Absolutely frustrated with this Mercruiser engine

Hoplite808 said:
Verified it when I hauled it out last year and double checked the serial numbers on both props when I replaced the entire leg with a new one. I know the gear ratio is correct in the leg because the certified Mercruiser mechanic I hired to do it verified it.
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On a new to me boat twin 496 bravo 3s one of My 2.0 Ratio bravo 3 with a tag was actually a 2.2 after I removed it and actually counted the input shaft to output shaft rotations. I chased my tail for a season. If it is a totally wrong ratio and mislabeled it would do almost everything you are experiencing. Pretty easy to check the ratio if you can haul boat.
 
Tri-County, what made you suspect you have different ratios just out of curiosity thank you
 
Wow that's pretty interesting that you caught that I'm glad you got it handled though but thank you for the explanation I appreciate it
 
PlayDate said:
His boat weighs 8,500 lbs. He needs all the ponies to get that boat on plane.
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Even a tired 7.4 should rumble a lot more than seen in the video . A 50 hp outboard would push this boat like seen in the video at ~ 9-10 kts. I do not think compression is the answer.

Do not give up ! Its not the ECU , lets think a minute what else it can be.

Edit : it feels strange for me how the engine kicks in at the beginning . Its hissing for a moment and then starts to push the boat a bit jerky . Not sure how to judge it .
 
I downloaded the video and striped the sound off, and did a FFT on it (fast fourier transform) which shows the frequency peaks of the engine. In the first part of the video while camera is pointing at engine there isn't anything significant but I can see freq peaks at 68ish and 135ish hz and a ton of mechanical/background noise which I would expect. 135hz would be just over 2krpm firing frequency and all the engine noice, cams, etc. for all 8 cylinders. When you turn away I would suspect you are just getting the engine intake/exhaust noice and the 135 hz drops away and the peak is the 68hz, so just 4 cylinders of intake/exhaust noise. It could be nothing, a sound recording from the back of the boat (just exhaust) would be better for this analysis. It does sure sound like a 4cly engine struggling though...

One other reason I think it's running on 4cyl is the whole safe mode argument. What that ECU was designed (stone ages of controls) there is no ETC (electronic throttle control) or individual fuel injector control (just the 2 injectors). They wanted a simple way to provide limp home and dropping one of the injectors is the way they did it. That provided about 2krpm which is in the manual as an indication of safe mode active.

So, I think you are getting 4cyl of power for some reason:
1. Power to only one bank of injectors (ECM thinks its firing but they really aren't opening up). Remember, ECM fires the injector bank by going to ground. It could even be that a corroded connector makes it look like there is 12v on the high side of the injector, but when trying to pass current, there is some voltage drop on a connector and not enough current (energy) to open the injectors.
2. Exhaust restriction on one side
3. Fuel blocking on one bank.


Back as a kid we had a blocked fuel filter and idled the boat to shore by pouring gas down the carb (from a beer can stolen from parents fridge). Can you get a can of starting fluid (ether) and squirt it into the engine while it's struggling at 2krpm? If it jumps some in RPM like it's making more power, that to me would provide a clue that it's fuel.

I'll attach the FFT for those that are interested! I did knock sensor calibrations on a couple packages, got pretty good doing FFTs using excel. It's amazing what is available for free now vs 20 years ago...
 

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That is an excellent idea 2000 RPMs spray ether in there at wide open throttle and see what happens that'll tell you right there Great idea you came up with
 
Have we ruled out a bad rectifier on the alternator, if it is outputting too high a level of AC voltage that will upset the ECU. Easy check to make. DVOM on AC volts, measure at alternator. Should be 0.2 volts or less.
 
Scott215 said:
That is an excellent idea 2000 RPMs spray ether in there at wide open throttle and see what happens that'll tell you right there Great idea you came up with
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Naah . Ether is for kids .

Spray nitromethane ( topfuel ) in the intake to make this sucker beg for his life and to teach him who's in charge here :D
 
britandy said:
Have we ruled out a bad rectifier on the alternator, if it is outputting too high a level of AC voltage that will upset the ECU. Easy check to make. DVOM on AC volts, measure at alternator. Should be 0.2 volts or less.
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Data dumps from both ecms are just under 14 volts, look correct. And if you are thinking an ac swing on that is causing the ecm to do bad thinks, it wouldn't run of the ecm was resetting from ac swings in the power supply.
 
@@Espos4 there's only 20 something pages and 10,000 posts I'm sorry I couldn't remember them all sorry I missed it
 
Assuming he manages to spray a combustible mixture into the intake of course the engine will jump up in rpm,s for a moment . But what do we learn from this ?
 
aerobat77 said:
Assuming he manages to spray a combustible mixture into the intake of course the engine will jump up in rpm,s for a moment . But what do we learn from this ?
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If the engine makes more power when you hit the intake with starter fluid wouldn’t this indicate a fuel delivery issue?
 
aerobat77 said:
Naah . Ether is for kids .

Spray nitromethane ( topfuel ) in the intake to make this sucker beg for his life and to teach him who's in charge here :D
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That would probably get it up on plane even if it was running on four cylinders!
 
Espos4 said:
Ive suggested that twice and he won’t do it.
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It's not that I won't its that I can't. I'm by myself mostly and the issue is that I can't steer the boat while also being in the engine bay.
 
18436572
My friends 2012 205 bow rider with a 5.0 Carb'd engine just couldn't get on plane after she had a "tune-up". Then she had another mechanic go over it who said the carb need rebuilding. That engine simply stalled at a given RPM and wouldn't allow the boat to nose over and get on plane. I rebuilt the little Mercarb for her but after apart it was clean; didn't need anything.
It didn't make sense.
Then going over the spark plug wires I noticed cylinders 4 and 6 wires were swapped on the distributor.
This was last Friday and Saturday.
Runs like a new boat....

@Hoplite808 - her engine's idle sounded smooth but under acceleration sounded just like yours in the video. Check your firing order on the plug wires.
 
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