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I have a 2008 115hp mercury optimax. It starts up fine

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Hi I have a 2008 115hp...
Hi
I have a 2008 115hp mercury optimax. It starts up fine and idles good. When I first take off it planes good and runs well. After a minute or so the RPMs start fluctuating up and down. Then the RPMs start to drop. If I hit the trim button up or down the rpm will go up.
Submitted: 2 years ago.Category: Boat
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Answered in 16 minutes by:
7/19/2015
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 4,048
Experience: US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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Hello, my name is***** you for allowing me to assist you today.
Do you have issues with the battery dying when you run the engine?
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Customer reply replied 2 years ago
No battery don't die and I've had it checked. I have two batterys.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Are you near the engine now and can do some testing?
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Customer reply replied 2 years ago
Yes I am
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Alright,
Can you get it running on the hose, then using a screwdriver, depress the valve stem that is in the center of the fuel rail? Does water or fuel come out of it?
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Customer reply replied 2 years ago
There were two of them. Both had fuel at high pressure.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Ok the lower one is supposed to be air only. Since it had fuel in it, you have a failed diaphragm in one of the regs. This will require that the rail be replaced.
Or you can contact http://www.thepelicanservices.com/Fuel%20Rail%20Page.html. They can rebuild your diaphragm.
to determine which diaphragm it is , you will need a 4mm allen wrench and a small C-clamp
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Customer reply replied 2 years ago
Why would it not happen right away and why does pushing the trim switch make it run fine?
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
The tear will be a small one, so it takes a bit of time to get the excess fuel into the rail. Trimming the engine places a different load on it, thus changing the RPM. The engine will run better on top end than at idle with a torn diaphragm.
If you have the tools, I can guide you through looking at them to confirm this.
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Customer reply replied 2 years ago
I just doubled check and the bottom one is air only. The gas that I smelled was from the top one.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Ok,
I only have the info that you give me to go on.
Do you have a multimeter?
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Customer reply replied 2 years ago
It runs perfect at idle. It runs perfect for about one minute than start acting up but if I go back into idle it runs fine. If I turn it off for a couple of minutes. It will get up and plane again. I can always keep it going with the trim switch.
Customer reply replied 2 years ago
Yes
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
On page 3 of this PDF is a test of the coils. There are two bulletins on them. Can you do this test?
http://www.marinepartsexpress.com/Merc_Prod_Bull/PCM%20Failures%20Due%20to%20Failed%20Ignition%20Coil%20Bulletin%20No.%202007-25R2.pdf
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Customer reply replied 2 years ago
I've replaced all 3 coils,plugs, and plug wires.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
About how long ago?
I have to ask, Are the wires going to the correct plugs?
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Customer reply replied 2 years ago
Yes I've had a coil go out a few years back so that was the first thing I checked. When the coil went out it ran fine but it would only turn 4000 rpms. I've also changed out battery and ground and power cable.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Alright,
What is the CCA of the battery?
Can you run the engine with your meter attached to the battery. Watch the voltage to see how high it gets to. Bring the engine up to 1500 RPM. How high does the voltage get to?
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Customer reply replied 2 years ago
1000 cca 12.8 off 14.4 running
Customer reply replied 2 years ago
It starts up good every time.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
What RPM are you running when the engine starts to surge?
How many RPM are you gaining or loosing?
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Customer reply replied 2 years ago
When I take off it planes fine and gets up to 5200 rpms which is its normal range. After a minute or so it wil start to fluctuate between 5000 and 5200. Speeds starts dropping. It gradually get worse. If I push the trim switch it will go back to 5200 but will go back down as sone as I release the switch. If go back to idle and try to take off again it will sputter and not get up on plane until I hit the trim switch and it will take off again. It will do that over and over again until I shut it down for 10 minutes or so and it will do the same thing all over again.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Really odd issue.
Make sure that the throttle cable and linkage are free to move.
Can you post a pic of the batteries?
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Customer reply replied 2 years ago
Not able to take pics of batteries. They are in boxes and in my console. One of them is a starting battery and one is a deep cycle. I had them out and both checked at local auto parts store. All cables are free.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
They are not wired together are they?
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Customer reply replied 2 years ago
There is a battery switch. I took the starter battery out and ran new wires directly from the engine to the battery and it did the same thing.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Is it only at 5200 that it does this?
Do you have a digital tach or analog?
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Customer reply replied 2 years ago
No it will eventually do it at any rpms besides idle. I have an analog tach.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Well that blows that idea. Was thinking of overspeed.
Can you take a vid of the engine so that I can see and hear it when it is running.
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Customer reply replied 2 years ago
It's 10:00 here on the east coast, but I could do it tomorrow.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
That is what I was meaning. I will be online after 530 PM CST. I have a day job as well.
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Customer reply replied 2 years ago
I can't seem to download. Would a audio work.
Customer reply replied 2 years ago
I can't get the audio to work either
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Are you using this procedure? http://ww2.justanswer.com/help/how-do-i-send-photo-or-file-expert-0
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Customer reply replied 2 years ago
I'm doing on a phone and its telling me it's the wrong type of file.
Marine Mechanic: Charles, Marine Mechanic replied 2 years ago
Oh. If you wish to talk to me directly, here is an offer for you to consider. We can also text or email.
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Charles
Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 4,048
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Experience: US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.

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