Mystery stalling issue on 7.4 mercruiser

[BomoBoater]

The boat is a 1994 Formula 252ss w/ mercruiser 454 carbed (Rochester)b3 outdrive and 425 hours.

Occasionally the motor will stall. There is no sputtering, backfiring, or indication it's going to stall; it just dies as though I switched it off. This has happened at idle, chugging up the channel at 1100 rpm, at speed (3100rpm), and with a cold or hot motor. It's gone weeks without issue, but then happens at the least convenient moment
When it dies at higher rpms the motor quits for a brief moment then springs right back to life. When it dies at idle or low rpm, I can restart it but it will die within a second or two, or as soon as I try to throttle up (in gear). If I put it in neutral and bring the rpms up and rev it up and down a few times it will usually run fine immediately thereafter. I did notice the last time when it died, I could start it, it would die, then a quick turn of the key gave nothing (no starter, no audible click). A 5-10 second wait and the starter would engage properly.

Jiggling the throttle around does not stall the motor, jiggling the key has no effect.

Some history. I had a strange short the first couple weeks of this season that kept kicking off the ignition breaker. I traced that to a spark plug wire that was never replaced properly by my marina.
The boat starts with difficulty when cold and having sat for 4+ days. Starts easily within 48 hours of use, and when warm
Occasionally the oil pressure gauge gives no reading from start up. Upon the next restart of the motor it springs back to life and operates as normal.
There has never been a buzzer or alarm tripped under my ownership

New within the past year:
Plug wires
Oil/filter
Divorced choke thermostat.
Water pump impeller

Within the past month
Fuel water filter (no water in it, tiniest amount of sediment)
Spark plugs mr43t gapped at .035
Gas - ethanol free 91 treated with marine stabil

I have not yet checked the fuel filter at the carb.

Any thoughts on this? Thank you

 

[alldodge]

Being a 94 it probably has a TB4 ignition. The blowing of the breaker was an indication of an issue, not so sure a plug wire would do it. In any case you have a bad connection. Being able to turn the key and nothing happens is the sign there is an issue in this area.

Pull the 10 pin engine connector off and check the pins for being clean and tight. Then go to battery and cable connections on both ends, Remove them, clean to shinny metal and reinstall. Make sure to do both ends

 

[tpenfield]

I am thinking electrical as well. since the boat is 23 years old, maybe go through the wiring in the engine bay and at the helm and look for signs of corroded connections, etc. Also, check the kill switch. I have also seen situation where the tachometer becomes defective and kills the ignition. You could test for that by disconnecting the tachometer temporarily,

 

[BomoBoater]

The motor died under power today. Cruising about 3200 rpm and it just quit. I gave it a minute and tried cranking. It took a couple turns of they key, but it cranked each time and fired right up.
I did have the stereo cut out at anchor just before this and had an alarm go off. I immediately switched off battery "2" and got under way. I usually start and cruise under "all", and anchor with "2". I have ignorantly switched battery positions in the past with the motor running (never to off).
I checked the 10 pin plug and pins seemed clean (I still gave them a little sanding). Negative grounds to block all look uncorroded, as do all the battery terminals except the positive to #2. Looked under the helm and connections have no visible corrosion. It's a freshwater boat if that makes a difference.

I have a new dist cap, rotor, ignition module and coil aboard the boat. Could either of them be my issue? Would you recommend replacing them at this point?

 

[BomoBoater]

No Title

 

[BomoBoater]

No Title

 

[alldodge]

I have a new dist cap, rotor, ignition module and coil aboard the boat. Could either of them be my issue? Would you recommend replacing them at this point?

Need to remove both ends of the battery cables, this includes the end attached to the block and starter terminal. Also strongly suggest removing the battery switch so you can get at all the ends. Look under the insulation jacket of the cables. Cables can look good but have major corrosion under the jacket

There could be an issue with some of the parts, but for right now, No I don't see it. the reason behind it is because some times when it dies you turn the key and have nothing

 

[Scott Danforth]

next time its running, simply wiggle the key - could be ignition switch

wiring from the boat goes from the 10 pin connector to potentially 2 connectors in the harness between motor and the helm. check all the connections

clean all the battery connections. use a 3M scotchbright pad or a 3M scotchbright 2" roloc disc in the die grinder to clean everything super shiny.

as AllDodge indicated, check the cables themselves. you will most likely have to cut off the heat shrink. if all looks good, re-heat shrink. if there is any green on the cable, replace the cable.

 

[BomoBoater]

The previous sparkplug wire issue had the wire off one plug and dangling into the bilge. It was enough to occasionally trip the ignition breaker upon first turning the key. The current stalling problem doesn't trip any breaker.

I've tried jiggling the key as well as the wires into the ignition. I think I jiggled just about every wire visible.
My first thought was a fuel issue. But, the no crank symptom is certainly electrical. It could be an entirely separate issue though. I'm going leave the radio on while under power and see if it dies at the same time as the motor.
Meanwhile today I'll begin closer examination of the connections. Starting at the starter and moving up. Maybe I'll get lucky and find something right off the bat. Thank you.

 

[Bondo]

The previous sparkplug wire issue had the wire off one plug and dangling into the bilge. It was enough to occasionally trip the ignition breaker upon first turning the key.

Ayuh,..... That will Not cause the breaker to break,...

Plug wires Always go to ground, through the spark plugs,....

I think yer still fightin' the same issue, a short in the Ignition circuit,.... but that plug wire ain't the cause,....

The failure to crank over is another issue, in the Startin' circuit,....

