Paddles

Hello... I wanted to say first off that this forum is awesome. I've read quite a few threads and I'm very impressed with the activity and the amount of members.

Here's my problem... I have a 2002 Formula with a Merc 496 HO with 112 hours on it. After I run the boat on plane for approx 5 to 10 mins around 3k or so, not WOT or anything, she starts to cut out, backfire, then stall out. Gauges all read normal. I get just a single beep at startup and nothing when it quits running. I can let it sit for 20 mins and she'll fire right up and run like a champ then... Same issue. I've replaced the plugs and low pressure fuel pump with no change in performance.

The boat was trailered to MI from north Tx... It ran fine down there on a small lake before I brought it back. Dont know if the long haul caused the issue or not. I've also pumped the fuel out of the tank and started fresh.

I think it's a fuel delivery issue but i dont know if its electrical or mechanical? Any feedback would be great!

gotstreetracin

probably need to take the fuel rails off and clean them out..i'm having fuel delivery issues to, and my rails were full of crud and water... sounds like your high pressure pump could be getting hot and not flowing enough fuel as well

Raylar

Paddles:

I've seen a few such fuel situations in quite a few early model Merc 496's. In those cases it was caused by the inside of the injector fuel rail rusting inside from moisture in the fuel system especially from sitting in winter to summer temperature variations. You have a 2002 engine with only about 100 hours use and that means it has sat way to much unused!
First put a fuel pressure guage on the Schraeder valve on the fuel rail and check the fuel pressure at , key on engine not running, engine on idling, vacuum line disconnected from fuel pressure regulator on Cool fuel regulator under the black plastic cover and then at speed when the engine sputters, and stalls.
The key on fuel pressure should be about 43-45 psi and about the same when engine on a idling with vacuum to the regulator pinched off or disconnected and should stay up between 40-43 psi when running at 3000 rpms. If there are issues with the fuel pressure from these readings then the problem could be in the fuel filter, fuel pump side of the system.
If however these readings all seem to be good then it could be the dreaded rust in the fuel rail problem! The check for this involves removing the fuel rail and injectors and carefully removing the injectors, giving a good look up inside the fuel rail for signs of rusting and debris. When take the injectors out of the rail, turn them upside down over a nice white paper towel and spray some good carb cleaner up into the inlet side of the injector where the fine screen is and then tapping the injector down onto the towel and see what comes out. If you get any real debris out of the injector this way you should have the injectors cleaned and re-flowed by an injector service shop or replace all the injectors. You will also have to replace the fuel rail itself with the newer ones which were finally plated by GM in later models to stop this problem.
I am not saying this is definitly what is wrong here, but with early 2000 Merc 496's that sat alot it has been a big problem.

A sitting unused for long periods marine engine is a problem waiting to happen.

Hope this helps and let us know what you find or figure out on the engine and its problem.

Best Regards,
Ray @ Raylar

Paddles

Thanks for the replys... I put the fuel pressure gauge on and with the key on it reads 43 psi and with it running the needle viberates around 42psi. I took it for a run and made it about 1/2 way to the middle channel from metro and it quit. I was alone so i couldnt read the gauge while driving. I'll take a friend along tomorrow. My question is, if it were something in the fuel rail... rust, debris, ect.... Wouldn't it run crappy all the time? I'm leaning more towards a hp fuel pump heating up and causing the issues.

I'm still going to pull the fuel rail and injectors just to verify it's not the issue and to eliminate the possibility.

NightHawk

Is it going into guardian mode? Do you have access to a scan tool?

Raylar

If it's a fuel pump the fuel pressure will drop and be low when it starts to quit. Sometimes debris in the injectors slowly drops back into the injector screens when fuel is flowing thru the injector and then when it's enough blockage at a certain higher rpm its just like cutting fuel off to the injector.
It could also be a problem with the crankshaft sensor as early Merc 496's had many failures and sometimes the sensor stops working at certain rpms. You can also try cleaning and replugging all sensor plugs and connectors and cleaning and tightening ground wires as loose or bad coonections there can show up at certain rpms and vibration levels with intermittant disconnection and ECM shut offs.

Best Regards,
Ray @ Raylar

Paddles

From what i'm being told guardian mode is more of a reduction in total power rather than a total loss or shut down/ stall. I have had a service shop hook up to it with a pc, low pressure fuel gauge and high pressure fuel gauge while out on the water which is how i arrived at changing plugs and the low pressure fuel pump. But they never experienced the shutdowns i am.

Ray, the problem isnt occuring at certain rpms, i can rip all through the revs and different speeds across the lake for a certain amount of time without a flaw, it seems to be more temp driven, but i will know more in the am when i take the fuel rail and injectors off.

Where abouts is the high pressure fuel pump located? I've googled images and the only results that seem to populate reference the low pressure pump that is located behind the water/fuel seperator.

Raylar

The high pressure fuel pump is under the black plastic cover of the cool fuel unit which is located on the opposite side from the low pressure lift pump by the side engine mount.
I don't think what you are describing is a guardian shutdown, but either a fuel cutoff or electronic/electrical shutoff or interrupt.
Hope you find it quickly and its fairly minor. That low engine hour usage over so many years is the ultimate culprit.

Best Regards,
Ray @ Raylar

Paddles

Thanks Ray

I bought the boat last month from a place in northern Tx. I had a survey done where they spent over an hr on the water with no issues. I then drove down to pickup the boat and at that time i also took the boat for a ride with no issues. I starting to believe the long haul back on a the trailer with an 1/8 tank of gas sloshing around caused the issue. The gas probably broke up all the sediment in the tank. The seperator was full of crud so i imagine everything after the seperator downstream still will have some debris in and around it. I'll know soon, heading to the boat now.

If it is electrical, where should i start? I've gone through the connection points already.

Thanks again

Paddles

Well... The injectors and fuel rail had debris as expected... I cleaned both really well with carb cleaner and re-installed. Then i turned the key to the on position and purged the fuel rail multiple times with my pressure gauge to help eliminate any air. She fired right up as expected.... Took it for a ride and still the same issue... Dies after a short run. I was able to grab a few readings of the fuel pressure gauge at 3000 rpm running fine and it read between 38-40 psi when it dies it def dropped somewhere between 25 and 30. I guess i will have to order a hp fuel pump and try that.

Im just glad its the beginning of the season.