1999 380 Mercruiser MAG MPI fall off after 2000 rpm 2450 MAX

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by 370Dancer, May 20, 2019.

  1. techmitch

    techmitch Well-Known Member SILVER Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    Still can't get past that low charging system voltage. You really need 13.8 - 14.2 to proceed. It's electronic engine control diagnosis 101.
     
  2. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    Agree. But the Starboard engine runs fine with the same voltage which leads me to suspect what the gateway is recording. Simple enough to monitor.....what do the helm gauges show? My bet the engines are seeing a lot more than 11 volts. I don't think the ECM will fire the injectors at 11 volts and based on the plugs.....he is getting plenty of fuel.
     
  3. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    Interesting they locked on to the voltage. I don't know how an engine starts and runs on 11 volts. The ECM shouldn't trigger the injectors which makes me suspect the reading. What does your multimeter read at the coil and what does your helm gauge read?
     
  4. techmitch

    techmitch Well-Known Member SILVER Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    They will run and fire the injectors in a low voltage situation, even much lower than 11 volts.
    Think about when an alternator stops charging. The engine will run until the battery voltage just gets too low and I've seen that threshold be as low as 7-8 volts on many occasions, even on fairly new engine systems. Will they run right? Absolutely not, but they will fire the injectors and run.
    Really shouldn't, (and I wouldn't) go any further until you have proper voltage to the coil, dist., and ECM/U.

    Edit to ad the word battery.
     
    Last edited: May 30, 2019
  5. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    My gauges are ALWAYS low for as long as I have had her, and I have not put a lead on the alternators yet. Happy to go voltage hunting.
    Might even do that before I start swapping injectors and regulators. I know, and you know, that it's only 4 things that matter whether it's ECM controlled or not. Air, fuel, spark, and compression. at the right time.
    Been thinking about how to introduce correct voltage. I'm pretty convinced that the alternators are not doing their job, or there is a sig drop across some connection. Nothing's smoking, and the Stbd continues to run with a crappy B+. Voltmeter in the shade in the morning. Thanks for the group think.
     
  6. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    We are all trying. It is hard to diagnose engine issues because you find yourself dropping into the middle of something. There is always a more complicated story to understand. I don't think it is electrical but Mitch does. Seems like a reasonable thing to put a volt meter on it and see what the story is. If you only have 11 volts on the coil with the engine running and shore power disconnected....that is no good and means that your alternators and/or batteries are kaput.

    Still....it doesn't explain why the Starboard engine runs other than it has a different ECM.
     
  7. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    OK, voltage checks this morning.
    First, the voltage being reported by the Fox Marine Gateway is 1 volt low from actual. I will take that up with Allen separately.
    measurements at hot side of 50 amp breaker on each engine, with ground going to the breaker mounting bolt. Confirming measurements taken at the starter hot side, and the alternator output. All the same. Even went cross engine to make sure my grounds were good. They are
    Engines off, Converter on: 13.8 volts
    Engines off, Converter off: 12.8 volts
    Engines on Converter off: 12.7 volts (gateway reads 11.7 volts)
    Engines on, revved to 1200 rpm: 12.7 volts (gateway reads 11.7 volts)
    Diagnosis: neither alternator is doing its job. Gateway misreading by 1 volt
    I have 1 or 2 alternators as spares. I'll be testing them first, and then replacing to see if I can get back to a charge state.

    This also explains a different issue I was having a while back but it was being masked. We always run the generator from start to finish of our outings. One day I lost the generator and we sat at a dock for a couple of hours with lights and stereo running before I realized the genny was off. One engine refused to start, so we headed home on one. 3 hours later, the running engine just died (along with everything else). Without the genset running the converter, we were just using up battery AH without recharging.
    Another note, where I said my gauges read low, they actually are right on. The marking that I thought was 12V is actually 13V.
     
  8. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    Makes sense. Frustrating that both alternators are dead. I'm curious whether it is a wiring issue or the alternator itself. I believe the exciter wire that triggers the alternator also feeds the ignition coil on each engine.

