5.7 merc voltage spike

[waybuc]

I have 1989 boat with 5.7 260 hp V8 merc ThunderboltIV ignigtion alpha 1 drive
The engine was rebuilt 100hours ago. Runs great for about 1 hour then lost power, can drive at idle for a while then the volt meter starts jumping and motor stops. Will start right back up but can't go faster than idle without powerloss misfire, If let cool for an hour will run fine.
Tried fuel filter, new rotor & cap begining of season, Replaced ignition coil and wire. Side note temp gauge was jumping now pinned full hot thought it was the gauge, tested it yesterday and its fine, bad sensor?? I don't think the two problems are related because the engine is raw water cooled. Looking for ideas. Thank you

 

[Haut Medoc]

Re: 5.7 merc voltage spike

Welcome to iboats.....
Sounds like an electrical problem.....
I'll bet your temp sensor is fine.....
I would start looking at all of the wiring on the engine harness for signs of chafing, especially around the alternator & igniton amplifier, but you will need to inspect the whole harness.......
Check the cannon plug for signs of moisture.....​

 

[Bondo]

Re: 5.7 merc voltage spike

Ayuh,.........

Bad Ground or a Cannon Plug problems were My 1st thoughts...........

 

[waybuc]

Re: 5.7 merc voltage spike

Cannon plug looked clean spayed it with CRQ Temp guage worked then started to jump, misfire above 1200rpm, finnaly died when voltage started to jump. Not sure were to look for a bad ground. Could it be a silly pvc valve? Thanks

 

[Bondo]

Re: 5.7 merc voltage spike

Could it be a silly pvc valve?

Nope......... It's Got to be Electrical........

This shoud be the Service Manual for Your motor,.........