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

370Dancer said:
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The results are attached as an Excel file. If you need it in another format, just ask.

While you are looking at this, I am going to recheck fuel pressure compared with Starboard, and put another fresh set of plugs in it. There's a txt file of next stuff to try. The only mechanical thing I found so far was a valve rocker that had been punched thru by the lifter push rod, which was preventing the valve from opening much. I thought that was really going to be it, but I still can't top out in RPMs.

Ask away, I'm all ears.
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I am also posting on marineengine.com's forum. Someone was confused about the excel sheet data readings. I posted this clarification. Now I'll post it here if anyone missed what I was doing.

I took the boat out for a ride.
I have Fox Marine real time engine monitoring gateways for each 1999 MEFI engine that display over NMEA 2000, and a Bluetooth app on my iphone. During the ride, I took 5 samples (screenshots of the app displaying all these parameters)
the notes below each set of readings on the excel page describe what I was doing.
In the first sample engine run time of 11.5 minutes, I set the throttles so each engine's TPS was reading the same, 10.16%, looking for variations in other readings
In the second sample engine run time of 12.5 minutes, I set the throttles to synch (or near it) of 1671 RPMs, looking for variations in other readings
In the third sample engine run time of 15.5 minutes, I set the throttles to around 2000 rpm, where the Port engine starts to really need more throttle than the Starboard to keep that RPM.
in the fourth sample engine run time of 18.5 minutes, I tried to plane out the boat. The Port engine got to 2430 RPM at full throttle, while the Starboard easily got to 3006 RPM, with only 45% throttle being applied. I saw no point in running the Starboard up to full throttle as I know that already is doing fine at 4600+. Fearing potential damage to the Port engine, I backed off to my normal slow cruise where both engines are "kind of" acting the same.
In the 5th sample engine run time of 58.5 minutes, I took one more reading at around 1800 synched RPM to see if anything had drastically changed over time.
The charts were just me looking at various parameters over the samples comparing the Port to the Starboard engine, looking for patterns.
The list of observations under the sample data are my notes of what I see looking at the comparisons.
 
And now I'm really bothered by the fuel rail pressure. Spec is 43 psi running. I'm 1/3 that on the good engine, and 50% down on the bad engine.
 
370Dancer said:
And now I'm really bothered by the fuel rail pressure. Spec is 43 psi running. I'm 1/3 that on the good engine, and 50% down on the bad engine.
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Seems like it's pointing you there... Check for restrictions in supply and return. Maybe you have a bad pump?
 
Edit - you stated 38-40 at idle, correct? That's probably within spec at idle. If you're dropping at throttle, there's the issue. It should be increasing with throttle.
 
douglee25 said:
Edit - you stated 38-40 at idle, correct? That's probably within spec at idle. If you're dropping at throttle, there's the issue. It should be increasing with throttle.
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Agree, so why does one work great at these low pressures, and the other one doesn't?
I think I'm going to throw the harbor freight one away. If I squeeze the hose, or stretch it, pressure goes up.
Today, I restarted after a few hours, and both pressures were at 20 at idle. This is really starting to smell like fuel delivery.
Pressing on. Thanks for all the comments so far. I came across an old post of my on the previous set of engines in 2012 where I was chasing the vapor lock issues before Mercruiser admitted it in a Tech Bulletin, and apparently I had a similar issue that was caused either by a crushed fuel line by the mechanics replacing the engines, or the fuel regulator. I did both at the same time, so I never figured out which one did the trick.
It bothers me that I can pull the vacuum line to the regulator on both engines, and the fuel pressure doesn't flinch.
 
So pulling the vacuum line on the fuel pressure regulator doesn't make a difference? Man something is goofy. Can you try a new fuel pump? Can you try a new fuel regulator? What do those cost?
 
Regulators looking like 150 or so. I can probably do better through some sources, and will likely pick one up just to have it. Fuel pump $65 to $275 depending on whether you go aftermarket or OEM. Here's a fun note. There is a screen on the regulator. Clogged? We'll see. Pulling the Port engine Cool Fuel II isn't too bad as it's in the deep part of the bilge, and accessible from in between the engines. I've been trying to diagnose with tools and analysis before just pulling things apart and swapping out suspects, which makes the boat unusable while I'm doing that. Trying not to disable her for more than a day here and there. We can still run, although at fast trawler speed. Happy wife, well you know the rest.....
So, if this were a run of the mill failure, I'd already be done with it. It's possible that I still have multiple issues, each one screwing with the other. These motors, new to me, were 11K for the pair. New period, would have been more like 40K for the pair. I still have lots of room for fixing before I get anywhere near what the drop in new re-power would have cost. On a 20 year old boat, love it or not, it's still a boat. Best part, I know more about Gen VI Mag MPI MIE inboards than I ever wanted to. Maybe that's my next career, but I digress....
 
