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Mercruiser 454 Mag random stalling and won't start back up after stalling.

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91stingray

Contributing Member
Hi everyone, I'm having a stalling and not starting issue with my carbureted 7.4 mag. It has a Rochester 4 barrel, Thunderbolt 4 Ignition, and is all stock. So it happened the first time last week on the boat ramp, while still strapped to the trailer but in the water enough to suck water, it fired up good as usual but after idling at about 900rpm and stalled it wouldn't restart until about an hour later. I suspected a flooded carb due to the angle of the ramp and not having it at a higher rpm to warm up as I usually keep it at 1200rpm until it gets up to temp.

The second time it happened was Saturday, after running fine all day in the evening I go to leave the dock and i'm putting along at low speed at around 1000-1200rpm after about a minute it misfired once and shut off and would not start again. I paddled it back to the dock and let it sit over night. next morning it fired up easily first try as it usually does.

Now this last time it stalled is completely different than the other 2. I was running along about 3000-3200rpm and the engine just shut off like I turned the key off, it didn't misfire or stumble at all. Wouldn't start back up, I should also note that I was getting a warning alarm when I attempted to start it back up, but everything was fine with the engine prior to stalling, no overheating, oil pressure was good.

I had to get towed 10 miles to the boat ramp where after sitting an hour being towed I was able to get it to start again and get it on the trailer, where it stalled again once i got it on the trailer. I don't know where to start but I'm suspecting it could be the mechanical fuel pump? carburetor leaking internally? ignition coil or thunderbolt module shorting out? timing advance sticking? temp or oil pressure sensors shorting out the ignition coil?
 
sensors dont have any impact only warning.

Based on TBIV means '80's to early 90's, does your large black wiring connector at engine have a hose clamp on it?
 
kghost said:
sensors dont have any impact only warning.

Based on TBIV means '80's to early 90's, does your large black wiring connector at engine have a hose clamp on it?
Click to expand...
It kind of looks like a hose clamp the main harness plus looks like a 7 pin trailer plug, not literally just the size and shape.
 
91stingray said:
It kind of looks like a hose clamp the main harness plus looks like a 7 pin trailer plug, not literally just the size and shape.
Click to expand...
The purpose of the hose clamp is,
The connector has two sides, one is male pins and the other is female pins/sockets.

The hose clamp should be tight so the pins stay in contact. If not, over time the male pins compress and have intermitten contact with the female side. (The male pins have a slot in them and thats what gets compressed)

You may want to loosen clamp, pull apart connector and look at pins and sockets to make sure they are not burned or oxidized. If ok reconnect and tighten hose clamp.

This is just a starting point. But I have seen this as a cause to random loss of power/ignition.
 
kghost said:
The purpose of the hose clamp is,
The connector has two sides, one is male pins and the other is female pins/sockets.

The hose clamp should be tight so the pins stay in contact. If not, over time the male pins compress and have intermitten contact with the female side. (The male pins have a slot in them and thats what gets compressed)

You may want to loosen clamp, pull apart connector and look at pins and sockets to make sure they are not burned or oxidized. If ok reconnect and tighten hose clamp.

This is just a starting point. But I have seen this as a cause to random loss of power/ignition.
Click to expand...
I checked my main harness it does not have a hose clamp, I didn't unplug the harness and check the connections.
 
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