2001 Mercury 250 EFI Instantly Cuts In and Out

[StingerFan 314]

Geeetings!
I hope everyone had a fun and safe July 4th!

I bought an old Chris Craft Stinger 314 from a friend that had been sitting in a barn for several years.
The engines are 2001 (I’m told) 250 EFI 2-stroke Saltwater series.
I looked for a serial number and cannot find the original one. The transom bracket engine data tag is missing from both engines and it seems both powerheads have been replaced as well. The serial number on the troublesome power head is Quicksilver OSGW00803051. I hope this helps…

The issue started when under load, not necessarily WOT, but with the throttles up a ways, the port engine will suddenly and instantly cut waaaaay back on rpm. The tach bounces around dramatically, but the engine never dies. If I throttle back to somewhere around 2500-3000 it runs fine. No buzzer ever goes off. (I know it works, it goes off when I turn the key on.)
I have determined that *this only occurs when encountering bumps* on the water. If the water is glassy smooth it runs perfect, but as soon as the boat hits some smallish bumps, like wake from another boat, the cutting out happens instantly. I never left the throttles open, I figured it can’t be too good on it to let it keep doing that. I was able to repeat this scenario several times, so I’m fairly confident in this failure mode.

Heres what I’ve done so far:
The previous owner disabled the oil injection, it’s premix at 50:1
Fuel lines and bulbs replaced from tank to engine
Filters replaced in boat and on engine
New flywheel
New stator
Rebuilt diaphragm fuel pump
Cleaned vst/fsm
(all the above was performed this year before I ever put it in the water)
Then after experiencing the engine cutting out:
Swapped Fuel ECU between engines
Swapped Ignition ECU between engines
Inspected every ground I can find
Disconnect and reconnect all the wiring harness connectors I could find, thinking maybe there was a corroded pin or poor connection somewhere.
The problem persists on the port engine. And the cutting out is instant, like a switch. Not slowly losing power like running out of gas.

One thing sticks out to me, and that is when I unhooked and reconnected all kinds of plugs on the wiring harness, it ran great for an hour or so. I thought I fixed it!!
Then, I turned the boat around in a very tight circle, turning the helm all the way over to turn the boat slowly around.
The cutting out came back immediately (like literally a few seconds) after turning around like this. (Just as I was getting back to speed.)
Could I have pulled some wiring turning so sharp? If so, what would be pulled that could cause this?

I’m Stumped as of right now.
Thanks in advance!
Eric

[lpugh]

Check the cannon plug and spread the male pins with an exacto knife for starters
Voltage drop test all power and ground ckts while problem is occurring
Wiggle test the harness that goes thru the rigging tube

[StingerFan 314]

Thanks Leon,

Funny you said that about spreading the pins out, after work yesterday I went over to the marina and had another look at that motor and did just exactly that.
I didn't get a chance to try it out yet though.
It's gonna be tough to check the voltage drops while the problem is happening, I need to be under power and bumpy water.

I did think about putting it on a trailer and backing it down a ramp, starting the engine and opening the throttle. Then with it trying to push my truck up the ramp, I can jiggle the harness around and see if I can recreate the problem that way.

Thanks Again!
Eric

[lpugh]

Good luck, find a lot of problems with wiggle testing

[StingerFan 314]

It’s Resolved!!

After slightly spreading the male pins out on the 8-pin connector, and re-securing the harness that runs from that connector up into the boat it’s been running good!
The harness wants to slowly pull itself back into the boat and eventually pull on the connector. Several zip ties are holding it in place better now and it hasn’t moved an inch.

(Hopefully I didn’t just jinx it… lol!)

Thanks again all!

[lpugh]