Twin 4.3s died. Please read

[Tbmac]

Hi all,

I recently bought an 07 280 Sundancer with twin 4.3 mercruisers(480 hours). The sea trial and survey went perfectly. I did not get a compression test, I know I should have. Fast forward to a month later(last Saturday) it gets put in the saltwater and I attempt to drive it home, motors keep shutting off. I have to get towed in.


my reputable mechanic comes and checks it out and finds the end of some of the spark plugs disintegrated and 3 of my 12 cylinders dead with 0, 15 and 30 on compression(spread out between 2 motors). He and another separate mechanic agree my pistons are probably shot and I need new long blocks.

What on earth could have caused this after a perfect survey and sea trial? Literally 10 minutes on the Chesapeake bay and it starts to fail. No alarms, all gauges stayed normal. I have another mechanic coming to borescope the motor. However, my mechanic is pretty sure my engines are screwed. They both fire up and idle fine now. Any help is appreciated, I am losing my mind.

 

[Tbmac]

Also, new manifolds risers and transom assemblies were installed before I bought the boat.

 

[hughespat57]

Always hard to say remotely
But those plugs look like burned up from detonation. What cylinders did they come from?
Two adjacent cylinders low could be blow head gasket
Did they do a leak down test with compressed air?
I would definitely pull the heads before committing to blocks

 

[Tbmac]

Starboard they were adjacent but port it was a single. What causes “detonation”? That’s what my mechanic said as well. He has not done a leak down test yet.

 

[hughespat57]

Detonation is when the fuel ignites violently rather then burning smoothly
Normally caused by a lean mixture

Like the plug damage it can damage the pistons
But also can damage the valves so if lucky might just need head work

 

[Hoplite808]

@Tbmac I just bought a new to me 1996 270 Sundancer that has a single 7.4 mercruiser with low hours. Paid $1000 for a survey and it surveyed well. Sea trial and everything suggested no major issues. After purchasing I made it about 1/2 mile from the marina before the engine failed. The #5 cylinder exhaust valve seized and bent the push rod like a pretzel. Also the #4 cylinder valves were leaking badly too. Was able to limp it to its new slip at barely faster than idle and now I’m waiting for the heads to get back from the machine shop so I can put the motor back together. I feel your pain but you gotta look at the upside which is once you’ve fixed them they should provide many years of worry free service.

 

[Golfman25]

I don't get it. When you guys sea trialed, to you check the cruise and max RPM numbers? I find it hard to believe that engines which passed a good sea trail would "blow" after the check is cashed and on the trip home.

 

[Tbmac]

I don't get it. When you guys sea trialed, to you check the cruise and max RPM numbers? I find it hard to believe that engines which passed a good sea trail would "blow" after the check is cashed and on the trip home.

Sea trial was an hour long. We worked the boat and it did great. I have no clue, even the mechanics are stumped.

 

[ErikE]

Did you fuel it after the trial? Curious what fuel went in. Only common thing between the two motors.

 

[hughespat57]

Basically the sellers got lucky...
Even a good engine survey may not have found anything at the time.

Maybe it sat a while before he decided to sell. Maybe he didnt fog it last year when put away because he no longer cared he already decided to sell. Who knows.
It made it thru a short run for the sea trial; then you work it a bit harder for a bit longer and a valve sticks, or the rings stick or an injector is clogged and running lean.

Every broker/dealer sales agreement for a used boat has an "as is, where is" clause.
Unless some guarantee is explicitly spelled out you bought it you own it.

 

[mrsrobinson]

I agree with the others, you may get lucky and just need the heads redone. Leak down test and borescope should tell you more.

 

[Nater Potater]

Did you fuel it after the trial? Curious what fuel went in. Only common thing between the two motors.

This needs to be addressed. You said it sat for a month after the sea trial; did you fill the tanks prior to trying to run it home? And, if so, what grade of fuel did you use? As @hughespat57 noted, those plugs look to be damaged by detonation, with the most likely cause being too low of octane of the fuel.

 

[Fill Ess]

Who had the helm at the sea trial? There are people out there who will sell a boat (or car) with a known issue. Maybe the seller didn't know, but maybe they did and either hoped/prayed it didn't happen during the trial or were able to cover it up by keeping the idle high.

Like @Golfman25 said, the surveyor should have put the performance numbers. I got these from my last sea trial:

 

[Bill Curtis]

A vacuum leak will cause detonation / lean mixture, as well. I have seen many and the vacuum leak was the main offender.

 

[ErikE]

This needs to be addressed. You said it sat for a month after the sea trial; did you fill the tanks prior to trying to run it home? And, if so, what grade of fuel did you use? As @hughespat57 noted, those plugs look to be damaged by detonation, with the most likely cause being too low of octane of the fuel.

Agree. Remembering that this happened to both motors. At the same (roughly) time. Have to look for things in common with both motors. If a vacuum leak...one would think that would effect only one motor (unless sharing a vacuum canister?) etc.

 

[mrsrobinson]

Yeah, good point about both motors, which suggests fuel.

 

[Tbmac]

This needs to be addressed. You said it sat for a month after the sea trial; did you fill the tanks prior to trying to run it home? And, if so, what grade of fuel did you use? As @hughespat57 noted, those plugs look to be damaged by detonation, with the most likely cause being too low of octane of the fuel.

I did not, the fuel could have been in there for 8-9 months.

 

[blaster]

When the ethanol separates the octane drops. Sounds like that’s what may have happened.

 

[Nater Potater]

...the fuel could have been in there for 8-9 months.

As @blaster mentioned, fuel can go bad. If it's 8-9 months old (or older), unless you added the proper ratio of stabilizer, the octane rating is one of the first things to suffer. If the detonation was bad enough to trash the plugs like that you normally get pistons with broken ring lands, caved-in domes, and/or rod bearings pounded out. At the tail end of your original post, you mentioned "They both fire up and idle fine now." If you have a number of cylinders with that low of compression on a six-cylinder, I'd be curious how they can idle just fine. Usually, they're rougher than a cob with even one dead cylinder. Anxiously awaiting results of the bore scope!

 

[Gunn]

Starboard they were adjacent but port it was a single. What causes “detonation”? That’s what my mechanic said as well. He has not done a leak down test yet.

The #2 and #3 cylinders on the starboard engine are not adjacent to one other; they are on opposite sides. Pretty bizarre readings.

My starboard engine failed in the fall of 2020. I had started to ingest water a couple of times into the starboard bank of cylinders (cylinders 2, 4 had lots of water when I pulled the plugs). Finally gave up the ghost. Compression test revealed 0 compression on those two cylinders. Never did find the smoking gun (I had assumed it was a rusted cylinder head between cylinders 2, 4), but the whole engine was trashed. I ordered a re-manufactured long block and swapped over the remainder of the parts. Hoping to get it back in the water soon!

Good luck with yours, that really sucks. I feel your pain...