Does this seem right? Looking for advice

Still LauCo

New Member
Sep 20, 2007
408
Grosse Pointe Farms, MI
Boat Info
300 Sundancer 1996
Engines
Twin 5.7L EFI Alpha One Drives
Good Afternoon,

(1996 300DA/twin 5.7L Mercs, 600hrs)

I'm completely lost. Here is the story-

I was headed out on a trip, boat running great. Went to get fuel, leaving the gas dock my port engine was hard to start... it took about 3 separate 10 second uses of the starter for it to fire, which has never happened before.

Went to get up on plane, and it felt sluggish. I then heard a loud knocking from the port side rear. I assumed it was the outdrive. I brought it home on one engine, and checked everything I know to check. At idle, and without load it seems to run just fine at all RPM ranges. Under load, the knocking seems severe. I also noticed black smoke coming from the outdrive.

I brought it (on the other engine) to a repair facility, and told them I suspected something in the outdrive, explained the symptoms etc... They pulled the boat, the outdrive checked out fine. They put the boat back in the water to do further diagnosis. It was determined that the port side fuel injector was only getting about 19lbs PSI, when it should get over 30. They replaced the fuel pump/fuel cooler, and advised to change the oil and plugs, because a rich fuel condition was causing the black smoke out of the exhaust, so I had them do both. (Both engines were tuned up less than 50 hrs ago, and the oil was fresh, but what's a little extra maintenance?)

They sea trialed the boat today, and the knock is still there. they are now telling me i need a new long block, and suspect that a piston wrist pin might have gave out.

So now I am out the $1200 for the pump, cooler, oil change and plugs, and they are telling me it didn't fix the problem. Furthermore, they are telling me that I need a new long block.

I am tempted to have the boat put back together, use it as a dock queen for the rest of the year (we love our marina, and can use our dinghy for buzzing around) and deal with this over the winter.

Any thoughts would be appreciated, mechanical stuff is not my forte.
Thanks!
 
Low rail pressure causes a lean condition, not a too rich condition. They screwed up the first diagnosis. No cure, no pay in my book. I wouldn't trust their second diagnosis. Wrist pin is a strange thing to suddenly fail during engine start. Not sure what kind of knocking you hear. Is it detonation or metal to metal? What color smoke? Black is excess fuel. Blue is oil. If you have black smoke you have a fuel problem. If it's blue you have a problem with the engine's internals that's allowing oil into the combustion chamber.

Hard to diagnose these problems from 1,000 miles away. One thing is for certain, you need a better mechanic.
 
It's definetely black smoke. As for the low pressure, the way he explained it to me originally (and again, I'm very far from mechanical) was that it the low pressure condition caused the fuel to dump, and not have a fine spray to ignite.

As for what it sounds like, it doesn't sound like detonation or a backfire, rather more of a very loud click click clik sound... Something like hitting a frying pan with a WOODEN spoon. It does not sound metal to metal if that makes any sense.
thanks for the advice!
 
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Which engine's tank did you fuel first?

I'm thinking that you just got bad fuel with the fuel in the vendor's tank phase separated because of water. One tank got okay fuel or enough okay fuel and the other tank got stuff that had separated or mostly was separated. Weird problem but possible.

Can you switch fuel lines from engine to engine or run the engine that's acting badly on a outboard engine's tank? (Like Merc recommends for fogging EFI engines.). Dump the fuel filter and install a new one, then connect to a different fuel source. Run the engine for a while at idle to burn off old fuel (the engine burns about a gallon per hour at idle, so you have to idle a good long time) then try running her.

Best regards,
Frank
 
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How old is the fuel? Could there be water in the fuel? The Mechanics second diagnoses seems strange to me. Did they check the pressure after they change the fuel pump? Could the injector need to be cleaned? I would hold off on a long block until you have a second opinion...
 
Which engine's tank did you fuel first?

I'm thinking that you just got bad fuel with the fuel in the vendor's tank phase separated because of water. One tank got okay fuel or enough okay fuel and the other tank got stuff that had separated or mostly was separated. Weird problem but possible.

Can you switch fuel lines from engine to engine or run the engine that's acting badly on a outboard engine's tank? (Like Merc recommends for fogging EFI engines.). Dump the fuel filter and install a new one, then connect to a different fuel source. Run the engine for a while at idle to burn off old fuel (the engine burns about a gallon per hour at idle, so you have to idle a good long time) then try running her.

Best regards,
Frank

Best regards,
Frank

Thanks Frank, I can't remember which one got filled first, but it's a great idea. I can run one off of the other, so I will try that as soon as I get their paws off my boat tomorrow.
 
Ditto...I would be thinking fuel issues and would be replacing mechanics before long blocks. I guess its possible, but If it is a wrist pin, thinking about it...it holds the piston to the connecting rod. It wouldn't knock on load anymore than it would knock not under load. However, your guy is on the ground, and if he correctly diagnosed that incredibly obscure condition from the set of symptoms you provided, he needs to be working on space shuttles.
 
Ditto...I would be thinking fuel issues and would be replacing mechanics before long blocks. I guess its possible, but If it is a wrist pin, thinking about it...it holds the piston to the connecting rod. It wouldn't knock on load anymore than it would knock not under load. However, your guy is on the ground, and if he correctly diagnosed that incredibly obscure condition from the set of symptoms you provided, he needs to be working on space shuttles.

Good points. To be completely fair, he did say something "like a wrist pin" but he wouldn't know until he yanked it and tore it down.
 
No way a GM engine lets go a wrist pin without other noises first. Wrist pins come out when theres no oil to them. You should have heard the lifters clattering like a sewing machine first.

Completely agree with Frank and the others on fuel related knock and smoke. Even if it was the wrist pins or rod bearings, why a long block when the fuel induction system is in good condition. Sounds like you have parts changers looking at it.
 
Which engine's tank did you fuel first?

I'm thinking that you just got bad fuel with the fuel in the vendor's tank phase separated because of water. One tank got okay fuel or enough okay fuel and the other tank got stuff that had separated or mostly was separated. Weird problem but possible.

Can you switch fuel lines from engine to engine or run the engine that's acting badly on a outboard engine's tank? (Like Merc recommends for fogging EFI engines.). Dump the fuel filter and install a new one, then connect to a different fuel source. Run the engine for a while at idle to burn off old fuel (the engine burns about a gallon per hour at idle, so you have to idle a good long time) then try running her.

Best regards,
Frank
Remember the OP said the first indication of the problem was when he went to start the engine after fueling up - if it was bad gas it would not affect the starting that quickly as he still had "old" gas in the injectors/lines - could be valvetrain noise or a bearing - I doubt its a wristpin too - I also agree that this mechanic is just taking a shot in the dark.......With YOUR money
 
I'd try to rule out bad fuel as the problem. Seems the mechanic was thinking about fuel as well since they checked injector pressure first.
 
Isnt there another thread going on here about a knocking noise that cant be fixed?
 
Isnt there another thread going on here about a knocking noise that cant be fixed?



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