Merc 350 Mag MPI with intermittent electrical issue

[twgroom]

Have had this issue happen last 3 times out and would like some recommendations on approach to find root issue. Motor starts up and runs great... ride for a while to anchor spot or other dock.... .hang out for 1 or more hours... then issue pops up... when starting starter motor turns motor over... it will always have the initial "start"... plugs fire get momentary (about the length of the time the key is turn over) power .... key goes to on position and then one of 2 things happens... it will continue running very rough with voltage gauge jumping around or hovering around 12 v with barely any power or it just plain will not continue to run at all.... in all cases so far waiting for some more time after attempting to start it "comes back" voltage meter is steady at 13.8... starts and runs with no issues again to get me back. So far I replaced the connectors from the alternator as the + one was corroded (1st trip where I thought that was issue) and looked like it could be intermittently connecting.. I replaced the alternator as the old one was pushing out 14.9 v and figured the voltage regulator was near its end. (2nd trip where I tested) I still have the issue after these 2 changes. Some other thoughts... when it does not start after riding somewhere and sitting for a while I have the engine cover open and it is possibly cooling some portion of the electrical system that once it gets back down to some temp it works... wiggling the harness on the engine and wiring to the switch don't seem to change the predicament. Given that it runs well until it doesn't it seems like a may need to replace some or all of the wires involved in supplying current to the ignition.... I am hoping one of you has seen something like this and could point me to better diagnostics or ideas on where to start to trace this down.

 

[Bt Doctur]

Start at the beginning where the power comes from, the main battery connection both pos and neg at the battery and at the engine

 

[alldodge]

It only happens after a good run and sits for a while, and my guess is its hot outside

Vapor Lock

 

[twgroom]

Thanks for the suggestion AllDodge...since I definitely had an issue with the primary + connection I have been thinking electrical but had 0 volts and nothing... dead engine... prior to that fix. Voltage meter fluctuation could be just reporting the poor running engine that doesn't spin the alt up fast enough to get full voltage... I will try to recreate issue without actually taking it out and having to sit somewhere with my engine compartment open... I will confirm if I have spark when it will not start. Do you recommend some vapor lock tests and if an issue corrections?

 

[Scott Danforth]

I would start at the battery and clean every connection to the motor and back, then clean every connection from the big circuit breaker to the helm and back.

 

[kenny nunez]

You said that you shook the harness going to the engine, try removing the large plug and gently spreading the pins, this may not be the problem but it eliminates the possibility. Look at all the grounds under the instrument panel.

 

[dubs283]

Check under dist cap, if its the flat "crab" style there is most likely corrosion on the contacts, very common issue with those caps

Also your description sounds like the IAC valve may be having issues, a very common occurrence on MPI engines. They can actually have issues without triggering a fault code.

You can remove it and check for debris build-up located inside the valve assy, clean with brake cleaning solvent and re install. Careful with the gasket they tend to rip when removing original IAC assy

I only mention these two things as you say this happens when the engine is warm after a decent run, a couple causes of electrical issues: poor connections, (e.g. corrosion) and heat

 

[alldodge]

Thanks for the suggestion AllDodge...since I definitely had an issue with the primary + connection I have been thinking electrical but had 0 volts and nothing... dead engine... prior to that fix. Voltage meter fluctuation could be just reporting the poor running engine that doesn't spin the alt up fast enough to get full voltage... I will try to recreate issue without actually taking it out and having to sit somewhere with my engine compartment open... I will confirm if I have spark when it will not start. Do you recommend some vapor lock tests and if an issue corrections?

While it could very well be electrical, most of that stuff doesn't come and go so often. Guess I need more of what happens and when so far as what was happening prior, and does it ever do anything different once the motor is running.

Does it only happen when its hot

and agree do check that dizzy cap

 

[tpenfield]

Do you have the engine serial number handy? I’m thinking the issue has to do with how the ignition timing is handled by the ECM in ‘start’ mode vs. ‘running’ mode, but knowing the serial number would help chase that.

 

[twgroom]

The serial number is 0M348..... I took it out again yesterday and ran it hard around the lake and came back... waited 45 min and then started it... ran perfect.... going on AllDodge suggestion that the issue could be vapor lock... I am thinking that I have been tunnel vision on electric issue since I definitively had an issue with the bad connection... When I was stuck at the dock last time before I could get it started again I filled it up with premium gas.... In retrospect the gas that was in there is what has been in there most of the winter... (generally use the boat year round but did not go out as much so this was the remaining 1/8 of a tank)... never experienced vapor lock but looking online it appears that old winter fuel could contribute to the issue.... I will need to run this thing several more times without issue until I trust that vapor lock was the issue... but if I can't get it running again after running for a while I will at least do some diagnoses on that (check spark... cool the fuel pump/line)

 

[scoflaw]

Advancing the throttle while cranking will start a vapor locked engine

 

[alldodge]

Advancing the throttle while cranking will start a vapor locked engine

I disagree

 

[scoflaw]

works on my 502 in my sig and my OX66 yammi on hot days. Both fuel injected. Try it

 

[alldodge]

Glad it worked for ya, have not heard it working that way for anyone else.

Never worked on my 502 MPI, happened twice. Fixed most of it by using only non-E fuel, and permanent fix during repower with fuel line back to tank

 

[scoflaw]

i'm assuming it's heat soak. Runs perfect, shut it of for 5 or 10 min. starts right up. Let it sit for 40 minutes and that's when it might act up. Switched gas brands on the OX 2 stroke to top tier gas and that helped considerably. In Florida top tier is the same price as no name 7-11 gas. When I get back north I will try that on the 502, at a substantial price increase. Both of those motors will not start in that condition unless I open the throttle