    Still....the generator is capable of providing enough amps to keep the batteries fed even though both alternators are not working. It's not ideal but that means there is enough voltage at the coils for both engines as evidenced by the Starboard engine working.


    So....that brings us back to the performance problem on the port engine.
     
  9. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    And, I actually have to go TDY, so it's going to be a week before I get back on it.

    I'm thinking the crud that suddenly showed up in plug #4 may have come out of the old injector if there is an old one there (which I think there is). That was the reason I changed them on the STBD engine months ago. This might have been one of them.
    So, back to the fuel rail, and gain some sanity on which injectors fire, which ones stick, and which ones don't fire.
     
  10. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    I'll be back on this tomorrow. Been following a couple of other similar threads, and I am confident that I have not thoroughly gone through the fuel system. I only cleared lines from the tank to the water sep. My first task is to yank the cool fuel II assembly, and check the regulator/screen along with the feed and return lines. I did get my 2012 replacement regulator off the old engine, but I wound up digging a black goo from the vacumn side, which I will assume is the diaphram, destroyed by ethanol. That is one vow I have taken with these motors. They will never see a drop of corn as long as I own them.
    More to come......
     
  11. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    I do think you are on the right track in making sure the fuel system is working correctly. That said, your rail pressure seemed okay which is usually where you see fuel pump problems appearing. Clogged injectors create a mix of rich and lean plugs which arguably you have. When you swapped injectors....at least on #4 the problem moved.

    You are fortunate you know how to do this work. Most of the mechanics I know gave up trying to fix problems like this and replace everything between the tank and the intake ($3k). The amount of time chasing debris is far more effort than replacing everything. That approach is expensive but having to service the boat multiple times for the same problem is far more expensive if you are in the business.

    I just can't figure out why Mercruiser takes so long to fix obvious issues that they have had with Cool Fuel and their Zeus pod drives.
     
  12. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    Yesterday, I pulled the injectors, and cool fuel system from the water separator to the rail.
    cleaned and tested every injector for sticking open or closed, and operation. Back to my 8 new ones.
    I did find one of the o-rings on the fuel line to the rail (rail side) was pinched, and compressed. I replaced all of the 0 rings on that fuel line, and on the 90 elbow from the fuel cooler to the high pressure pump. I blew out all lines, and checked the fuel pump for operation. I don't have a good way to test for pressure on the bench though. The regulator still looks pretty new (300 hour engine), and I did not find any fuel, goo, or rust in the vacuum line fitting. I did notice that the fuel feed line has a slight kink in it near the pump inlet side, which was what I found years ago on the other motors. I didn't have any replacements, and it's not too bad getting to that cool fuel assembly, so I put it all back together.
    I grabbed one of my alternators from the old motors (which was almost new when I pulled the engines), and replaced the port side. I immediately got 14 volts out of the alternator. So the old one was definitely bad, and I assume the Stbd is too. Two new ones on order, will be here tomorrow.

    This morning, I took it out for a test run to see if anything had changed.
    I snapped screenshots of the Fox Engine Monitors at 1250 RPM going out, and then at 3600 RPM on plane. I was a very happy guy, until I decided it was time to firewall them and see what I get. The Port engine started losing RPMs as soon as I pushed up, and I couldn't recover until I dropped to 2000 or so, and then got back to about 3100 RPM, but no more without it falling off. Stbd ran up like nothing was going on.

    Reported fuel burn went back up on the Port, and I have to give more throttle to synch the engines, although I now think that is a misnomer because of how fuel rate is calculated on these vintage engines. Got back to the dock. Ran the engines up to 3000 rpm, and then shut them down. Tried to restart 2 minutes later, and the Stbd started right up. The port labored turning the engine over, and would never catch, like out of fuel.

    This HAS to be fuel starvation, and I'm leaning towards the fuel line collapsing at that kink under high demand, and not relaxing until I'm at idle if it does at all. The other 2 possiblities will be the fuel pump not being able to keep up with demand, and leaning out the engine, or the regulator not doing its job.

