4.3 MPI bogging down at WOT

[Iro]

Hi boys. Recently purchased a bow rider with a 4.3MPI Alpha drive in the back. She had spent the past 5 years in a barn in outback Australia rarely seeing water due to drought so knew I was up for some maintenance on her. Clear signs she had spent a long time in salt water as well before this as the anodes were pitted heavily.

Resealed the gearbox, new bellows and shift cable, engine oil and filters, replaced the alternator (**** itself) and she ran like a dream.

Took her out a few weeks back and she started bogging down anywhere over about 3500 RPM. I’ve since changed the cap, rotor and fuel filter and currently waiting on new plugs and leads to arrive. Launched her with new cap and rotor and still getting the same issue.

Now I obviously haven’t done much in the way of troubleshooting yet. As a lot of you know getting a chance to source parts, install and sea trial between work can be difficult. Im a Diesel fitter so not entirely clueless but far from an expert on the old Mercrusier here. Hoping someone can guide me through running a multi metre over relevant points on the ignition system, where to test fuel pressure from and what pressure I’m chasing and any other tricks you blokes might have up ya sleeve in dealing with this issue so I can hopefully diagnose her quickly as she’s parked on her trailer half an hour from me. Any help is much appreciated

Engine serial:
M333652

 

[chs marine service]

what year is the boat/engine? if itsbogging down, it could be fuel or ignition, the more likely cause is fuel. i would look into taking a fuel pressure test, the fuel pumps will allow the engine to run decent and totally bog down when under load. pressure shoudl be over 32psi. if the pump is bad, you will see the pressure do a nose dive when the bogging occurs. also the fuel regulator should be looked at. lastly, before you disconnect the pressure gauge,, open the fuel cap to allow air in and test the boat with the cap off, you may have a blocked vent line.
the other thing it could be is the ignition breaking down. which could be ignition amplifier or coil (coil is ALOT cheaper)

 

[alldodge]

Howdy

Thanks for the serial number.
Your fuel pressure should be 43 psi (296 kPa) at 1800 rpm, and should stay about there clear to WOT

Did you get rid of the gas in the tank?

Measure pressure at the fuel rail item 13 after removing cap
https://www.mercruiserparts.com/bam/subassembly/31499/7801/80

 

[Iro]

Thanks for the reply there boys. The hull year is 1995 but the engine was repowered at some stage. I’ve tried crunching the serial number through any online search engine and it’ll only tell me its post 2002.

Just tested off the valve on the fuel rail and getting 42 PSI with key on (when pump kicks on for 2 seconds before ignition) but drops to 36PSI at idle then to 32PSI at 1800 RPM. She’s running off the muffs though so no load on her but it’s still a fair way off 43 PSI. Sounds like the pumps gone then. Can’t test voltage at the pump yet as I have a full tank of fuel and I’ll need to remove the tank to squeeze in but seems voltage is good as I doubt the pump would get up to 42 PSI at key on with low volts.

Fuel pressure regulator seems fine. Screen was clean and no leak at the diaphragm. I drained the tank when I got her just before changing out the filter so only fresh clean fuel since I’ve owned it. Should be launching this week so I’ll take another reading at WOT and see what happens when she starts bogging down again but sounds like the pump is the culprit.

 

[alldodge]

Your can read pump volts at the fuel pump relay. Right now I don't think its a voltage issue

I think its either a pump, regulator or restriction

Pump - could be weak. Would need to bench test to see if it can produce and hold 60 psi

Reg - it starts off with key ON but is leaking internally. If pump checks out then its the Reg. If pump is not tested, then remove Reg and apply air pressure slowly until it reaches 43 psi to see if it opens on or before. If screen was clogged then pressure would be high, since its clean then the screen is not the issue

Restriction - Any where from the tank pickup tube, antisiphon valve and to the pump could have a restriction which keeps the pump from being able to draw the fuel