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Merc 5.7 randomly dies

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Nickzlb77

New member
Hello All,

Recently installed new merc 5.7 carb, alpha with HEI ignition and 4 barrel 1409. Vortec motor.

Boat is an 02 Bayliner 2252 I rescued from the scrap yard. Brand new drive as well.

once dialed in and broken motor seems to run great (although I think I'm over propped at the moment. She only sees 4200 and 34 mph.

Although I'm not ruling out carb or fuel issues, every now and then she just does as if the kill switch was hit. No sputter no stumble no backfire. I believe it's electric due to the way gauges bottom out momentarily.

Side note it was doing this regularly until I completely bypassed the facory engine harness plug. Must have had a short or poor connections. Everything on the engine side of the harness has now been replaced at this point with the exception of the main ground splice within the factory engine harness.. Next attempt will be to run new purple wire to helm, and new main power feed.

Although I may be answering my own questions here would a bad fuel/water separator be a cause of these symptoms? I would expect it to behave differently if so.

With the HEI setup, do they cut out completely at a specific voltage?
 
Pull the filter, pour contents into a clear container,let sit a few hours and see if there's separation of fluids. Keep checking grounds,just because they look nice means nothing,pull them off and sand again! Internal corrosion can not be seen and testing for continuity sometimes does not show problems.Running new grounds to fuse box can't hurt,ditto on positive feeds. Look for loose ignition connections at coil,distributor,wires on cap, plugs.Check inside of dist. cap,check rotor. possible ignition switch?
 
Ground connections at motor all replaced and sanded, but definitely need to split the harness and redo all connections at that splice. Coil and distributor wiring all new, plugs and plug wires good. Ignition switch connections definitely a good place to start.

For arguments sake if water was in gas would you expect to just completely stop dead sometimes even from WOT?

Thanks for the reply will replace the few remaining old connection mentioned and get back with results!
 
Usually a fuel related issue gradually happens, If it's electrical usually a dead stop like turning key off. Even though ignition parts are new check them anyway, Never assume there still good
 
Docksidemarineservices said:
Do not repair the harness, just replace it. The repaired harness will just fail again.
Click to expand...

By bypassed I meant cut it out, replaced basically every wire on the engine side of the harness and connected to the boat side with quick connects. Thinking the few straggler ground wires left behind may be the issue or the feed and ground heading up to the helm. Going to pull a new bundle and just get rid of all of it. The idea of my new motor and drive somehow being damaged by this old wiring drives
me insane. Thanks!
 
Fuel issues won't zero your gauges.

Re: "quick connects"... do not use. Use insulated heat shrink marine butt splices IF you HAVE to connect to an existing wire. The last time I was involved in a problem like this, I replaced ALL of the boat's wiring using marine grade wire and marine grade heat shrink splices and ring terminals. The boat I did this to has been running weekly for the last 6 years without electrical problems from June to Sept as a race committee boat for sail boat races. It got washed off its blocks, moved 100 ft and swamped by Hurricane Sandy ( boat lives 1 mile north of "Ground Zero" for that storm).
 
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Same here, long beach NY! Something I've been trying to forget. Sorry to hear. Hope the boat issues weren't the least of your worries and your life wasn't washed away as well as mine was.

I only would use marine grade stuff. By quick connects I mean insulated heat shrink connectors followed up with a large burial grade heat shrink tube... I'd like to be able to disconnect the engine harness completely if ever needed. Would require splitting all of the tubing but not as bad as cutting a bunch of butt connectors. Thanks for the advice. Kind of what I felt with the nature of the stalling. It seems to be a brief short or loss of power. I'm in the electrical engineering field so familiar with circuitry and wiring practices.
 
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