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

I would still love to see fuel pressure under cruise now that you have a good gauge on it.
 
You are using fuel flow as one of your variables in troubleshooting. That value is a calculated value based on the expected fuel pressure, rpm, throttle position, pulse width to the injectors etc. So if you have low fuel pressure you will still show a normal flow rate but that is not true as the flow would be much lower. If you had bad injectors the flow would be lower but again the flow rate you see displayed would be normal. Restricted fuel from tank, same thing. There is not a true flow meter built in.
 
Ruff Life said:
You are using fuel flow as one of your variables in troubleshooting. That value is a calculated value based on the expected fuel pressure, rpm, throttle position, pulse width to the injectors etc. So if you have low fuel pressure you will still show a normal flow rate but that is not true as the flow would be much lower. If you had bad injectors the flow would be lower but again the flow rate you see displayed would be normal. Restricted fuel from tank, same thing. There is not a true flow meter built in.
Click to expand...


All true. Modern ECMs also have a fuel pressure sensor which basically solves the missing component in the calculation. That said, the tolerances for EFI operation help contain the equation. The injectors require at least 35 psi to fire on the low end and even a defective stock regulator and stock pump won't push much more than 45 psi. So the equation is limited to that range. Older ECMs operate under the assumption if the injectors are firing and the engine is running.....then fuel pressure is within the equation.

In this case, the plugs tell the story. Four are fuel contaminated and four look perfect. I don't know of another explanation that fits the story. A low pressure fuel rail pressure would cause a lean condition and a higher pressure would cause a rich condition on all the spark plugs. Same thing goes for a restriction from or to the fuel tank.....all plugs would look the same.

Finally, you have to look at all the data to see what is going on. Having the Starboard data for the same operating conditions shows the Port engine is struggling to keep up after 1000 rpm. If the mechanicals are solid.....then it has to be a condition which is related to cylinder combustion.
 
Eventful yesterday and this morning.
I pulled the rail, moved 2 to 8, and re-tested temp gun on the exhaust ports. 2 moved to 8.
Pulled the rail again, and swapped out a couple of injectors with replacements. Still misfiring, but a new run test got me up to 2600 RPM. I have collected that data, and will post it shortly.
I think I had at least one "new" injector that wasn't firing. Maybe 2. The reason I say that is because this morning, I pulled all the plugs and they are looking better than they did with one exception, which I have held suspect all along.
https://www.dropbox.com/sh/b60mnjpowt06wyx/AAA030YVgJ8xXa19MUVIVzn9a?dl=0
I also redid compression, and the same cylinder has all arrows pointing to it. And yes, I think it's mechanical. Scoped the cylinder the best I could, and shot a bit of video while I am bumping the engine over.
I will leave analysis to you guys.
I continue to suspect that I have/had multiple issues, pointing us in all kinds of directions. I appreciate the systematic thinking that has been applied.
 
Here is yesterdays run data and the one from 5-19
 

Attachments

  • Engine Run Data 5-19-2019.pdf
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Definitely an issue with #4, but #1 still isn't right either. Maybe I missed it in an earlier post.
Which cylinder had the rocker that was broken?
 
Number 4? That plug was one of the better ones of the bunch. I don't think a 120 cylinder is causing the performance issue. Shoot some oil in it and see if the compression comes up alternatively you can do a leak down test. I have seen plenty of 120 cylinders that will spin up to 4,000 rpm under load.

When you have a dead cylinder the throttle advances about halfway and then the engine runs out of power. A dead mechanical cylinder means the plug is firing and gas is flowing. Usually the engine is producing a cloud of blue smoke or backfiring because of a damaged valve or rings. What I still think you have is a misfire(s) which are combustion related.


Was there any mechanical work done to the engine (timing chain for example)?

#4 Plug

p.jpeg
 
PlayDate said:
Number 4? That plug was one of the better ones of the bunch. I don't think a 120 cylinder is causing the performance issue. Shoot some oil in it and see if the compression comes up alternatively you can do a leak down test. I have seen plenty of 120 cylinders that will spin up to 4,000 rpm under load.

When you have a dead cylinder the throttle advances about halfway and then the engine runs out of power. A dead mechanical cylinder means the plug is firing and gas is flowing. Usually the engine is producing a cloud of blue smoke or backfiring because of a damaged valve or rings. What I still think you have is a misfire(s) which are combustion related.


Was there any mechanical work done to the engine (timing chain for example)?

#4 Plug

p.jpeg
Click to expand...
It was a better one until this morning's pics. It was about the only one full of crap, and black. The crap around the anode had NO metallic material in it. Still lovin engines. Thank God my charge rate is really low. I did move some injectors around, and 4 might have gotten mixed in that. I am committing to video/pics of anything I do now. It's getting hard to remember what I did last week vs last night.
I could get the Port engine to backfire once each time I tried to run it up full throttle after 10 seconds or so. Not constant, just once.
 
PlayDate said:
Was there any mechanical work done to the engine (timing chain for example)?