 

[BomoBoater]

No Title

I cleaned and inspected all +\- connections in the engine bay, battery cables (both ends), and replaced the coil and coil wire (because I was in there). I ran out of time before reaching the battery selector switch. Boat fired up and ran smoothly at the dock for a few minutes and then stalled. Radio and blowers appeared uninterrupted. I played around with the throttle and waited a bit and eventually it died again. In 20 minutes of idling, cycling rpms up and down, the motor died 3 times and had one "hiccup" but didn't stall. Only once would the starter not immediately engage. Eventually it stopped stalling and I gave up for the night.
I did find this sensor in a hidden spot of the bilge. It's for my safe-t-alert. It must've been knocked out at some point. I'm assuming this has nothing to do with my current issues?

 

[tpenfield]

That is the fume detector . . has nothing to do with the engine. It is a good thing to have and make sure it is working.

 

[BomoBoater]

Tpenfield - what's the best way to disconnect the tach, at the tach or coil? Which grey wire is it, there are two?

 

[alldodge]

There should only be one gray wire on the negative side of the coil, it will go to the tach. If there are 2 then something else has been added, and would be good to trace it down

 

[BomoBoater]

No Title

Two grey wires. One to the TBIV ignition module and the other into the wiring harness. I assume it's the one to the harness I disconnect to disable the tach.

I discovered a small (2"x3") box connected to the alternator that was poorly mounted. Is it an external voltage regulator? I've never seen one. It was quite loose, so I tightened the single bolt up. What's the proper attachment look like? I know there's no way it supposed to be mounted with a single bolt to the alternator bracket as it is.

No stalling yesterday at the dock. I'm going to do a few hot laps today and see if I've found a resolution yet.

 

[alldodge]

The box is a noise suppressor and is used to keep alternator noise out of the radio.

 

[BomoBoater]

It's been 2 weeks now without a single stall. I don't want to curse myself, but I think I fixed it. The last things I did were clean the connections at the perko switch, the ignition switch, the trim pump, and tighten the bolt holding the noise suppressor to the alternator bracket. If I had to guess what it was I'd say the ground on the trim pump, (It was the only connection I ran into with any corrosion at all, and it wasn't what I would've called bad). The noise suppressor was definitely jiggling loosely around, but the wires were still fully sheathed and unbroken.

Thank you for all the advice. On to the next issue

 

[BomoBoater]

This intermittent issue has been plaguing me for almost 3 seasons. It’s taken most of the relaxation out of my boating. I’m getting desperate.
I’ll restate the problem:
The boat starts/runs/idles normally. There’s a “random” stalling issue. It will stall at idle, under light throttle, at heavy throttle. It will randomly stall when hot or cold. I cannot get it to stall at will or regularly.
When it stalls there is no sputtering, it’s as though someone just cut the power. There are no blown fuses to replace or breakers tripped. It’s gone days or weeks without issue, only to return and stall multiple times a day.
The dash instruments all go dead, but every accessory still has power, (radio, blowers etc). Within 15-60seconds power returns to the dash and the motor will crank and fire up as normal. It may stall out shortly thereafter or not at all.

I replaced the eta ignition breaker last year. This seemed to fix the problem for several weeks, but then it returned.
I bypassed the kill switch, it made no difference
I disconnected the tach, no difference
I cleaned all the grounds and it seemed to work for a few weeks, and then it started stalling again.
I replaced the ignition switch, no change. Still stalling.
Jiggling wires or throttle makes no difference when running
Plug wires, coil, sensor in distributor, cap/rotor, and battery, have all been replaced.

I had had it at the shop, and they cleaned the banana plug connections and also jiggled everything but couldn’t get the boat to stall.

I figure it’s something in the ignition circuit, but I’m at a loss.
Could it be the ignition control module? The 50amp breaker at the starter going bad? Breaker on the engine? Voltage regulator going bad?
What would cause a loss of power to the dash, not trip breakers, and then disappear.
I just put her in the water today and I’ve got to solve this.
Thank you.

 

[alldodge]

With everything that has to do with the motor loosing power, its either the red, red/purple, ignition switch or grounds.

The radio, blower and others are powered by a different circuit

Red - unfused battery voltage
Red/Purple - Fused (50 amp) battery voltage
Ground

Red comes from the battery to starter, to 90 amp fuse, to 50amp breaker
Red/Purple from 50 amp to cannon plug, to 20 amp fuse, to ignition switch
Abreak in connection between the bat to ignition, and including the ignition switch is the issue

I would look real close at the Red/Purple wire connections in and thru the 10 pin connector

 

[BomoBoater]

Today I let the engine run for 20min till it cut out, then restarted. It cut out sooner, restarted...etc. eventually however, power didn’t return quickly, it took 10 min. The next time it didn’t come back at all. Perfect, now It’s really broken and I can put a meter to it.
I started at the ignition switch.

No power at the purple wire, (from kill switch) terminal I
Power at the B terminal
No power at the S terminal
I believe the cut off wire is from a halon system... which doesn’t seem to exist as far as I can tell

Then I went back to the slave solenoid.
No power on C
Power on A
No power on B
Connecting A&D gives power, so ground is good
Jumping A&B does crank engine
tested the connection from switch wire to solenoid and it’s connected.

I bypassed the kill switch yesterday when she was still starting and the engine continued to die. I ruled that out.

Could it it be the neutral safety switch? Jiggling the handle never appeared to cause stalling, or bringing back power.
this is my throttle/shifter. Any clues on how to get in there and test the switch?

Am I overlooking something?