2005 4.3L mercruiser surge and reduced wide open rpm

[Murfalert]

200 hours on this 18 Crownline. Has run perfect with a 52mph top speed. All of a sudden it surges at part throttle and won't rev past 3800. Replaced fuel filter, gutted anti siphon, blew air through fuel hose from tank to filter- all with no change. Engine doesn't misfire, runs smooth, just acts lean. Any ideas? BTW; air filter clean, runs slightly slower with it off- again seeming lean. What would make EFI lean?
Thanks,
Murf

 

[Scott Danforth]

Welcome aboard.

Restricted fuel line, plugged filter, blocked vent..... Many things can make a motor run lean

 

[alldodge]

Howdy

What do the plugs look like?

 

[Murfalert]

Plugs look excellent, though white- again a lean indicator.
New filter, checked filter at throttle body and it's clean, blew air through tank vent and fuel line with my mouth.
Throttle body looks immaculate. I'm baffled.
Thanks,
Murf

 

[achris]

You have an MPI. There are a lot of sensors and such that can cause problems like that. One the springs to mind is the IAC. If that jams open at high revs, then you have a lean condition.

Get a scantool on it and see if any codes are set.

Chris.......

 

[alldodge]

Agree scan tool would be great. Can also check the pressure at the fuel rail, if its low it cause a lean condition and also loss of power.

 

[Murfalert]

It's a throttle body system. No rail.
Murf

 

[alldodge]

It's a throttle body system. No rail.
Murf

OK, then you need one of these to check pressure, part number 81-806901

 

[achris]

It's a throttle body system. No rail.
Murf

The last year Merc produced the 4.3EFI was 2001 (from 2002 on it was MPI). Can we have the serial number of your engine? (difficult to know what to suggest if we don't know what we are working with)

Thanks,

Chris.....

 

[Murfalert]

OW317627 is the serial. Any help is appreciated. Maybe my lungs can't blow hard enough through the fuel pick up. Though it's hard to imagine it's plugged bad enough.
Murf

 

[alldodge]

OW317627 is the serial. Any help is appreciated. Maybe my lungs can't blow hard enough through the fuel pick up. Though it's hard to imagine it's plugged bad enough.
Murf

With that serial number you have a TKS carb not EFI
http://www.mercruiserparts.com/4-3l-carb-alpha-bravo-0w310000-thru-0w

http://www.mercruiserparts.com/bam/subassembly/31492/11812/80

For it to run lean, the carb has some crud in it, fuel pump pressure is weak (3 to 7 psi), of a fuel restriction between tank and carb. To check fuel pressure you need to install a Tee between pump and carb. The cheap vacuum fuel pump tester will work and you don't need the fitting I posted above.

https://www.amazon.com/Performance-Tool-W80594-Vacuum-Tester/dp/B000FW2O1G

 

[Murfalert]

Makes sense if it's a carb. A pilot or low speed jet would cause that. Any idea where those are? Do I yank the top off and I'll see them?
Thanks
Murf

 

[alldodge]

You can see them with the top off, and the exploded view is in the second link down in my post 11.

 

[Murfalert]

Wonderful- thanks a bunch. Don't know why I thought it was throttle body injected. It acts exactly like a motorcycle with a plugged pilot (low speed) jet. I'm not sure which jet controls low rpm but I'll just make sure they're all clean. And I'll check the emulsion tubes.
Much appreciate the assistance.
Murf

 

[Murfalert]

Update; sure enough there was a flake of dirt half plugging a main jet. Runs like new again. There was a small amount of scale in the float bowl from when ethanol fuel evaporated sometime in the past. A piece must have loosened and floated into the jet.
Thanks for all the help,
Murf