4.3L Mercruiser quit running after a loud noise

[Todd in NY]

My friend was out in his boat yesterday to do some fishing. He had just got his boat up on plane when the engine made a loud bang and shut off. He could turn the motor over but it won't start. He had to limp back to the dock with his kicker motor. It sounded like it was running fine right before it broke, and it gave no indication that something was amiss. The engine is either new or rebuilt with less than 100 hours on it this year since he bought the boat. Any ideas what might be the problem? This is all the information I have at this point. I wasn't with him when the motor broke. Thanks in advance!

 

[alldodge]

Check the oil level, and then do a compression test

 

[Todd in NY]

Thanks AllDodge! It really caught him off guard because he's had the boat out about 6 or more times this year and didn't notice any problems. The motor started just fine and got up on plane right before the problem happened.

Could it be an issue with the gimbal bearing, U-joint issue, or alignment issue? This motor and drive had to be installed on a new transom because the boat had a different make and model motor in it that had to be replaced, so the previous owner had this 4.3L mercruiser installed by a marina.

 

[alldodge]

So the motor had a bang, died and would not restart, and now it is starting and running fine?

 

[Rick Stephens]

Only time I ever had a bang that preceded total instant shutdown, I broke the roll pin holding the gear to the distributor shaft. I was running a high volume oil pump on a new Ford Windsor motor. Eventually had to drop the HV pump. Not that many things that can fail with a bang. If it was something in the outdrive that was enough to stop the engine, the motor would not turn over afterwards until you removed the outdrive. Don't see it being there. Seems to me it has to be either something like a failed ignition module or a broken part.

 

[Bt Doctur]

Last one I saw was off the VZ that sent a rod out the pan and into the hull

 

[Bt Doctur]

Now THIS produced a very BIG BANG and stopped the motor

 

[Todd in NY]

So the motor had a bang, died and would not restart, and now it is starting and running fine?

No, it's not running fine. It turns over but will not start. It was running fine before the big bang. He drove out 9 miles to do some trolling, trolled with his kicker motor, then fired up the main motor, got on plane, then the bang. Sorry for the confusion.

 

[Todd in NY]

Only time I ever had a bang that preceded total instant shutdown, I broke the roll pin holding the gear to the distributor shaft. I was running a high volume oil pump on a new Ford Windsor motor. Eventually had to drop the HV pump. Not that many things that can fail with a bang. If it was something in the outdrive that was enough to stop the engine, the motor would not turn over afterwards until you removed the outdrive. Don't see it being there. Seems to me it has to be either something like a failed ignition module or a broken part.

Thanks for the reply Rick. I'll pass this on to him.

 

[Todd in NY]

Last one I saw was off the VZ that sent a rod out the pan and into the hull

Fortunately it wasn't that catastrophic. That sounds like a really bad day.

 

[Todd in NY]

Now THIS produced a very BIG BANG and stopped the motor

Wow! I take it the engine wouldn't turn over after this? His motor still turns over but will not start. He didn't keep cranking on it though, just to avoid more damage.

 

[khe]

Does it have spark? Is this a carbed engine or fuel injection?

 

[alldodge]

Thanks AllDodge! It really caught him off guard because he's had the boat out about 6 or more times this year and didn't notice any problems. The motor started just fine and got up on plane right before the problem happened.

Could it be an issue with the gimbal bearing, U-joint issue, or alignment issue? This motor and drive had to be installed on a new transom because the boat had a different make and model motor in it that had to be replaced, so the previous owner had this 4.3L mercruiser installed by a marina.

No, it's not running fine. It turns over but will not start. It was running fine before the big bang. He drove out 9 miles to do some trolling, trolled with his kicker motor, then fired up the main motor, got on plane, then the bang. Sorry for the confusion.

OK, then I go back to post 2
Check oil level and do a compression test

 

[Bt Doctur]

1 does the motor pully turn when the motor is cranking over No broken crankshaft If Yes go to 2
2 Does the dist rotor turn as the motor is cranking No ,broken crankshaft, camshaft, dist gear Yes got to 3
3 Do the cylinders all have compression No Broken connecting rod

 

[Maclin]

When it cranks now, is the cranking real even, or does it struggle on a couple of cylinders? Like the timing chain let go? Also try taking the cap off the distributor and watch the rotor while cranking.

 

[Stingray1961]

Did the motor hydrolock when the last time gaskets whee changed exhaust elbows happened to me last year lucky no damage

 

[Todd in NY]

Thanks AllDodge, it's on my list for him to check.

Thanks Bt Doctur, I've added your checks to the list.

Thanks Maclin, your feedback is also added to the list of things to look for.

Thanks Khe, one more thing to check.

 

[Todd in NY]

Did the motor hydrolock when the last time gaskets whee changed exhaust elbows happened to me last year lucky no damage

We'll take a look at it this week. All of the replies here have given us a good list of things to looks at. Thanks for your reply.

 

[Todd in NY]

It's a new motor and outdrive, just installed this year by a marina in the northeast. It seems like whatever failed, or caused the failure, has something to do with improper installation, improper adjustment, or a factory defect. The boat (a mid 80's Bayliner Trophy 2159) originally had an OMC motor in it, so the transom had to be rebuilt to align the outdrive with the motor. I've fished with him 3 or 4 times this year on his boat and the only thing that caught my eye is how LOW his outdrive hangs down compared to my Bayliner (same boat) with a 5L OMC. But his motor always sounded like it was running good. I wasn't with him when the motor shut down on him, but he said that nothing strange happened before the bang.

He'll most likely just take the boat to his marine mechanic who he has used for many years. He fishes Lake Ontario every weekend and would rather put his trust in a trained mechanic and close friend than trust his own work. We'll take a look at the checks that all of you mentioned so he has a better idea of the cost to get the boat back in good working condition.

Thanks again for all the great feedback!

 

[Rick Stephens]

I think it is a leap to just blame a faulty install. While possible, there is literally no evidence. Blame the installer after figuring out what went wrong would be a more ethical approach.