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1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

GAS engine, transmission and generator repair and maintenance discussion forum.
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grizer
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Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner 5.0 Merc 3000RPM WOT

Postby grizer » September 5th, 2018, 8:23 pm

@bud37, we have had the boat 4 years and this issue is new (last month or so), the engines are mpi w merc coolfuel2 pump, batteries new last year and seem to be in great shape (no slow cranking or anything like that), ECM not throwing any codes.

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Re: 1998 350 Mariner 5.0 Merc 3000RPM WOT

Postby CptBacardi » September 6th, 2018, 11:16 pm

My GUESS: low voltage from the batteries. Seen similar issues before. Free for you to diagnose yourself. Put a multimeter on it.
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Topic author United States of America
grizer
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Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner 5.0 Merc 3000RPM WOT

Postby grizer » September 15th, 2018, 8:18 am

@tomschauer - I understand that but am trying to understand what else could be the issue, what conditions need to be satisfied for the fuel pump to work? Only voltage and fuel in? It would seem like there is something else and it is odd that both engines behave the same way. I guess I am trying to see if there is something else that is common to both engines that could be the culprit.
@bud37 - my original post covered all of your questions, we have had the boat 4 yrs, it just started doing this summer, these are MPI engines (Merc 350s w Cool Fuel 2 pumps), batteries are perfect.
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Topic author United States of America
grizer
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Vessel Info: 1998 Carver 350 Mariner
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1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby grizer » September 17th, 2018, 5:01 pm

Hello, I am reposting this same issue as it is not resolved but I the forum also will not let me enter new post to it or updates (just hangs).
Here is my situation:
1. Boat will not go over 2800 RPM under WOT, in idle engines rev fine which seems to eliminate throttle or throttle cable.
2. Normal things all seem to check out ok; fuel filters, fuel vents & lines, fuel looks good, even pulled it and checked props, shafts and did compression check, also NO ECU codes are being thrown.
3. BOTH engines are behaving exactly the same
4. Under WOT, fuel pressure guage only reads 20-25psi and I believe it should be mid 40s according to Mercruiser. Voltage at the pump is also good.
5. Boat is a 98 with twin 350s with 600hrs (I believe the pumps are the Merc Cool Fuel Gen II, we have owned it 3 yrs and it just started doing it this season, worse is that 2 weeks ago it was limited to about 3000 RPM WOT, yesterday it seems to be closer to 2400 RPM (again BOTH engines)
I know this SEEMs like it must be the fuel pumps but it just seems so random that both fail the exact same way at the same time. I am just wondering if there is something else I should check or look at...looking for any and all suggestions PLEASE HELP. Thanks!

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Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby Viper » September 18th, 2018, 6:54 am

Did you have the fuel regulators and the little filter under them checked? They're in the cool fuel module. Did you have any water in the fuel in the past? Do the fuel pumps have a loud whine?

If you have a sync gauge, disconnect it and go out for a test run. Then disconnect one tach at a time and test.

DO NOT do WOT in neutral :-O
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Topic author United States of America
grizer
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Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby grizer » September 18th, 2018, 7:29 am

Hello, thanks for the input. When you say "disconnect the sync guage", where would I do that at, right at the instrument? Does that actually have anything to do with the system or just a simple analog read out? No I don't hear a whine and I have not had the regulators checked. Do you have any graphic or a way to show me where they are located and how to check and also where the "little filter" is located?

When I say I check WOT in neutral, I simply revved it for half a second just to make sure it would go above 3000 RPM to rule out throttle cable problem haha

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Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby Viper » September 18th, 2018, 11:40 am

The regulator is in the cool fuel module usually on the bottom of port side of the engine. Also make sure the vacuum hose going to the regular is connected at both ends and that it's intact.

Disconnect the sync and tachs at the gauges. Sometimes tachs can fail by shorting/leaking to ground and cause puzzling symptoms. The sync gauge hooks up to both tachs, and it's not impossible for a defective sync to pass on a problematic electrical issue to both engines. Depends on the unit you have but when I see something pop up that suddenly affects both engines at the same time and certain things have been ruled out that are common to both engines like fuel, source voltage, etc., considering how easy it is to eliminate the tachs and sync, it's a quick way to take that possibility out of the equation.

I recall seeing a bulletin about Cool Fuel II fuel pumps. Can you post one of your engine serial numbers.
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Topic author United States of America
grizer
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Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby grizer » September 23rd, 2018, 8:28 am

Sure I can post serial numbers, i will locate.
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Topic author United States of America
grizer
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Joined: August 7th, 2018, 2:48 pm
Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby grizer » September 30th, 2018, 8:53 am

I cant seem to find the engine ser numbers in any literature but found a tag on one of the engines that says “L002390” as the serial number.
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Topic author United States of America
grizer
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Posts: 35
Joined: August 7th, 2018, 2:48 pm
Vessel Info: 1998 Carver 350 Mariner
Location: Port Clinton, OH

Re: 1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

Postby grizer » April 30th, 2019, 10:59 am

I have eliminated many suggestions and finally broke down and replaced both fuel pumps. Same issue still persists. We are going to try some more things but curious if anyone else has any suggestions or has seen something similar.

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