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

370Dancer said:
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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I'm old school and have been knocked across the Engine Room on more than one occasion trying to find a dead cylinder! I wish I could take credit for the idea in this video because it is far safer than pulling spark plug wires on a live marine engine.


I know it works on cars and it should work on boat engines as well. It is such a simple idea to determine if a cylinder is misfiring after all F1 cars monitor the exhaust on every cylinder to determine how the engine is running. I would use your Starboard engine as a baseline.

Years ago we used to have a Sun oscilloscope that showed whether a plug was firing or not based on spark discharge. In the age of fuel injection....those tools became antiques. You can still find them looking at Ignition Waveform analyzers.

https://www.amazon.com/d/Automotive...-Technologies-Corp-GTC505-Ignition/B001TD5JYE

Also one other question: which cylinder had the damaged rocker? The reason I ask is that a sticking valve would also explain a high speed miss. I have seen valves seal fine for a compression test but not seal well at higher rpms. Given the engines sat for a while....that thought is still rattling around in my head. A stuck valve or weak/broken valve spring usually only shows up higher rpms. Let's leave that alone for a while and focus on the easy things.;)
 
Playdate - l too thought ignition at first, but that doesn't explain the stalling? Fuel pressure was not increasing off idle which should be the case. That told me fuel related.
 
+1 on the fuel issue.

If you can’t maintain the proper pressure, that need to be corrected first.

Low fuel pressure creates a lean condition because less fuel volume flows out of the injector during the short time that it’s spraying fuel.

Oxygen sensor equipped engines can increase injector pulse width to compensate for the lean condition. Without any sensor to provide feedback, your stuck with the preprogrammed pulse width which is based on MAP sensor, throttle position and rpm inputs.
 
douglee25 said:
Playdate - l too thought ignition at first, but that doesn't explain the stalling? Fuel pressure was not increasing off idle which should be the case. That told me fuel related.
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Agree....but his fuel pressure gauges don't seem to be reliable. If you look at the fuel flows in his spreadsheet....the ECM on the port engine shows 14.72 GPH at 2430 rpm. The starboard shows 13.44 GPH at 3000 rpm. He is dumping fuel into the port engine and the engine is losing power. He either is losing ignition or compression as the engine spins faster.

No EFI that I have worked on would run below 30 psi at the fuel rail.

Further, the problem starts showing up at 1671 rpm when the fuel consumption of the port engine rises 1.5 GPH over the starboard and it get worse from there. You can see that the port injectors are staying on longer and the IAC values are higher than the starboard engine. Spark advance is also higher......all signs showing the engine is struggling to make power.

In regards to the stalling.....it could be a number of related things maybe related or not. Once we solve why it is losing power between 1600 and 2000 .....the rest may fix itself. I think it is something electrical or mechanical (sticking valve). I would definitely check the firing order again and the secondary ignition system (wires and plugs) to eliminate an electrical problem.

I want to complement 370Dancer on posting the run data. Most of the time we drop into the middle of the story and posting run data helps narrow down the problem.
 
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I haven't completely ruled out a fuel pressure problem but he needs a reliable setup that matches what we see in the run data. It is a pain to connect to the Schrader valve on that fuel rail which can cause the hose to kink and throw the readings off. I have a 90 degree Schrader fitting that makes it easier to attach a hose. Once we get some reliable numbers testing the pump and the regulator.....that will really help.
 
PlayDate said:
I haven't completely ruled out a fuel pressure problem but he needs a reliable setup that matches what we see in the run data. It is a pain to connect to the Schrader valve on that fuel rail which can cause the hose to kink and throw the readings off. I have a 90 degree Schrader fitting that makes it easier to attach a hose. Once we get some reliable numbers testing the pump and the regulator.....that will really help.
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Please source your 90 degree fitting. I'd really like to set up some permanent fuel flow meters for the pair. Not sure I can trust the adapter hose that comes with the flow gauges long term.
A piece of good news. The Starboard engine is showing 40 psi now. That is on my good old gauge, not the Harbor Freight POS that I have on the Port engine. When they cool down, I'm going to switch them.
A little more. I disconnected the inline fuel filter, and low pressure pump, and ran straight from the fuel shutoff solenoid to the fuel water separator. That almost immediately let the port engine startup and idle. New fuel water separator this morning as well. Now I'm back to vapor lock when trying to restart hot. That I expect. It's 96 degrees out, and I experienced this for years before installing the low pressure pumps. So, I will pull and bench test the low pressure pump later today. Right now, I'm going to pull the fuel tank vent line and run some air through it to make sure it's not clogged up. Then I will do the same to the fuel line (with the shutoff bypassed) to make sure I hear bubbles in the fuel tank.
I did do the laser temp gun thing but that was a while back. I will repeat, and yes good idea to use STBD as a baseline as one side tends to be a bit hotter. My target point would be the bit of manifold piping to each cylinder you can see between the manifold body and the block.
What else..... I still think I have more than one problem. The punched valve rocker was number 8.
I'm going to post my spark plugs when I pulled them this morning. Stand by.
 
Also....more of a curiosity ......when you say these were LA Fire Rescue engines.......did they come out of vehicles that had O2 sensors attached and are you using your original ECMs or the ones that came with the engines?
 
My 90 degree adapter came as part of a kit. However, this looks like the same one.


https://actron.com/content/right-angle-large-schrader-adapter




0180-000-1299
Right Angle Large Schrader Adapter
Right Angle Large Schrader Adapter (Honda, Daihatsu, Hyundai, Mazda, Mitsubishi, Sterling) for Professional Fuel Pressure Tester Kit CP7838



$9.00
free ground shipping
 
Do you run some sort of fuel additive? I can't tell if that's fuel additive or water burning off on the spark plugs?
 