    Fuel pump on order, and I have to source these fuel lines. Big money from Marineengine.com
    May just get some made locally.

    So my analysis is everything is groovy until I put the engines under max power demand. Then the Port goes flaky and stays that way until fuel is allowed to the engine again. Since I do not have my low pressure fuel pump in the loop at the moment for the port side, the restart could just be vapor locking. It is 87-90 degrees out. I have zero confidence in my fuel pressure gauge, which was reading zero through the whole test. Something is wrong with the connection to the rail, I'm guessing.

    Here's the run data from this morning, right before it all went bad.
    https://www.dropbox.com/sh/7of9p07aevnit51/AADrnu19o2ZG0C5U_OgpfpiOa?dl=0
     
    Chris-380 likes this.
  13. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    That is encouraging. It must have felt great to the point that you bumped the throttles up. :(

    The one thing you need is a fuel pressure gauge that works. It is the missing component from all the hard work you have put in. It would tell you in a heartbeat if you are running out of fuel pressure or the regulator is malfunctioning and would have saved you a ton of diagnostic work.

    I don't see anything notable from the performance data. Both engines look like they were doing well at 3600 rpm.

    The hard start should be resolved with the new pump. I am curious....aren't you tempted to swap fuel pumps with the Starboard side?
     
    Chris-380 likes this.
  14. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    There is no easy access to the Cool Fuel from the Stbd side of a 370. I'm sure it would involve laying over the top of the engine with the manifold and riser removed for clearance for a couple of hours, and lots of band-aids. I'd rather buy a pump and leave the good one alone for now. Yes I was happy until throttle up. Recommend a "good" fuel pressure gauge, please. Really, I'd like to set up a permanent rig to monitor fuel pressure, but that's another project.
     
  15. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    I thought I remembered seeing a bluetooth sensor that could plug into the fuel rail. I haven't been able to find it but that would make your life easier running the boat.

    Most of the quality fuel pressure test kits should work if they have a flexible hose to mate with the schrader valve. I really don't understand why GM put the valve where they did.

    https://actron.com/content/professional-fuel-pressure-tester-kit

    https://www.jegs.com/i/JEGS+Perform...VCR-GCh3fBggZEAQYAyABEgK7__D_BwE&gclsrc=aw.ds
     
  16. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    I didn't fall off the earth. Father's Day weekend intervened. Looking to run tomorrow am with 2 new alternators (already showing good charge voltage at the dock), and a new high pressure fuel pump on Port side. The regulator filter was really clean. The regulator will be about the only thing in the fuel feed I haven't changed. Fuel lines made local is a no go. If I am still in the woods, they are next along with the regulator. Scoflaw will be pleased......
     
    amajamar, Chris-380 and douglee25 like this.
  17. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    LOL, man I don't have your kind of patience. That puppy would be out, stripped clean, and off to the machine shop,..a month ago. Put back together and back on the water by now.

    Good luck with it though
     
    amajamar likes this.
  18. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    Couldn't wait. Ran out about 2 hours ago, and I am back to even throttle, 4000 plus on both engines, 20 knots at 40 gal per hour total. That's what it's always been. Maybe if you don't lie in the water 24/7/365 you might be a bit lighter, and get better than 1/2 MPG at cruise. I never have.
    Since we only really move a few times a year, that's ok. Our 46 Post was 1/3 MPG.
    I did cheat, and put a couple of small hose clamps on the fuel lines where they were kinking. Maybe that was it. We'll never know.
    Thanks for all the sanity. I'm happy enough. Beach House lives for a while longer.....
     
  19. douglee25

    douglee25 Well-Known Member

    Jan 13, 2008
    Dallas, TX
    Cruisers 3575
    Twin 7.4l
    Nice work!
     
  20. 370Dancer

    370Dancer Active Member

    854
    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    I'd like to figure out how to replace the fuel pressure schrader valves on the front of the fuel rail with permanent rigid piping out the side or front from under the plenum that will have a full time fuel pressure gauge on the other end. Surely the racers are doing this. Any ideas or experience?
     

Share This Page

Show Sidebar