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

scoflaw said:
Head over to OSO, where the racers hang
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Been there, but haven't done an exhaustive search. Google stuff is 99% throttle body carb stuff.
Did pick up an interesting thread that has the 2001 tech bulletin about vapor locking. Might be something in there. I have the actual tech bulletin. This text one seems more detailed.
Has a few dud characters in it. It did remind me that the original fuel line from the shut off solenoid to the fuel water separator was 12.5 mm ID, and it was real soft when I got the boat in 03. All other fittings and pass thrus are 3/8" hose, so when I replaced the line, I used 3/8". Maybe I do need to go back to 1/2" for that stretch (about 6 ft). All newer models showed using 3/8" line across the entire feed, so I figured I was ok, and this 1/2" thing was to try to avoid vapor lock, which it didn't. Now that I have low pressure pumps in front of the fuel water separator, it might not matter anymore.
 

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  • 99-7 Mercruiser tech bulletin on vapor locking from OSO.txt
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So for racing engines we use an adjustable fuel regulator that has a port that accepts a mechanical fuel pressure gauge rated for 100 psi. You can find them on ebay some come with braided hoses and AN fittings. I am a bit concerned about using them on recreational boats. First.....it isn't exactly ignition proof and second.....vibrations causing a leak that sprays fuel over the engine just will end in a bad place.

If I was going to do something.....I would probably modify the rail to accept an electrical fuel sensor and go that route.
https://www.summitracing.com/parts/atm-4371
 
That's what I posted above. Something to attach to the rail directly. In my opinion, you're over thinking and complicating the solution. You rarely need to monitor fuel pressure on a factory type engine. ItsI really for diagnosis only. So put a mechanical gauge at the rail and only check it randomly or if a problem crops up.
 
douglee25 said:
That's what I posted above. Something to attach to the rail directly. In my opinion, you're over thinking and complicating the solution. You rarely need to monitor fuel pressure on a factory type engine. ItsI really for diagnosis only. So put a mechanical gauge at the rail and only check it randomly or if a problem crops up.
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Actually I was thinking more about how to get out from under the plenum, and if I do it with piping, then I have to start worrying about supporting it and the gauge at the end. I would likely leave the gauges in the engine room, unless there's an ignition issue. Once I go to electric sensor, there's lots of mounting/monitoring possibilities. I do appreciate your comments, and I know the schraders are only for temporary connections. I have some spare fuel rails to prototype with, so I'll figure something out. Not in a rush. Need to go boating for a while first. ;-) Let's call this thread closed with a happy ending. I'll start something else about the fuel pressure gauges if I pursue it.
 
#4 low compression is indicative of water intrusion from the exhaust manifold, risers and/or elbows. #’s 4, 6, 3 and 5 are generally the first to go. Get a video scope from AutoZone, separate the exhaust system where the exhaust hose clamp onto the elbows and take a peak inside. If you see evidence of water you have a problem.
 
Wanderer 290 said:
#4 low compression is indicative of water intrusion from the exhaust manifold, risers and/or elbows. #’s 4, 6, 3 and 5 are generally the first to go. Get a video scope from AutoZone, separate the exhaust system where the exhaust hose clamp onto the elbows and take a peak inside. If you see evidence of water you have a problem.
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You couldn't possibly have read this thread....
 
We'll call it a year later........
Up and running great didn't last past about October.
Back to not getting past 2800 or so, except this time, it's not bogging down to nothing; it just won't get anymore. So, back to the basics.
Port is still the culprit. Stbd still runs like a rock.
So, for starters, I'm looking for clues. Here are the spark plugs, and some bore scoping of the cylinders. Not looking for solutions, that will come. Looking for experienced eyeballs to read the plugs, and the isms in the cylinders. Note that I have been running around at idle to 2000 rpm for 8 months, although not a whole lot. Starts fine, idles fine, runs up fine until...... Still needs some help restarting after hot shutdown, but with the low pressure fuel pumps, I just power through the vapor lock until it catches. Then, like nothing ever happened.

https://www.dropbox.com/sh/jst4tu3hzdwl44t/AAA6ILvdJGH1rTvpWzMZbaLBa?dl=0

Next action is to compression check while everything is out, and then pull the elbows and scope down to the exhaust ports. Time for manifolds and risers anyways.
 
