5.7 MEFI 5 stumbles and chugs

2008 Tige Z1 5.7 MEFI 5 (180 hours since boat bought new). Boat filled and stabilizer added before storage in dry un heated garage last year. First day on the water it stumbled and wouldn't respond to throttle from idle after about an hour and quit, it would re start but not idle, towed to the dock (30 minutes) would start and run fine. Emptied 5 gals fuel from tank and put fuel/ water additive in it.
NO CHANGE
Emptied the tank thinking fuel contamination, added new Premium fuel from different station, added water filter before the low press pump.
NO CHANGE
Found elec boost pump intermittent when powered to the battery, Replaced it!,
NO CHANGE
NOTE: Boat starts and idles fine but then stumbles when throttle applied, we can then get it running at higher RPMs but then all of a sudden drops power and chugs. Also if idling for short periods then again it won't respond to throttle and stumbles and back fires.
Oil press sits at 54-55, eng temp at 140-148.
What should fuel press normally be from the high press pump when running?. This seems to be a fuel problem.
We are in Medicine Hat Alberta.
Thanks. Keith

Sounds like a fuel restriction. Maybe a collapsed line or anti siphon valve. I found the little piece of foil from the stabilizer bottle top clogging the fitting at the top of the tank. Start at one end and check everything thoroughly.

I am leaning to a restriction. The last thing I did last year was change the fuel filter in the high press pump which seems to have never been previously done (had boat one season) and I had to score the paint on the housing to get it apart. I will remove the housing tomorrow and drain the tank and check the tank pick up line and blow all the lines out. I should have looked at this before hand as generally I look at what was the last thing done as a possible culprit.
i will submit my findings.
Keith

IS there a check engine light in the dash instrumentation? I'd suspect there's an issue and the ECU has one or more codes stored in it....and you'll need a scanner (scan tool) to find out. would also be good to do a www search and get a MEFI-5 service manual - it's full of useful info. And when it comes to injected engines, verifying fuel pump pressure delivered in the rail is always prudent....check the factory service data for the specific value....most newer GM systems will require something around 50 PSI....marine power's www site appears to have been updated with most of what you'll need...

I have a scan tool but was unsure if it would work on a marine engine. I will give it a shot. I think I am leaning more towards the fuel issue, I don't want to get ahead of myself. Marine Power had suggested fuel blockage so I will start there. We tend to get crazy when the word "fuel injection or ECU" get thrown into the mix lol but generally fuel injection works well most of its life. Thanks for the info.

If its an OBDII (auto) tool, don't waste your time....the connectors won't play well together....

The MEFI-5 controller introduced the ability to monitor and control the fuel delivery system so its not like any of its predecessors...agreed fuel injection works well and has for decades.....its just a pain to troubleshoot when something goes wrong....especially if you are 50+ miles off the beach...

A vacuum gauge plumbed into the suction line (to the fuel pump) will give you a real good idea if a blockage is present...

Thanks for the reply. I rented a fuel press gauge and put it on the schrader valve between the fuel rails it measured 65 psi. I removed the fuel pump/ filter assembly and drained into clean container and found water, I believe the high press pump filter was water logged, changed it along with the extra water/ filter I installed previously. Blew out the lines and removed the anti siphon valve. It appears the pump filter might have been plugged with water. I will need to get it on the water to test it.

Kinda miffed my OBD2 scanner not MEFI 5, Marine Power people were very helpful, they suggested if fuel press low to replace the fuel press regulator. It was ok and thank god because it was hard to find and long to get

After re-reading your first post, I'm pretty sure you have the CANBus interface....it may work electrically with a newer scanner but the mechanical aspect of the connectors will be different....if you know how CANBus works, you can experiment....

a RINDA scan tool (tech mate) or their software (with an adapter/connector) will give you everything you need for the future...

Also, once this is resolved, for MPI systems, you need to make sure the fuel pressure is "within spec" at all engine RPMs - not just idle.....

Sorry for late reply, work work work. Replaced the fuel press reg as suggested by George at Marine Power, put porta- lake on it and ran it on shore at idle= 62 psi- Awesome. Then put it into the lake with fuel press guage hooked up at the fuel rail and press dropped to 52-55. Engine still stumbled but better. Installed NEW high press fuel pump and ran on shore= fuel press won't go above 50 except if vac line removed and still Max is only 58. I changed gauges = Same press.
I am going to remove the braided fuel return line at the pump canister, plug the canister and put the line into a container, I assume there shouldn't be fuel returning if the rail press shows only 50 psi, if so then the new reg is duff!
i have again blown all the fuel system lines out and also checked for kinks or collapsed braided lines whil running to see if press fluctuates.
Frustrating.

After reading a tech manual on MEFI 5 systems I discovered that if there is any sort of blockage in the return line then your fuel press will go above 60 psi and cause problems. There was a possibility something was lodged there. The GM manual stated the press to be around 50 psi with pump running and then increase by 5-8 psi when the vac line removed from the reg while running, it did this and is supposed to be a test for determining that the reg is good.
We didn't have any further problems with the motor all day except for a very minor hesitation from idle. The outside temp was 34 C today and we tubed most of the day. I am left to assume the high press pump I changed the night before was the culprit or one there of.

glad you got it running...