At a loss

Noah208

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Dec 9, 2022
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4.3 MPI new fuel cooler and pump, new pressure regulator, new IAC, no leaks fuel or vacuum, new MAP. Motor starts good idles good accelerates good but when coming off WOT to idle it just dies. No problem starting up and same thing happens again. Anyone have an idea on where to look next.
 

QBhoy

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Mar 10, 2016
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8,342
4.3 MPI new fuel cooler and pump, new pressure regulator, new IAC, no leaks fuel or vacuum, new MAP. Motor starts good idles good accelerates good but when coming off WOT to idle it just dies. No problem starting up and same thing happens again. Anyone have an idea on where to look next.
Hi. Be keen to hear more about the exact circumstances when it has the issue. Are we saying that you go from full throttle back to idle quickly and then it will die ? Does it happen if you pull back slowly ? Is it happening exactly when the following surge of water approaches the stern as the boat sinks down off plane and into the water ? Does the boat have a normal set up or does she have straight through transom exhausts?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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And when you plug into it with Rinda, what are you seeing?
 

Noah208

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Dec 9, 2022
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10
Hi. Be keen to hear more about the exact circumstances when it has the issue. Are we saying that you go from full throttle back to idle quickly and then it will die ? Does it happen if you pull back slowly ? Is it happening exactly when the following surge of water approaches the stern as the boat sinks down off plane and into the water ? Does the boat have a normal set up or does she have straight through transom exhausts?
Thanks for responding. I can only run the unit in the test tank so its not water surge. It is easier for the motor to stay running if gently pulled back from WOT but it sometimes dies anyway. Motor only has 300 or so hours and is in good shape. Fresh water boat with no corrosion. Basically a new boat. I work on boats every day and this one has me stumped. Don't like admitting I cant figure it out but there is always someone that knows more than I, especially with a unit I've never worked on.
There is nothing in the book that describes the failure that is occurring on this unit but I'm leaning towards rotor cap or a bad ground somewhere. Any thoughts are much appreciated since I am at a loss. Cant let the unit leave until I find it because it would be unsafe for use.
 

dubs283

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Jul 27, 2005
Messages
5,322
4.3 MPI new fuel cooler and pump, new pressure regulator, new IAC, no leaks fuel or vacuum, new MAP.
That's a lot
I'm leaning towards rotor cap
Probably the right direction. Best to start with a full tune up. Cap/rotor, plugs, wires, fuel filters.....surprised to see all the components replaced prior to your suspicion of the routine maintenance ignition components.

Would also be curious to see fuel pressures with/without vacuum line connected along with fuel system vacuum.

FYI, running in a test tank will never come close to simulating running on the water
 

jimmbo

Supreme Mariner
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May 24, 2004
Messages
13,447
A big tank? Do you have a Marine Dynamometer hooked up to it?
 

jimmbo

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May 24, 2004
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So you were revving it it up to WOT in Neutral, or was a Prop Turning?
 

jimmbo

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May 24, 2004
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13,447
I doubt the prop had a Solid Bite on the Water unless the Tank went under the Trailer and allowed a free flowing path for the water to get to the prop. Tank testing in gear is very limited.
Have you got the Mercury Service Manual for it? If Not, then get it.



Give me a Carbed engine with a Mechanical Advance Distributor any day of the week, over a Computer Controlled Mumble Jumbled Mess with FI
 

Noah208

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Dec 9, 2022
Messages
10
Tank is a pond basically with the bow secured plenty of water flow. I have the manual and still no progress. I'm bound to find the problem eventually it is just taking more time than I expected. I was just hoping someone has had the same issue and would give me a light bulb idea.
 

jimmbo

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May 24, 2004
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13,447
Is it dying as it slows down and remains in gear, or are you shifting to Neutral as it is Slowing down?
 

Noah208

Cadet
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Dec 9, 2022
Messages
10
Shifting in to neutral. Most 4.3s I've worked on will hiccup when brought back fast bought rarely have I had them die every time.,
 

jimmbo

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May 24, 2004
Messages
13,447
One minute is a Tank, cause the Lakes are Frozen, then it is a Pond. I think a Pond would freeze sooner than a Lake...

There is an Ignition Interrupter Switch on the Mercrusiers to make it a lot easier to shift out of Gear, perhaps it is sticking a bit and that kills the engine
 
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