Merc 350 sputters, dies, won't restart after heating up

[Longhorn-Boater]

Chasing an issue on a Mercruiser 350 inboard motor. Starts up fine, idles smooth, runs at speed and under load (e.g. pulling a wakeboarder or skier), then if speed is reduced or brought back to idle speed, it will begin to run rough, sputter and die. Starter will crank, but engine will not restart. The first couple of times, if it sat for 20-30 minutes, then it would restart and run (during which time it would be driven straight back to dock).

Based on the handful of occurrences, it seems to be getting worse; by worse I mean it starts fine but begins to run rough & then die sooner after launch. Also seems not to "recover" following a period of "rest" like it did previously. On the last outing, it would not restart, even after an hour. Towed back to dock and am now very motivated to solve this mysterious problem.

Based upon the sputtering fashion in which the engine dies, my first thought was that it was starving for either fuel or air. But the fact that it starts up upon first launch and then starts again after cooling down made me think perhaps it was just fuel delivery once hot, with the fuel pump changing it's operation once the engine heated up. After some research, I think it may be electrical - while still being adversely affected by temperature.

Engine details:

• Mercruiser direct-drive model: MIE 350 MAG MPI SKI
• Serial: 0L393706
• Motor is circa year 2000 and has ~900 hours.

Steps taken since the behavior first occurred:

1. Replaced fuel filter (fuel/water separator)
2. Compression check - good
3. Replaced distributor cap, rotor, and spark plugs - one cracked plug was noticed when the old ones were pulled, but behavior was consistent after this set of maintenance replacement activities.
4. Thoughts on possible root cause:
   1. Fuel tank filter sock - trapping sediment during extended operation, then clearing while the boat floats and sloshes back and forth?
   2. Bad ignition coil - operating differently once hot?
   3. Crankshaft position sensor - faulty? More finicky at temp?

The challenge has been performing any diagnosis when it happens, because it has happened 20-30 minutes after starting a day of watersports, so we're usually with a boatload of passengers in and/or behind the boat and the focus shifts to just getting back to the dock. Hopefully now that it happened sooner, it will mean I can go on a test run and bring some tools to check / diagnose a few details. First order of business is to get a new, working thermostat in so that operator can watch that gauge and read accurate temperatures. Next I'd like to try to troubleshoot the ignition coil. I'm open to other hypotheses and/or troubleshooting suggestions.

tags: Mercruiser, 5.7, inboard, stall, no-start, temperature problem

 

[achris]

TL;DR... (just the summary at the end)

But.... 1999 engine not 2000.
This sounds very much like a blocked fuel tank vent..

Chris

 

[Longhorn-Boater]

Thanks, @achris. Appreciate the quick reply. Yes, the engine is on a year 2000 inboard boat, so date of manufacture may have been '98-99. Was just trying to give an indication of age & hours when I said 'circa year 2000'.

I'll do some searching on 'blocked fuel tank vent'

 

[Scott06]

Thanks, @achris. Appreciate the quick reply. Yes, the engine is on a year 2000 inboard boat, so date of manufacture may have been '98-99. Was just trying to give an indication of age & hours when I said 'circa year 2000'.

I'll do some searching on 'blocked fuel tank vent'

Agree on fuel vent when it happens open the gas cap see if the affects it

 

[Drcoffee]

Open the fuel cap as its sputtering and see if it changes how it runs.

 

[dubs283]

1. Never seen a fuel tank "sock" but most tank pickups have a filter/screen which could be clogged/clogging

2. Not likely

3. Possible, but usually cps works or doesn't. Test would be to check for rpm signal when cranking during no start.

One test, as mentioned about fuel tank vent would be to remove fill cap. If vent line is blocked this will relieve vacuum and allow pump to draw fuel.

Have you checked fuel pump pressure and vacuum?

 

[Bt Doctur]

Remove the fuel pickup tube from the tank, unscrew the tube from the 90deg fitting , remove the stent filter

 

[Scott Danforth]

Define "heating up"

Check the dizzy trigger assembly. See if you have spark

 

[ScottinAZ]

Define "heating up"

Check the dizzy trigger assembly. See if you have spark

wouldnt be the first or last time that a pickup went wonky when hot, and then took an extended break before it worked again..... at the end of the day, its still a Chevy... starter solenoids and dizzy pickups dont like heat as they age

 

[todhunter]

1. Never seen a fuel tank "sock"

Common in import automotive. Same thing...just often times resembles a sock, so that's what it gets called.