370Dancer said:
We'll call it a year later........
Up and running great didn't last past about October.
Back to not getting past 2800 or so, except this time, it's not bogging down to nothing; it just won't get anymore. So, back to the basics.
Port is still the culprit. Stbd still runs like a rock.
So, for starters, I'm looking for clues. Here are the spark plugs, and some bore scoping of the cylinders. Not looking for solutions, that will come. Looking for experienced eyeballs to read the plugs, and the isms in the cylinders. Note that I have been running around at idle to 2000 rpm for 8 months, although not a whole lot. Starts fine, idles fine, runs up fine until...... Still needs some help restarting after hot shutdown, but with the low pressure fuel pumps, I just power through the vapor lock until it catches. Then, like nothing ever happened.

https://www.dropbox.com/sh/jst4tu3hzdwl44t/AAA6ILvdJGH1rTvpWzMZbaLBa?dl=0

Next action is to compression check while everything is out, and then pull the elbows and scope down to the exhaust ports. Time for manifolds and risers anyways.
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I'm having a hard time believing these are from the same engine. 1,3,5&7 have an orange deposit on them and 2,4,6 & 8 are fuel fouled. I would scrape off the orange stuff and test it with a magnet to see if it is rust. What I was expecting to see is plugs with a tan (tan/gray/white) colored insulator not these.

How many hours are on the engine and how old are these plugs? The video files didn't play without downloading them individually.

The only time I have seen an orange deposit like that is from an engine that suffered from reversion. So ....being an optimist ;) I hope it is not rust.

Also.....how do the plugs from the other engine look?
 
scoflaw said:
You didn't like my answer last year. Maybe them other optimists have a solution for you.

Best of luck to you
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You're my hero. Don't go away mad. I'll nurse it along for a bit longer, and then do top end on em, or convert to electric if they FU. But, why would it get running, and then slowly deteriorate? This was September....
IMG_5201.JPG
 
PlayDate said:
I'm having a hard time believing these are from the same engine. 1,3,5&7 have an orange deposit on them and 2,4,6 & 8 are fuel fouled. I would scrape off the orange stuff and test it with a magnet to see if it is rust. What I was expecting to see is plugs with a tan (tan/gray/white) colored insulator not these.

How many hours are on the engine and how old are these plugs? The video files didn't play without downloading them individually.

The only time I have seen an orange deposit like that is from an engine that suffered from reversion. So ....being an optimist ;) I hope it is not rust.

Also.....how do the plugs from the other engine look?
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This is the 4th set of engines for hull 523. The first three happened between 1998 and 2003, when this one became the prototype for water lift mufflers instead of log exhaust. I bought the boat then, and the 3rd set lasted until 2017, when a valve let go and beat the bejesus out of the piston. I put the 4th pair in during hurricane Irma. They've always more or less run. Even with 300 hours of probably running on a trailer at a Firestation in LA, they did come from salvage, so they may have turned over, may have gotten wet, I'll never know. On this port engine, I pulled a rocker off it in 2018 that had a hole beat through it, with the rod ball end rolling around in the head. That was #6.
I will pull the Starboard plugs. And the manifold, or at least have a look down the exhaust with the borescope.
The most frustrating thing is I can get her going, but it tends to develop other issues.
Might be long block time.
 
Read up on the orange color. Exxon Mobil uses additives that can do this. I use Exxon Mobil rec fuel exclusively in this boat because the gas station has a dock I can fuel from. Maybe?
Posted the compression readings to the dropbox, and removed the cylinder videos. this is cold, dry readings. Hadn't been run for days.
Even though 4 looks lower than 120, I may have had the gauge angled when trying to take a one hand picture. Don't recall anything being under 120, and let em sit for 30 seconds each. #1 was the high boy at 130.
scoping the exhaust manifolds next. Gonna warm it up first to see if expansion is playing a role.
 
370Dancer said:
Read up on the orange color. Exxon Mobil uses additives that can do this. I use Exxon Mobil rec fuel exclusively in this boat because the gas station has a dock I can fuel from. Maybe?
Posted the compression readings to the dropbox, and removed the cylinder videos. this is cold, dry readings. Hadn't been run for days.
Even though 4 looks lower than 120, I may have had the gauge angled when trying to take a one hand picture. Don't recall anything being under 120, and let em sit for 30 seconds each. #1 was the high boy at 130.
scoping the exhaust manifolds next. Gonna warm it up first to see if expansion is playing a role.
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Why would the color only be on the odd bank? Something is going on which causes the even bank to run rich (and probably misfire). If you pull some of the Starboard engine's plugs.....it will prove your theory if the color is from a fuel additive.
 
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