Map pressure sensor low would be the direction I would go first, It relays info to the TPS and controls the fuel.
Make sure it's getting the 5 volt input from the ecm.

Are your injectors original? If so , there due for a service at the very least.

 

Hey:
Yes, I ran the non-scan diagnostic steps in the Mercruiser Factory manual and it is getting the 5v reference signal.

I have only owned the boat for less than a year but the previous owner had the injectors service/rebuilt back in 10/2017 (have receipt). Do you think injectors need to be cleaned?
Thx!

 

Can't hurt. If those are 20 year old fuel lines I'd replace them all. Check your fuel pressure, them parts don't last forever.

Is it still throwing the MAP code ?

 

The "MAP voltage low" code was one time and has not thrown it again. I ran the boat a few days ago with the scan tool in "live" and MAP, TPS, ECT all were giving normal readings as they related to increasing and decreasing RPM's. While idling back to the home dock, the engine almost sounds like it is running on certain cylinders and not firing all?

 

Yup, does sound like a fuel problem and the old mefi won't throw a code for that. If it was my boat, and I have an old 1 too, I would start with a complete tune up and while the plugs are out, check compression.

I just replaced everything on my fuel system from A to Z, new lines, injectors , pump , regulator and it now runs like the beast it was.

 

Thx...
Compression was 150psi on all, right before I replaced the plugs (hoping plugs were the issue). The plugs that were in the engine was the NGK non-platinum tip and I replaced with the same ones (are the platinum tip necessary?)

If the new distributor cap and rotor doesnt clear it up, I guess I will pull the intake off and check fuel pressure and pull the injectors
Thx again

 

Start with the fuel pressure. Compare it to the other engine. There are lots of threads with similar symptoms always ending up with the fuel pumps/lines being the cause if they haven't been replaced.

You are fortunate it didn't quit completely in Bimini.

 

Thx....the more I read/research, the more it sounds like a fuel pressure regulator issue. Once I have new intake gaskets in hand, I will pull the intake and check the pressure at the fuel rail and then move on to injectors if necessary.

 

Plan on the fuel pump and regulator, and as Scoflaw says, while you're digging around that part of the engine, do the fuel lines between the separator going in and coming out. Mine had kinks in them from how they are plumbed. Still, you gotta pay attention to the fact you only had 1 hiccup and then nothing crossing the gulf stream? Then 2 weeks later of non use, a big difference? Pulling the plenum is not necessary to hook up a fuel pressure gauge, just don't try it hot, and as I learned, measuring fuel pressure while at the dock no matter how much you rev it is no test. You need to be underway at load, at speed to see what's really happening. Since you are going into the top, I would not hesitate to pull the rail, and injectors, and clean or replace them. Find my other thread that went on for months chasing pretty much what you are experiencing now. Pay Attention to Scoflaw, PlayDate, and Douglee if he shows up.
Good Luck! BTW, I'm sure you don't have a 7.1 liter MAG MPI. Engine serial numbers will help these guys know which plumbing you have. 2000 was a very transition year or two for Mercruiser.

 

Thx....
The vessel build date is 2000 but the engines were factory built in the latter part of 1999 apparently. They both are the 454 CID (7.4L) MAG MPI Horizon (Gen VI) with serial # 0L398572 with the MEFI 3 ECM.
In looking at the factory manual, it looks to me like the schraeder valve is on the fuel rail and buried under the intake? If I dont need to take the intake off at this point, I would rather not.

 

Not intake but probably plenum. Extend the gas fitting while you are there so to make a simple hook up for the next time you need to check it/

 

Yeah...reading 370Dancer thread and how he did it
Good Idea!

 

And, congrats to you for taking gassers to the islands. I am interested in your exploits. Used to do it a LOT, but in a 46 Post. Different time, and different parameters back then. Fresh water at less than 50 cents a gallon was hard to come by unless you made your own. We did. Not so much of an issue now, etc.

 

15" of vacuum is low if that reading was taken at idle with a known to be accurate gauge.
How old are the ignition wires and are they routed properly? (not just connected to the correct cylinders)

 

Thx Mitch
According to my Mercruiser Manual for this year and model engine, 15" at idle is the spec.

 

Thx Mitch:
I am in Palm Coast, Fl....just above Daytona. Since I am leaning towards a bad fuel pressure regulator or perhaps the vacuum line to the regulator being bad, it may affect the vacuum in the plenum?

 

Kinda the other way around, low engine/manifold vacuum will affect the performance of the regulator. The regulator should hold vacuum with a good vacuum pump/tester. The fuel pressure should increase approximately 10 psi with the vacuum hose disconnected from the regulator.

 

You need to have the fuel gauge on it when it hits the rpm wall underway. There is another rubber vacuum line under the plenum that could have failed.

 

Thx....I think I am going to pull the plenum and at least clean it out and inspect/replace all vacuum lines. Then I am going to do what you suggested and pipe the fuel rail schraeder valve out and above the plenum so that I can install the fuel pressure gauge and watch it while running the boat on a plane at 3200rpm and see what it is doing as I push the throttle.

 

So finally back at the boat....installed new distributor cap, new PCV valve, checked coil (ok) checked oil pressure sensor (ok). Fuel pressure remains constant at 43psi (manual spec) at idle and under load and when I remove vacuum line nothing changes....I even added all the way up to 25 inHg of vacuum to regulator with vacuum pump with no change in fuel pressure? Is this a sign the regulator is "stuck open and allow too much pressure at the rail for idling conditions?

I thought that the fuel pressure should change by 3-5psi with vacuum on and off?

If the fuel regulator wouldnt hold a vacuum doesnt this mean the diaphram is broken and as a result low fuel pressure?

Thx!

 

Yah the reg. isn't working. 39#'s at idle and 43 at speed is normal. So your fuel pressure is high at idle but doesn't explain why it doesn't rev out. What was the FP doing at WOT ?

 

The diaphragm in the FP regulator likely has a hole in it. That's why it doesn't respond to vacuum.

 

My dock is 30-40 mins from an area were I can open the throttles and get on a plane (canal system with no wake zone). I did change the plugs when this all started but I think the high fuel pressure has fouled them again before I can open it up?

Sounds like I need a new regulator and change the plugs again....any value in going to the Iridium/Platinum with this model motor? I am running the stock NGK's that the manual calls for.
Thx

 

NGKs are fine for that motor.

 

Hi just seeing if you found the problem? Thanks

 

Hi:
Well....the fuel pressure regulator was stuck “wide open” meaning fuel pump was delivering 43psi even at idle, I also took off the air plenum and found it was coated with oil inside as was the vacuum hose leading down to the regulator. Also the small hose from the plenum to the intake horn (closest to the flame arrestor was not sealing so I replaced it as well.
New fuel pump, regulator, vacuum hoses, cleaned out plenum and to be safe replaced intake sensor, ECT sensor and MAP sensor. I also went ahead and replaced the coil and Ignition module.

Engine starts right up and idles well, next week will take out for test at speed!

 

Thanks for the update, I hope all is good now I am in the process of what seems to be the same issue.

 

Cdb Hi just curious with the issue you were having did you get one long beep when you turned on the ignition switch on the dash? Thank you.

 

the starboard side should always give you 1 beep. Port side, 2 beeps.

 

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

 

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.

 

ALSO...

Hope this helps someone else.

 

You KNOW it will...... These are fragile beasts. Any little thing makes them squirm.