 

[dubs283]

Common in import automotive.

Cool

But we're not talking automotive here. This is a boating forum page concerning marine engine issues/repair and responses should be focused accordingly

 

[Bt Doctur]

plug in a outboard tank and retest.

 

[Longhorn-Boater]

wouldnt be the first or last time that a pickup went wonky when hot, and then took an extended break before it worked again..... at the end of the day, its still a Chevy... starter solenoids and dizzy pickups dont like heat as they age

dizzy trigger, dizzy pickup - some of the terminology may make other readers dizzy as they try to follow along.
I think y'all are referring to the distributor pickup, known by Mercruiser as
part # 87-892150Q02 SENSOR ASSEMBLY. It goes for about $100 and apparently there is an updated 3-wire version, which superseded the original 2-wire part # 861780A2 in my Merc 350.

It's #11 in the diagram below. I plan to swap that and if that doesn't resolve it I'll swap the coil & report back with updates.

 

[dubs283]

at the end of the day, its still a Chevy...

There's nothing "Chevy" about a thunderbolt ignition system. It's all mercruiser

OP: you are on the right track with replacing the ignition sensor in the distributor, it's the most fragile component of the system. Make sure you get oem and ground to the same flywheel stud as the main battery ground lead

 

[Scott Danforth]

Item 11. If you do not have the new and updated trigger assembly. Get it

 

[todhunter]

Cool

But we're not talking automotive here. This is a boating forum page concerning marine engine issues/repair and responses should be focused accordingly

Cool. I'm just saying it's a thing since you had never heard of it. We're not all marine mechanics with decades of experience who know all the marine lingo yet...many of us who are willing to tinker on our boat engines have automotive backgrounds, so we use the terms we know, and are completely understanding when corrected and will use the correct terms going forward. I'm not trying to change anyone's mind and say that we should all call it a sock...just explaining where OP is coming from.

Appreciate all the input from the marine gods who are patient enough to deal with us newbies.

 

[ScottinAZ]

Cool. I'm just saying it's a thing since you had never heard of it. We're not all marine mechanics with decades of experience who know all the marine lingo yet...many of us who are willing to tinker on our boat engines have automotive backgrounds, so we use the terms we know, and are completely understanding when corrected and will use the correct terms going forward. I'm not trying to change anyone's mind and say that we should all call it a sock...just explaining where OP is coming from.

Appreciate all the input from the marine gods who are patient enough to deal with us newbies.

my background is in automotive. Any in tank filter on the end of the pickup has been called a sock for as long as I can remember (better than 30 years at least). Most of us also realize that these engines have been "modified" with marine equipment, but their parentage is undeniable. While its not a good idea, you COULD take a small block 350 right out of a car, and drop it right into the hull of any of our boats, and call it a day. It very likely has been done many times before, and will be until the end of time and the engine will run just fine (for a while at least, until the head gaskets and freeze plugs rot out, and you get tired of the cam that doesnt produce power where you need it....We wont start on spark issues with the accessories). People are generally cheap. the basic premise is that the failure points remain largely the same, with Chevys, the starter solenoids still suck (and are still in a crap location), the distributor pickups on ALL BRANDS are a fairly safe failure point, and this is just two of the "common" automotive failures that do carry over to the boating world. Just because its no longer Dad's Chevelle and is now in a fiberglass hull doesnt mean the knowledge is useless.

 

[Longhorn-Boater]

TLDR: Replaced faulty ignition coil.

Circling back to close the loop on this issue. Thanks for all the suggestions. The behavior worsened to the point of a no-start condition, even when cold. Upon closer inspection while testing to see whether it was getting spark, it was discovered that the ignition coil was arcing from the positive terminal along the outside of the unit.

Replaced the ignition coil first and decided to test before replacing the ignition sensor within the distributor assembly. Ran the boat for a 20+ minute water test while the engine was hooked up using diagnostic software, and so far everything looks good. Will get back out for a longer test day on the water and will do some towing to ensure the original problem doesn't reappear under prolonged load conditions. I plan to keep the new 3-wire version of the ignition sensor handy as a spare until needed.