That looks like gas/oil on 2,3,5,6....definitely not water and fuel additive is usually a solvent (both clean the plug). The reason I asked if the photos lined up with the plugs is that you have two injector drivers on a GM ECM. One fires cylinders 2,3,5,8 and the other fires 1,4,6,7.

If the ECMs are out of a vehicle then they are operating in open loop since they don't have an O2 sensor reference. If you are using the Mercruiser ECMs then they should also be operating in a designed loop off of a stored fuel table. Let me know if the plugs line up with the photos.....that may give us a clue as to what the problem is.

The weird part is oil contamination is usually valve seals on the engine and it is more pronounced on one head versus another. In your case, random plugs on both heads have contamination on them and the other plugs are perfectly normal. Also if the photos line up with the plugs/cylinders....2,3,5,6 are the middle of the engine.

I believe if you pull a couple of the middle spark plugs off the starboard engine they will look normal like 1,4,7,8.
 
Yup oil,...nothing short of a rebuild will bring this thing back to life....crappy compression numbers told the story 50 posts ago.
 
Man, I take a nap and all hell breaks loose.
photo numbers are the cylinder numbers.
No additives except some marvel mystery oil a while back
These engines are a matched pair from a LA Fire Rescue boat, not vehicles.
The ECMs are the originals that I got. As stated previously, I assume the 4B pn the port engine was swapped out for the original MEFI3 very early in it's life. It is true that I don't know much about the boat they came out of, but the Stbd engine has no issue with the hull they are in now. I do have the MEFI-3 ECM from the old engine should I ever dare to replace the 4B, and remove the adapter harness.
Blew out the vent line and fuel line last night just to make sure.
The 4B does break out a reading for each driver (A and B), and they have identical on times.
 
New nugget for this morning.
Just did the temp gun on the exhaust ports.
1,2,4,5,7,8 reading about 190-200
3 and 6 a good 50 degrees cooler
It's kind of a guess test as the engine is heating up in the process, so I kept rechecking and looking for a temp delta rather than an absolute number.
Also fuel pressure at the rail before starting is 38 lbs now that I have the "good" gauge on it.
 
I believe you are getting closer to the actual problem. That is good news on the fuel pressure. Do you know if any of the injectors have been replaced on the port engine? The only explanation that fits the plugs is that either the injectors are mismatched or some are sticking open. It isn't an ECM problem. I simply was curious what type of vehicle (car/boat) they came out of. Transplanted Boat engines usually perform better than car engines since the default ECM fuel tables are tuned for marine work.

To test the injector theory....you can you swap injector #1 with injector #2 and put new plugs into each one. That should tell us in short order if the problem moves with the injector. If #1 gets fouled after swapping the injectors.....then we know we have an injector issue. If the problem doesn't move.....then we most likely have a mechanical issue. I know it is a major pain in the ass to pull the manifold to get to the injectors just to swap two of them. If I was on the clock for this......I probably would replace all the injectors since pulling two is is the same work to replace 8. Replacement injectors run $100-150 for a set.

If you go down the path of replacing all the injectors, make sure you look up your engine serial number to get the right injectors. On 7.4 engines.....make sure that the injectors are fully seated in the manifold. If they are not fully seated....they can trigger misfires.

Your temp test confirms what we see on the plugs with #3 & #6 running rich and cooler.

If you need words of encouragement.....I believe replacing the injectors could solve the problem. Nothing is certain but how engines work is not a mystery. In your case, the engines sat for a while before they were transplanted. The single system that gets compromised when an working engine sits is usually fuel related. Additives rarely solve mechanical problems. The fact that some injectors seem to be working properly and some are not based on the spark plug contamination is also a clue.
 
PlayDate said:
I believe you are getting closer to the actual problem. That is good news on the fuel pressure. Do you know if any of the injectors have been replaced on the port engine?
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All of the injectors are brand new. I did that a couple of months ago thinking the same thing. And, when I replaced the ones on the starboard engine, that was the fix that took it to top performance. They are aftermarket, but the same ones I used on the Starboard engine. Could I have pinched an o-ring? maybe. could I have cracked an injector plug? not likely but possible. I've lived through all of those mistakes made by me, and other mechanics before me.
It's easy enough to pull the rail and check. I'm just running out of plenum gaskets.....
 
370Dancer said:
All of the injectors are brand new. I did that a couple of months ago thinking the same thing. And, when I replaced the ones on the starboard engine, that was the fix that took it to top performance. They are aftermarket, but the same ones I used on the Starboard engine. Could I have pinched an o-ring? maybe. could I have cracked an injector plug? not likely but possible. I've lived through all of those mistakes made by me, and other mechanics before me.
It's easy enough to pull the rail and check. I'm just running out of plenum gaskets.....
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It is hard to imagine another problem that would cause the plugs to look like that. A rich carb or TBI blackens all the plugs. MPI is cylinder by cylinder which is why your plugs look like that. If the spark plug firing order is correct it has to be related to the injectors. As I mentioned.....you can swap #1 and #2 .....if the fouling follows the injector....you have your answer. The good news is that if this is the problem.....your hard restart issue might go away as well.

Sometimes replacement injectors are problematic. Meaning that a 36# injector could be a 38# or 40# due to mass production. No quicker way to get misfires than to dump fuel into a cylinder.

Just a comment on the regulator. If you were showing high pressures 50+psi when the engine is running ....you would see a rich condition on all the plugs. The injectors are set to work between 35-40 psi.
 
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