Engine symptom general rule of thumb?

[juicegoose]

Just as a general rule of thumb would random engine stumbling or complete dying suddenly at idle typically point to a fuel delivery problem over a spark or ignition problem?

[Savage]

No, could be either area.

[juicegoose]

Well crap.
Okay then here we go
Serial 1B505108
While out on the boat this past weekend and for the last couple years the boat has been running great. Came to a fishing spot and idled across a section for about 5 minutes and suddenly noticed the engine dropping rpms and then it died. It acted like it had lost fuel flow. Cranked back up after a quick pump of the bulb and restart.
After leaving that spot and while traveling around 3500rpm She started stumbling and dropping rpm. Again as if it was loosing fuel and then catching back up. I came out of the throttle and idled for a bit. Bulb was firm so i got back on plane and it didn't do it again for a 10 minute boat ride. Then while coming home it randomly stumbled again. Didn't loose it 100% but stumbled pretty good. Picked back up and continued on with 1 other incident.
I got home and ran the boat at idle. Ran fine with no hesitation or rpm drop for a solid 10-15 minutes then it killed suddenly. No real rpm drop this time. When I turned the key to on position i didn't hear the high pressure pump prime the engine. I then turned boat off and on a couple times and finally heard the high pressure pump prime up. Started the engine and all seemed to be idling fine. Started down the canal for a test run and after about 5 minutes at 800 rpm she started acting like she was loosing fuel again and died. I cranked her back up and she started but almost immediately began acting like she was starving again and died. I used my trolling motor to get back and tried to get her to replicate but she wouldn't kill over. I did notice a slightly loose plug wire but nothing else was out of the ordinary. Ran out for a test and everything seemed fine she ran for probably 20 minutes without an issue. Stopped and fished and then on the way home she started stumbling and catching back up again.
I went ahead and ordered some new plugs and water filter because i hadn't done that since last year.

Question though. Should I hear the high pressure pump prime up every time the key is set to on position or will it not activate if the system has been primed.

Thanks,

[EuropeanAM]

You should be hearing the fuel pump for approximately 1-2 seconds at each key-up. Check your battery, battery connections, keyswitch harness connections in the cowling, lanyard and keyswitch wiring harness connections, and perhaps the MPR (Main Power Relay).

[juicegoose]

Thanks Don. It is the high pressure pump i should hear correct? I checked voltage to the pump on startup and was getting 12v and change at the high pressure pump harness plug. I also checked it while running and even when the engine was sputtered while on plan I still saw 14volts.
But I can for sure tell you that when the engine died while idling and i turned the key to the start position i got voltage but didn't get a pump sound for a couple attempts. Once it did start making a sound it was rather loud and with a funny pitch.

[EuropeanAM]

Those are symptoms of two separate problems- One electrical (harness connections, MPR, battery voltage, etc) and the other possibly a fuel delivery problem.

Deal with them separately, but they are equally important.

[juicegoose]

Sorry Don i don't follow the electrical aspect of the problem. In testing the pump I'm not seeing a voltage issue. The pump receives power upon key on and the motor operates. When the engine is running there is constant power to the pump even when the motor tried to stumble. I haven't noticed that the voltage drops on the pump at any time to cause the pump to not work.

[EuropeanAM]

There are two pumps on that power circuit- so if you really DO have power and ground, but had no pump activity, you could potentially have two bad pumps (or a harness problem in the VST).

Better triple-check those results.

[juicegoose]

I was checking the voltage at only the high pressure pump.

[EuropeanAM]

May I ask "how" you accomplished that, given that the harness has a "Y" that branches to BOTH pumps, INSIDE the VST?

[juicegoose]

Ah remember Don I have the frankenstein vst system. I have the 2002 version vst with low pressure pump on the exterior of the pump. So in my case I'm able to check both pump individually. I am correct in that if i don't hear the high pitch whine upon key on then that is only the high pressure pump right?

[EuropeanAM]

No- you should be able to audibly hear both pumps. And with that earlier style VST, you can isolate one from the other when testing.

[juicegoose]

Well After a long weekend on the water I think i finally once and for all figured out the stinkin problem. Weekend started with me taking the VST apart to ensure no blockage on the high pressure pump screen. Everything was clean as a whistle. I cleaned everything, replaced the water separator and replaced the sparkplugs. Upon reconnection of the fuel pump connectors I happen to notice one had a little play but testing on the muffs didn't cause any problem. Placed gauge set on the fuel and air gave me almost perfect 90-80 split. Ran the boat like a mad man and she ran like a top. Next morning we loaded up to fish. Idled out of the marina and upon throttle up the stupid stumble occured. Went back to the ramp put gauges on and went again. This time i didn't have 1 issue for at least a 30 minute boat ride. On the ride back i had one stumble and knew it had to be something electrical at this point. While idling back in i went straight to that connector i noted as kinda loose and sure enough if you jiggled the wires just enough it would kick out the low pressure pump and the engine would starve. When i got home i took a good look at the connector and the female side pins over the years look to have lost their angle and weren't making great contact. I took the connector apart and adjusted the connection point out and never had another issue all weekend.
As Don would say. Start with the simple things and work up the line. In my case the connector on the low pressure pump seemed to be the culprit and when jiggling wires etc it was hard to diagnose because if the low pressure pump cuts out for a second while you jiggle the wires the engine won't immediately kill over.

On a secondary side note I noticed while connecting my gauge set that the cover for the fuel schrader valve had blown its top. Upon looking at the valve i was getting some fuel spray out of the valve which no doubt couldn't have helped anything. Quick turn with a valve tool and I tightened it back down with no leaks and replaced the caps with sealing high temp metallic covers.