I'm not a ''parts replacer" like some, however, the data your telling us is pushing us to a fueling issue. I would either start swapping parts from the working engine to the non-working engine or I'd just go new.
 
Agree. I will start at the tank, and work my way to the fuel rail. New gas, non ethanol, full tank.
 
I did have another thought. The ECM claims it is using more fuel on the Port than the Starboard. How is that calculation done? Counting how long the injectors pulse X RPM? Maybe I'm not really using more fuel because of a restriction, but the ECM thinks so, and starts introducing more air, and so forth.
 
370Dancer said:
I did have another thought. The ECM claims it is using more fuel on the Port than the Starboard. How is that calculation done? Counting how long the injectors pulse X RPM? Maybe I'm not really using more fuel because of a restriction, but the ECM thinks so, and starts introducing more air, and so forth.
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Yes, Injector pulse width calculations will give you your correct GPH assuming proper supply pressure and clean injectors.

However, the ecm cannot add more air. That is purely mechanical on your part and depends on throttle position.
 
Espos4 said:
Yes, Injector pulse width calculations will give you your correct GPH assuming proper supply pressure and clean injectors.

However, the ecm cannot add more air. That is purely mechanical on your part and depends on throttle position.
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Injectors are new. Supply pressure is acting like it's way low.

Except for the IAC (Idle Air Controller) which regulates air at idle, while the throttle butterflies are closed.
 
Yes, but the position of the iac, whether open or closed, would have extremely minimal effect (I’d even venture to say absolutely no effect) on your big block motor at 2,400 rpm under load.
 
Double check each injector. I had cracked one of mine one time while I was changing exhaust manifold. It cracked where the wire connects.
 
bone354 said:
Double check each injector. I had cracked one of mine one time while I was changing exhaust manifold. It cracked where the wire connects.
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Yes, they are notorious for that. I was careful, but you never know. I will check again. My experience is if you can wiggle the connector up and down (softly) and it moves, it is cracked, and making intermittent contact if any at all.
 
98 degrees and sunny in Florida this weekend. Gotta limit my time to the morning when the cockpit is in the shade. Went to the yard this morning and yanked some stuff off of the old motors. The cool fuel system that I wanted access to requires picking up the engine, so I'll have to wait for the fork lift operator. Tried rolling it over by hand, but a fully dressed 454 weighs a bit more than I do. ;-)
On a good note, the Port engine is getting worse. Lost it coming to the dock at idle last night, and really pushing me towards fuel now. Separators, filters, and low pressure pump in the morning. If I have time, i'll start moving parts from the Stbd engine over.
 
How about an external fuel tank test? Maybe a collapsing line, bad feed, etc. I assume you have two tanks?
 
Sorry I'm a bit late to your thread. I have read the comments and reviewed your data. I don't believe your rail fuel pressure gauges are reading correctly. No way an EFI motor runs on 15 psi. So I'm going to discount that because your fuel consumption readings show fuel is flowing on both engines even more so on the port engine at higher rpms.

My money is on a misfire. A misfire is consistent with almost all of the symptoms especially where the engine just can't produce rpms under load. If you pull the plugs....(I know you have probably already done that) one of them isn't firing under load. Could be anything from a cracked distributor cap, wires or the plug itself. Why? Because it is usually something simple. If you have new plugs in the engine....it is easy to not notice a plug that is misfiring because it hasn't had enough buildup to get your attention. I have also seen where the firing order was not matched to the cylinders....creating the same symptoms.

Just my thoughts.....compression numbers are okay and the run data you provided was very helpful.
 
PlayDate, thanks for your insight. I am not beyond thinking I have more than one issue. The fuel pressure gauges were really throwing me off. One day they were reading 38-40 and the next barely 20. I sure wish GM would have put the schrader valve in a more accessible place than halfway under the plenum.
What tool or technique do you use to perform a cylinder balance test without getting knocked across the room? The ECMs are too old to let me do it with a scan tool.
 
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