Engine dies suddenly after a few minutes & won't restart

[superbenk]

Last year I had been running the boat (2008 Four Winns H200 w/ 5.0L Mercruiser TKS) for several days at the lake without issue. Dropped off some family at the dock & started reversing out of the hoist when it just died suddenly & would not restart. I couldn't get it to restart the rest of the day so went into town & bought a replacement cap, rotor & ignition module. Replaced all those things & it still wouldn't start. Ended up having to get a tow to the ramp after 2 full days fiddling with as much as I could while in the hoist.

Brought boat home & tried it again in the driveway where it started right up! Let it sit running for awhile until it warmed up to temp at which point it suddenly died again. Couldn't get it to start again that day. Replaced the coil (I know, it's never the coil, right?) but that didn't help. Old coil *SEEMS* to test fine with a multimeter so put it back on but still won't start.

Now we're into the next year's season & I'm looking at it again. Started again, ran great until it warmed up & then suddenly died again. Same thing happened a couple days later (today) but this time it started a second time a few minutes after dying & ran for another 2-3 minutes before dying again. This last time I had a multimeter on the coil purple wire & ground to the engine. I saw ~13.5v while it was running & then it dropped to 0 for a second when it died then showed 13.5v again. Looks a lot like a short somewhere but I can't figure out where!

It's not sputtering & dying like a fuel starvation situation so I don't think it's fuel. I tried disconnecting the fuel pump while it was running & it eventually died a different way with that (sputtered & quit rather than a sudden quit). So far I can't find any shorts or obvious electrical problems & I'm at a loss! Any ideas?

 

[cyclops222]

That 0 volts is critical to me. I would be after a loose connection.
Is the starter motor cranking very strongly all the time with no engine sputters ?
I would check that fuel is being getting into the cylinders. If yes ? Your o volts is most important.
Gently pull on each low voltage wine connection from the coil terminal on back to see if you find a touchy connection. Do this with boat on the trailer. Back the boat in deep enough on the ramp. To not burn up the water pump.

 

[superbenk]

That 0 volts is critical to me. I would be after a loose connection.
Is the starter motor cranking very strongly all the time with no engine sputters ?
I would check that fuel is being getting into the cylinders. If yes ? Your o volts is most important.
Gently pull on each low voltage wine connection from the coil terminal on back to see if you find a touchy connection. Do this with boat on the trailer. Back the boat in deep enough on the ramp. To not burn up the water pump.

Yes, I'm definitely running it with muffs.

Cranks fine for awhile then clicks & catches but I assume that's just the battery getting run down. If I stop & give it a minute on the charger & try again it cranks fine again. But I could be wrong, maybe something is just a little loose somewhere.

I'm wondering about the remote control neutral switch and/or safety kill switch but I'm not sure how to remove the control lever to get inside it. Looking that up now.

I also wonder about the 50amp main breaker but it seems fine from what I can tell so far. I'll do more testing.

 

[cyclops222]

I like your logic on the safety switches. Boat is old enough for them to get erratic. With engine running. GENTILY tap the switches from different angles for the problem to occur.

 

[superbenk]

Pulled kill switch & remote control wires & checked them at the connectors - all seems fine. Got the engine running & shook/tugged on all the low-voltage wires/connections I could reach without any effect.

I noticed that the breaker at the ignition switch seems to be tripping (I think). I replaced the breaker with a 15A fuse & it blew pretty quickly. Put a 20A in & it didn't blow but I think it was getting warm. I'm not sure what that's about. The 50A breaker at the engine is not popping. I don't think the 30A helm power breaker is tripping either (also not sure that would affect the engine running).

Connections at the starter & alternator *SEEM* ok but I'm still tracing wires.

 

[alldodge]

We all know no volts on coil it's going to die
I would look closer at the key switch

 

[superbenk]

Ignition breaker was a false positive. Must have just been a weak fuse. Engine died without tripping any breakers.

Starting to second guess dismissing fuel issues now. Maybe too many years of ethanol has an idle jet clogged & when it idles down after warming up it dies. I’ll have to go through that stuff another day. Probably clean/rebuild carb, check float, carb inlet filter, anti-siphon valve & new water separator.

I’m at a loss. Anyone wanna buy a boat?

