Mercruiser 7.4 MPI - Fuel Starvation?

[jmagrann]

Greetings,

Carver 400 CMY, twin Mercruiser 7.4 MPI. 500 hours.

I saw a similar post, but that was for a carbureted boat.

Port engine, has been running well since I purchased the boat Oct 2015. Had both engines fully serviced, including new OEM manifolds and risers. Had an issue with Starboard engine, but looks like it was solved with new MAP sensor.

On a recent cruise, ran for about an hour at 3,200 rpm. Both engines running smoothly, then suddenly the Port engine 'stalls', rpm drops to 1,000rpm, no change to throttle. Bring both engines down to idle, port stalls. I'm able to restart, and fortunately was at the mouth of the harbor (Newport Beach, CA). Motored to slip at 5 kts, both engines seem to be running fine. Did several marina cruises with friends. Engines start and run fine at 5kts.

On way to home port (Huntington Harbor, CA), came up the 3,200 cruise, seemed to be fine. After 5 minutes, port engine begins 'stalling' again. doesn't quite die, but rpm drops to 1,000, again, throttle not changed.

This pattern continues, and I figured out that I could cruise at 2,000 rpm. At 2,500 rpm or above, the engine would start to stall. Made it to home port, no issues motoring to slip.

Come 4th July weekend. I switch Port engine to Aux fuel tank.

Had friends on boat Saturday. Port engine would not start. Sunday morning, port engine fires right up, and was able to do a marina cruise. Monday (July 4th). Port engine reluctant to start, but it does, and was able to do a marina cruise. Like day before, running normally at 5kts, smooth idle.

Following weekend, Port would not start. Had engine guy check it out, showing good fuel pressure, spark at the plugs, no error codes. He could not get the engine to start.

I installed a new fuel filter (primed with fuel), starts right up. Runs normally, and went for sea trial, and ran fine at WOT, cruise for about 20 minutes. Headed back into harbor, and did one last WOT run, and sure enough, stalling starts again. Seems like running out of gas at higher rpm, very smooth down low and at idle.

I think fuel starvation, it feels like it's running out of gas at higher rpm. I think I need to get my engine guy with the Diacom out on the water, and see what's happening when it starts to stall.

Anyone experienced similar problems?

Thanks!

[bud37]

Before you spend a pile on this , make sure the anti syphon valve is clear, these things get partially plugged and cause symptoms like that.....should be a fitting in the fuel line by the tank ...simple first...

[Wkuhilltopperfan]

I'm guessing you looked over all your gauges and verified everything was at its normal operation? I've not experienced these issue with mine but I've heard people talk about fuel vapor lock during extreme temperatures. Make sure no fuel lines are laying against the manifold ect. I rarely run my engines above 2000 rpm for two reasons 1 fuel consumption 2 I feel like I'm stressing the boat (probably not). You said you was at WOT for around a hour? I'm guessing that has to be stressful on the engines. These boats have tons of safety features to protect and it's always possible a sensor could be going bad but I don't know how you test for that. If all your gauges was normal.....I would be looking at vapor lock as a possibility

[Viper]

I agree with checking the anti-siphon valves. They should be right on top of the tank where your fuel line is connected to. It'll just look like a fitting.

You should also check to ensure your tank vent isn't clogged with a spider's nest or something. Follow the vent hose from the tank and make sure it is going up hill all the way and that there is no low area that will trap fuel in it and slow down venting. You can confirm if it's a venting issue by simply opening up the fuel filler cap when the engine starts to act up.

When you replaced the fuel filter, was the filter only half full when you took it off?

When the engine stalls, does it just quit without warning or does it run rough, cough, etc. before it quits?

Did the fuel pressure test get done out on the water under load at high rpm or just in the slip?

[jmagrann]

Thanks for the feedback. Additional information, based on questions:

Temperature, Oil Pressure all withing normal range throughout.

Fuel filter was about 1/2 full of fuel. I poured into a clear jar and no sign of water.

Engine would start to stall suddenly. Running smoothly, then sudden drop to 200-300 rpm.

Fuel pressure test was done in the slip, not under load.

I'll check the anti-siphon valves and vents, but note that during the last testing, I had both port and starboard engines running off of the same Aux tank. Starboard engine ran fine throughout.

I only ran at WOT for 10-15 seconds each time. The 1-hour cruise was at 3,200, which was the 'sweet spot' according to the prior owner.

I'll do some additional research, but may go for swapping the fuel pump. Looks like $250 or so. I have a GEN VI-L29 (serial #0L004074), so don't have the low pressure pump, near as I can tell.

I appreciate any additional throughts.

Thanks!

[bud37]

Source of the post I'll check the anti-siphon valves and vents, but note that during the last testing, I had both port and starboard engines running off of the same Aux tank. Starboard engine ran fine throughout

Sometimes with both off the same tank one could get full fuel flow on one while the other would be restricted, especially if the fuel flow is somewhat compromised somewhere ....had this happen to me with a bad fitting,and valve leaking air into the fuel system. Just cross things off one at a time. When you put your filter back on ,fill it first maybe some air lock.

Anything is possible on a boat near as I can tell

[jmagrann]

Solved - It was the coil.

I was sure it was a fuel problem, so to confirm, I sprayed some starting fluid into the intake while cranking, and nothing. I had a brand new coil my mechanic lent me, installed it and engine fired right up.

I ran the boat for 30-minutes, at cruise with a couple of WOT runs for a few seconds, and no hiccups or stumbles.

Thanks for the feedback!

[Wkuhilltopperfan]

Good stuff! Really glad you got it fixed