#4 Plug

p.jpeg
Click to expand...
Not on my watch. I bought a pair of used engines that would be plug and play, with 1/4 the hours that my last set had. We ran them on the ground before purchasing, and compression tests were near what we are seeing now. I've never seen them under load until they were dropped in my boat. I'm a salt water boater, so I had lost confidence in all of the bolt ons on my old engines. In fact I was getting hose clamped to death on the previous pair. I don't have any non-warranty information on what these particular power plants went through, but they are 20 years old, and they have been sitting for at least a year and a half before I installed them.They were in Sausalito, California, so they may have experienced some lower temps, but I see no indication of water getting where it shouldn't. I gave 11K for the pair, so I still have some room to rebuild if need be. I just want to rule out/fix all the other issues as well. And, if I don't have to tear down to the block, all the better. The Starboard had plenty of issues too, but I have managed to get past all of its drama. Port, not so much.
 
the throttle advances about halfway and then the engine runs out of power. A dead mechanical cylinder means the plug is firing and gas is flowing. Usually the engine is producing a cloud of blue smoke or backfiring because of a damaged valve or rings. What I still think you have is a misfire(s) which are combustion related.

Roger. I have exactly this symptom, with none of the sheen in the exhaust, smoke, or anything else I would expect from a non firing cylinder. Most puzzling.
 
Ruff Life said:
You are using fuel flow as one of your variables in troubleshooting. That value is a calculated value based on the expected fuel pressure, rpm, throttle position, pulse width to the injectors etc. So if you have low fuel pressure you will still show a normal flow rate but that is not true as the flow would be much lower. If you had bad injectors the flow would be lower but again the flow rate you see displayed would be normal. Restricted fuel from tank, same thing. There is not a true flow meter built in.
Click to expand...
I have one. Time to put it inline? I had this exact question about how the ECM is coming up with the flow rate since there's no sender that I am aware of in a 1999 engine.
 
370Dancer said:
I have one. Time to put it inline? I had this exact question about how the ECM is coming up with the flow rate since there's no sender that I am aware of in a 1999 engine.
Click to expand...

It uses the fuel table in the ECM and approximates GPH which is usually pretty accurate. The equation includes fuel pressure (nominal), injector rating (nominal) and how long the injector is open. It also includes engine speed and a couple of other variables to come up with the GPH. Newer ECMs have a pressure feed to make the estimate more accurate.
 
I'm pretty sure you are on the right track....the problem you are chasing may be the reason they pulled the engines in the first place. You got a great deal on them so let's figure out what is wrong.

If only #4 was heavily fouled.....that usually means the spark plug was dead and not firing at all. That is weird since the original plug photo looked great. I agree it is most likely an injector that you moved.....which calls into question four of the aftermarket injectors. I still believe that we need to get the injectors sorted out before you yank the heads off. If we don't, the injector problem will reappear the minute the reconditioned heads are put back on the motor.
 
370 I owned those exact motors for 10 years-then boat was totaled and sold at a auction site. I had countless problems with valve seat and stem corrosion caused by the reversion issues. Reversion at idle speed is so great that it will suck water directly back into cylinders and wash plugs, usually in the "center cylinders that are directly under the exhaust elbow.
I believe the fuel issue is being caused by the fuel pressure regulator which has internal corrosion. Fuel pressure at low rpm around 30 psi, above 2200 rpm you need 45 psi. Probably loosing vacuum in the regulator and not allowing it to flow the full 45 psi. You did say that pulling the vac hose off had no effect

Also -are you sure they came from local boat or is that what you were told?
 
Sublimetime said:
370 I owned those exact motors for 10 years-then boat was totaled and sold at a auction site. I had countless problems with valve seat and stem corrosion caused by the reversion issues. Reversion at idle speed is so great that it will suck water directly back into cylinders and wash plugs, usually in the "center cylinders that are directly under the exhaust elbow.
I believe the fuel issue is being caused by the fuel pressure regulator which has internal corrosion. Fuel pressure at low rpm around 30 psi, above 2200 rpm you need 45 psi. Probably loosing vacuum in the regulator and not allowing it to flow the full 45 psi. You did say that pulling the vac hose off had no effect

Also -are you sure they came from local boat or is that what you were told?
Click to expand...


You are correct that those SR model years had major problems with reversion and the center cylinders saw the water first. In this case though the plugs and combustion don't look steam cleaned which is a characteristic of water reversion. To your point that it might have occurred on the previous owner's watch.....depending on the boat they were in ......it is a possibility.

In fact the video shows the combustion chamber and the valves opening and closing. Both show buildup that looks more than 300 hours but if they are sealing....they should make power. Also...the Starboard engine isn't showing any signs of a similar issue. Almost every reversion issue I have seen affected both engines.

You do raise an interesting question: 370Dancer what happened to the previous engines?
 
PlayDate said:
I'm pretty sure you are on the right track....the problem you are chasing may be the reason they pulled the engines in the first place. You got a great deal on them so let's figure out what is wrong.

If only #4 was heavily fouled.....that usually means the spark plug was dead and not firing at all. That is weird since the original plug photo looked great. I agree it is most likely an injector that you moved.....which calls into question four of the aftermarket injectors. I still believe that we need to get the injectors sorted out before you yank the heads off. If we don't, the injector problem will reappear the minute the reconditioned heads are put back on the motor.
Click to expand...
I have a DIY injector cleaner rig. Admittedly, I did not test the new ones for sticking or non operation before putting them in. I still have lots of old injectors too. Fuel rail has already been on and off a dozen times. A couple more won't hurt.
 
I am also gaining access to my cool fuel II tomorrow on the old engine. Hopefully I'll have a fuel pressure regulator that is still good to try out.
 
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