 

[cyclops222]

My old brain just remembered a sneaky source of erratic shutdowns and engine cranking but no ignition running.
Engine was a 2002 Mercruiser 5.0L 220hp V8. I found out by trial and error.
That the engine would continue to run as long as I held the key in the full STARTER position. I then quickly and slightly moved the throttle upto 1000 rpms and back to idle throttle. Worked every time.
I told my mechanic what was wrong & how I cold cure the problem. He smiled and said I had a very common engine problem. We went to the boat and he pulled apart the biggest engine electrical plug. Then turned off the battery main switch. Pulled out his pocket knife. set it with the smallest blade. The proceeded to ...... GENTILY ... spread each of those SPLIT PIN gaps a few thousandths WIDER. Reconnected the big plug. Engine restarted several times in a row. It has never had another running problem. With age and vibration The split pins start to drift closed.

Print this description for your mechanic to read and maybe Your engine plug is developing the same problem. Get lucky

 

[cyclops222]

I changed drift apart...............to drift CLOSED. Looser fit
Do not get really old.

 

[superbenk]

How do they drift closed? That doesn't make sense. I could see the act of pulling the plug apart & putting it back together again dislodging some corrosion but I'm skeptical about adding more clearance. Maybe spreading them scraped away some corrosion as well & that was what actually fixed it?

I'll take a look at those larger connectors though on the engine harness & see how they look. I have some electrical cleaner I can use on them as well.
The ignition switch itself may be another good point. Maybe something in there is intermittently shorting/breaking connection in a really weird way.

 

[Chillythe1]

Those tks motors had a bit of problems and no offense to you at all but was a poor idea from Mercury. We had quite a bit of them have wrong float height and drop from the factory. They also seem to foul spark plugs like crazy. In your case though do you have a resetable breaker on top of the intake next to or behind the carburetor that has a yellow "flag?" Has a little red button that flips the yellow flag downward when tripped and you push upward to reset? Those were known to have issues making contact. What happens is from you out bouncing on the water it's enough at times to move downward far enough to break contact even though it's not tripped. An on the water fix or temporary solution is to take and flip that breaker upside down so you'd have to push downward to reset it .....but that keeps contacts closed internally. I also had one years ago that would run for hours but then all of a sudden die and I mean like 6-8 hours. That I believe was the tks diode. Both can be checked with a meter, with the diode having a certain resistance range that I cannot remember off the top of my head.

 

[superbenk]

I’ve had this boat for about 10 years now & it’s been nearly perfect up until now. It’s never had any problems starting or running since I’ve had it.

I’ve checked that breaker many times but that’s not tripping. I’ve tested it as well & it works as expected.

I did read about the TKS diode so I need to check that as well. I haven’t done that yet.

 

[superbenk]

Pretty sure I found the problem. That TKS diode seems to have failed. I hacked together a fix but this one blew too (IN5404 diode - 3A 400V).

 

[superbenk]

It's the coolant sensor shorting that's blowing the diode & choking the engine. Time to order a new sensor!

 

[alldodge]

It does not appear you understand a diode or TKS system

Here is a quick pic to help

 

[superbenk]

It does not appear you understand a diode or TKS system

Can you give me more info about what you're trying to say? What am I missing? The diode is being burned up (I can physically see it crack & smoke as well as test it has failed). It looks like when that happens the TKS module floods the engine stalling it.

If I unplug the coolant sensor while the engine is running but before it stalls it continues to run & work as expected. Pretty sure I've narrowed it down to the sensor but I'm happy to be corrected.

 

[alldodge]

If I unplug the coolant sensor while the engine is running but before it stalls it continues to run & work as expected. Pretty sure I've narrowed it down to the sensor but I'm happy to be corrected.

The Temp switch is shorted out
When running excess current is drawn through the diode to the Temp switch causing the diode to heat up.

You should be able to start and stop the motor with the Temp switch disconnected. The Temp switch is there to keep power on the TKS module when motor is OFF so long as motor is warm. When it goes cold it disconnects

 

[superbenk]

The Temp switch is shorted out
When running excess current is drawn through the diode to the Temp switch causing the diode to heat up.

You should be able to start and stop the motor with the Temp switch disconnected. The Temp switch is there to keep power on the TKS module when motor is OFF so long as motor is warm. When it goes cold it disconnects

I think we’re saying the same things.

 

[superbenk]

Just to close the loop on this, I got the new coolant temp sensor & put it in yesterday. No more blown diode & no more stalling! Certainly looks like that fixed the problem.