Low RPM, Slowly decreasing above 2k

My 2003 6.2l had a similar problem, my main fuel pump was failing so that I could only get 28 psi under load. The 2003’s have a helper pump to prevent vapor lock in addition to the cool fuel 2. Between the two pumps I could rev it up high in neutral, but only the mid 2000’s under load. The mechanic finally figured it out by putting it on the dyno, maybe your buddy could attach the fuel tester to the rail when you are driving around.
 
Before you change anything, try this test... you may have a clogged vent system. This can be an actual clog in the vent, but since your vent is internal to the fuel fill, that is less likely. However, a very real possibility is that a low spot developed (sag) in the vent line over time and it got filled with fuel when you filled up - especially if you "topped off". So take the boat out and run it with the fuel cap off. If that solves the issue, it's your vent system. If you still have issues, then proceed with the other advice already given.
 
Thanks guys! Once I get this plug replaced I am going to have the engine scanned. Depending on what that says, I’ll take her out, Try the vent cap thing (yes I did inadvertently top off as the pump didn’t stop when full). And have the mechanic check the fuel pressure.

I am just wondering though. Water squirting through a pipe plug develops then I start having this issue. Maybe it’s not a fuil pressure thing… maybe a sensor or the water got somewhere it should not have. Honestly it seemed like water would have been squirting right into the air filter….
 
It's hard to tell from the photos but is that one of the pipe plugs in the intake manifold? If so I have spares on the boat and if I go today I can get the size/threads info for you. I think it takes a 3/8 square ratchet.
 
Phil S said:
It's hard to tell from the photos but is that one of the pipe plugs in the intake manifold? If so I have spares on the boat and if I go today I can get the size/threads info for you. I think it takes a 3/8 square ratchet.
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Hey Phill. Here is a closer photo. Yes seems to be on the intake or coolant crossover.
423C8B54-F1CE-4992-9A12-4D58F40ECFD2.jpeg
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Although on second thought -- that takes the dreaded allen key :eek: I'm sure you could get one locally

.375-18 x .475 brass
 
Phil S said:
Although on second thought -- that takes the dreaded allen key :eek: I'm sure you could get one locally

.375-18 x .475 brass
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Thank you! Mine has a square female socket. Which I only found on Amazon. I’ll get one similar in brass with as normal a socket as possible.
 
Many of those plugs are the same size as your garboard drain plug - meaning, one of those can be used - 1/2" pipe.

If you suspect you're injesting water, make a quick diverter/cover so the water goes somewhere else. Check your spark plugs.
 
Eddie B said:
Thank you! Mine has a square female socket.
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It "should" be a 3/8" square socket, which allows to to stick a 3/8" extension directly in the plug. Pretty robust, actually. Better in my eyes than one with a square head on it, as you pretty much need an eight-point socket to be able to apply the correct torque.
 
So I got the old plug out. Came out easily. Needed one of these sockets. Nothing else would fit but the smaller one of these fit perfect.

CTA Tools 2049 Square Head Drain Plug Sockets, 2 Piece Set https://www.amazon.com/dp/B008TOHAV...abc_F3FJBSY115KX47WZZ81Q?_encoding=UTF8&psc=1

and there it is. A hole like through the rim of a clam shell. Mech is coming tomorrow with a new plug (unless the one I ordered gets here first) and to read the codes… let’s see what this little bastard messed up. My auto mechanic called it a “freeze plug”. As if it were supposed to sacrifice itself should the water in the manifold freeze?
E956946E-998B-4BDB-B54F-409F201C1966.jpeg
213CB42B-B0CD-433A-9DE4-D3F30AB7BEBE.jpeg
 
Eddie B said:
Mech is coming tomorrow with a new plug (unless the one I ordered gets here first) and to read the codes… let’s see what this little bastard messed up. My auto mechanic called it a “freeze plug”. As if it were supposed to sacrifice itself should the water in the manifold freeze?View attachment 109039View attachment 109040
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Okay, two totally different items. What you have there is a MNPT (Male National Pipe Thread) plug. This is used where an external water passage might be desired. They are most certainly not designed to push out if the cooling jacket freezes. That, and I wish people (especially mechanics who should know better) would stop using the term "freeze plugs"! They're Welsh plugs! These are not designed to push out when the water freezes. Rather, they're access holes utilized during the casting process. Typically, if the water freezes hard enough to push a Welsh plug out, it'll have already cracked the block somewhere else. Rant over. Thank you for your attention. I should go to bed to see if my attitude will improve after a full night's rest...
 
Nater Potater said:
Okay, two totally different items. What you have there is a MNPT (Male National Pipe Thread) plug. This is used where an external water passage might be desired. They are most certainly not designed to push out if the cooling jacket freezes. That, and I wish people (especially mechanics who should know better) would stop using the term "freeze plugs"! They're Welsh plugs! These are not designed to push out when the water freezes. Rather, they're access holes utilized during the casting process. Typically, if the water freezes hard enough to push a Welsh plug out, it'll have already cracked the block somewhere else. Rant over. Thank you for your attention. I should go to bed to see if my attitude will improve after a full night's rest...
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He was an auto mechanic ;-)
 
Eddie B said:
He was an auto mechanic ;-)
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No excuse. They're all the same thing. Yeah, I know it's semantics, but the term tends to lull people into thinking the plugs will save the block from cracking, then they're mad when it does crack. btw, my daughter calls me the "Grammar Nazi"...
 
Eddie B said:
Worth noting I’m not just loosing rpms but applying full throttle has not effect. Will not climb before the steadily decreasing limit.
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This is usually from a dirty fuel filter. As it continues to get dirty, less fuel gets thru, causing rpms to drop. Change the fuel filter before doing anything else.

Another hint to the problem being a dirty fuel filter is that that you just added fuel which could have stirred up sediment from the bottom of your fuel tank, and/or you just got a dirty batch of fuel.
 
Dani-Lu said:
This is usually from a dirty fuel filter. As it continues to get dirty, less fuel gets thru, causing rpms to drop. Change the fuel filter before doing anything else.

Another hint to the problem being a dirty fuel filter is that that you just added fuel which could have stirred up sediment from the bottom of your fuel tank, and/or you just got a dirty batch of fuel.
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Thanks Dani. Odd that it occurred at the same time this leak was discovered. I am hoping to get the plug in and change the filter tomorrow and take the boat out before I have to leave for the weekend. Will let you know how it goes.
 
Hey folks. 2 weeks. Replaced the plug but the boat has not left the dock. I can’t get ANYONE here in Stamford to take her in, read the codes or troubleshoot. Thinking I am going to have to do it myself at this point to save my season. Any recommendations on a decent (code reader)?
 
You would need a Rinda scanner to get the codes, they are $600ish. Problem is that your boat may not be throwing codes if the pump is failing. Mine wasn't, the tech had to put it on the dyno and do a pressure test under load.

You might want to look into the mercruiser vessel view mobile. It connects the engine computer to your phone via bluetooth and gives you fault codes in actual text rather than beeps. Its about $200
 
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