Slow Acceleration Mercruiser 4.3 tbi
 

Hi all,
Have been scratching my head with this issue for days now. My Mercruiser 4.3 efi engine suddenly is experiencing slow acceleration. It will start fine, runs perfectly on idle , when on plane it will reach WOT Rpm and have very good top speed. Issue is that it lacks power to go on plane but when it does it will continue to accelerate perfectly. Sometimes when it will not manage to get on plane.

Boat weight distribution is always the same, Propellor has been the same pitch from new.
Compression is 120 on all cylinders
Fuel pressure steady on 18psi
New spark plugs 2021
New leads 2021
New ignition pick up the sender 2021
Injectors and Throttle body serviced and cleaned
Throttle body gasket new 2021
All Fuel hoses new 2021
Knock sensor new 2021
No fault codes
Battery full
Grounds all good
Alternator charging perfectly
IAC valve New 2021
Exhaust riser and Manifolds new 2021
Exhaust Shutters new 2021
Fuel pump new 2021
Fuel filter New 2021
Spark plugs running good fuel combustion color, not lean and not rich, just right
Distributor cap in very good condition
Distributor coil New 2021
Oil pressure sender new 2021
Changed propeller to the same pitch as it was always been from new
Water Thermostat new 2021
Engine temperature is always perfect at 160
Engine oil pressure between 50-70psi



Engine is TBI.
Does anyone have any ideas why the lack of power during acceleration and hard to get on plane?

Not really, but is your bottom dirty ? Hopefully not your mind ? :)

Also, what you are describing is typical of a fuel system with vacuum line on regulator, where regulator is faulty, or vacuum line is slit/cut/not attached/etc . Most tbi's don't, but does yours ?

Edit: double check timing too

Edit: "Suddenly" as in fine one day(s) and then not ?

Thanks for your input..
No Bottom is good conditions nd very clean , boat has been in water for two months and new antifouling before.

Yes Fuel system has vacuum line from TB to Fuel pressure regulator, Vacuum pipe is secure and not leaking at all.

Yes suddenly as it was fine on a Sunday and not the following day.

Will check timing to be sure as this I have never thought could be an issue.

is 18psi of fuel pressure typical on these efi engines............seems low in my world

Fuel pressure should be 30 psi at 1800 rpm

If you added fuel ⛽️ in the tank or an additive to the fuel that could be the problem ??

Fuel is new with no addivitves and also tested engine running from another small outboard tank with fresh clean fuel.
Engine works perfectly in all rpm ranges , just very poor acceleration and won't plane on load.. if load is removed boat will plane and ride perfectly.. Issue is that load was always the same and this reduction of acceleration happened suddenly .

Cannot understand..

That what I was thinking too, but according to specs I found on the net for my engine it should run between 9-12psi -- 18 is max..

what boat is this, looks like you have approx 220 hp in the perfect world ? how much weight do you have to remove to get it to run

It's a Rio 700 Crusier Cabin. Yes, put a lot of work and new parts into engine.


I have to remove about 80ltrs of freshwater to plane.. If I load with the same amount as we usually do every weekend it will not plane.. The boat is lacking power on acceleration only, when on plane it runs beautifully .
My only concern is fuel pressure as I'm not sure if 18psi is correct.

You sure that is s vacuum line to regulator ? Maybe to send fuel back into tbi in case psi reg diaphragm ruptures/leaks. If it is hooked to vacuum and not supposed to, this can cause improper psi under different loads

https://cimg3.ibsrv.net/gimg/www.off...84e4ff6b9a.jpg

THIS IS GOOD INPUT.. YOU ARE CORRECT! my current vacuum line is coming from TB and not from flame arrestor.. checked service manual and it's how you say... will try to reconnect correctly and see the difference next Monday..
Still unsure why low fuel pressure though-- Maybe the regulator got busted due to wrongly connected hoses?

who would have changed the vacuum line without your knowledge?

Purchased boat from another user 3 years ago with the engine "rebuild". Could be they did not wire vacuum hoses correctly but ironically it worked for 3 seasons this way.. TBH I always felt engine was to small for acceleration on this boat.. This summer decided to lack of power during acceleration, could be that the pump or pressure regulator got busted now..

If the engine is supposed to have 30psi fuel pressure it surely has to be a pump or regulator so need to check these on Monday.. but vacuum lines not connected in the correct order would make sense for loss of power at a different speed.

Have you ran the engine on external fuel can to check if tank issues such as pickup screen, etc.?

Yes , I tried with an an external tank , new fuel filter and fresh fuel

UPDATE:
Replaced Fuel Pressure regulator and Fue pressure is always on 30psi , it only fluctuates about +/- 3 psi . Tested boat and it feels to operate much better. Will try with Full Load next Thursday and will ensure that problem has been solved.
Only concern is if I have the vacuum tube from TB to FPR correct.. Should the vacuum line be from Flame arrestor to FPR or from TB? Tried to install it from the Flame arrestor and it makes little difference in performance.

When line is to flame arrestor, it is a vent hose, not a vacuum line. When hooked to a vacuum nipple on tbi, it will become a vacuum hose and it will reduce psi when under vacuum. As throttle blades open there will be less vacuum and psi will go up. The more throttle, the less the vac and more psi. Psi will not be higher than when hose is too atmospheric ie flame arrestor

thus my original question.

Yes it's fitted on the TB , you can even here good vacuum when on idle. So all should be good now in terms of Fuel delivery and pressure. On WOT the fuel pressure is always constant on 30psi, this means vacuum and fuel pressure regulator operating accordingly..

Will test the boat with full load next Thursday and update with resuts

Fixed my typing on my PC. Sorry. My fingers and cellphone touchpad don't mix well. Doh !

So the issue all a long was a misplaced vacuum line ?

Hi All,
Seems that all has been resolved. Boat is now getting on plane. Fuel pressure was indeed causing the issue of loss of power.
Vacuum tubes are in correct order and 30psi is constant on all rpm ranges.

I thank all of you for your input in solving this issue.

Many Thanks