2003 Mercruiser 5.0 EFI back firing at WOT and total loss of power

Late last season my 2003 Mercruiser, 5.0 EFI Bravo III developed a backfiring sound at WOT, a "pop, pop, pop" sound. It would stop when I backed of on the throttle. I replaced plug wires, plugs, fuel filters to no avail. This season, the problem persisted with difficult starting, bucking and sputtering. Once running, the engine would idle fine and seemed to accelerate ok in neutral. However, under load the backfiring continued and yesterday, a total loss of power occurred. The engine just bogged down when the throttle was advanced, no power and the engine eventually stalled. Now it will not start and just sputters and bucks when you roll it over. The oil pressure and temperature seemed ok, there were no alarms. There is no water in the oil and no leaks. However, the tach was floating up and down occasionally. Am I looking at a rebuild? or could it be something more simple like a faulty oil pressure switch, electrical or fuel issue. Could there be a fault code that would point me in the right direction? The engine is raw water cooled and has 350 hrs on it. All comments appreciated.

Re: 2003 Mercruiser 5.0 EFI back firing at WOT and total loss of power

You'll want to check your fuel pressure, It should be right around 43 psi, There is a good chance that it's not at 43 psi at this time from your description above.

The vacuum line going to the fuel pressure regulator will need to be disconnected to see your 43ish psi though or the gauge should show somewhere around 38+ psi on average with the vacuum line still hooked up.

Good point Mac525, The distributor cap and rotor could very well be the problem in this particular case. There is a Mercruiser service bulletin (2003-06) that might be around your engine time frame. You will need your engine serial number to confirm this though.

http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/gmefi-nostart.PDF

http://www.thehulltruth.com/boating-forum/172302-mercruiser-350-mag-horizon-problem.html

Good luck to you with it.

Re: 2003 Mercruiser 5.0 EFI back firing at WOT and total loss of power

This is probably not a fuel issue all the merc engines with the crab style distributor cap are prone to this. Did you replace the plugs if so where they gaped to .060 try changing the gap to .035 this helps the crossfire in the cap issue also a new volvo cap with the brass pins msd also makes a cap with brass pins. I hope this saves you alot of time and money there is talk of a recall but details are few and far between

Re: 2003 Mercruiser 5.0 EFI back firing at WOT and total loss of power

Dealing with the same thing here with my 1999 5.7L Black Scorpion. 2500 - 2700 rpm is it. After that with load - same symptoms. Tach drops out, loss of power and occasional backfire. I was told to check a number of things before plugging in the computer.

1: Check fuel pressure. should be 45-50 psi for EFI. (mine read low... 22-25 psi so that was my starting point)
2: Check fuel filter/water separator
3: Check inline fuel filter (not all have them - mine was by the gas tank)

if it still runs rough - then probably a fuel pressure or vacuum issue.

4: Start engine and let it idle out of gear

5a: Disconnect the MAP sensor from the hose line (3/8th rubber hose I think). It should immediately start to run really rich, like the choke is on. Reconnect it. If there was no change, you most likely have a bad MAP sensor (Manifold Absolute Pressure). It's not telling the computer when there's a vacuum change so it's not letting more gas in at higher speeds and loads.

5b: If the MAP does seem to be working, then while still running, disconnect the smaller vacuum hose line from the EFI manifold that leads to the fuel pressure regulator. Should do the same as above. If it doesn't, then it could be that the FPR failed and the diaphram is frozen or stuck closed.

I replaced the high pressure fuel pump and did all the checks above. Disconnecting/reconnecting the FPR vacuum line did nothing so that's the suspected failure. I'm replacing that today - I'll let you know how it goes.

Some of the newer Mercruisers also have a fuel boost pump that looks almost identical to the high pressure fuel pump. It's located near the water separator mounted to the same bracket. I'm told those tend to go first, then high pressure pump then FRP. Since I dont have a boost pump, I went to the regulator.

To be continued...