Mercruiser 4.3L Loud Knocking at 1200 + RPM

[MSRTBUDWISER]

Hey guys,

My first time on iBoats so here goes....

I have a 2001 Maxum 1800 with a Mercruiser 4.3L with an alpha 1. Just picked it up from my girlfriends uncle. He said that it had sat for about a year. I checked the oil and it looked brand new so I didn't change it. I got it home and ran it on the trailer with muffs and she ran great, purred like a happy cat. I took it out to the lake yesterday for her "shakedown cruise" and here's what transpired.

Backed it into the water and let her idle on the trailer for about 2-3 minutes before pushing off (ran great). Idled over to the dock and left idling while I parked the truck (ran great). I started out away from the dock after checking the bilges and making sure we were good to go. I idled out to the no wake buoy, then kicked her up to about 25-35 mph. About 2 minutes into cruising, I began to hear a loud knock from the engine compartment. I had my girlfriend take the wheel while I listened. It seemed to be getting worse over about a minute so we shut her down. I pulled the hatch and I had steam or smoke coming out of the breather tubes.

I drifted her over to the beach and towed it back to the dock :-( ......

When I got home I started it on the trailer with the muffs. At idle it is as quiet as could be, just as smooth as you could want. But.... When I increase the RPMs in neutral, once I hit about 1200 RPMs, (to steal someone elses phrase) it sounds "like someone is sending me morse code by hitting my engine with a ball peen hammer." As I increase RPMs the knocking sound increases in speed also.

I have just under 40 psi of oil pressure at 2000 RPMs according to the dash guages. It has plenty of drive oil and engine oil. Temp never rose over 175.

Any ideas as to what this might be?!? I am thinking maybe a stuck lifter? Please help... before I pay someone like $$$$ to look at it... thanks.

 

[Bondo]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

It has plenty of drive oil and engine oil.

Ayuh,... Welcome Aboard,... Is the motor's oil AT the full mark,..??
Is the gas, what was in the tank,..??

My Guess is, ya detonated it from Bad Gas....
I'd run a Compression test to start, 'n inspect the sparkplugs while yer at it...
Probably take a peek at the contents of the fuel Filter too...

 

[dirtdog38]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

u still gt the blowby 2 or just the knock. sounds like a rod rapping to be

 

[04fxdwgi]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

#1 on what Bondo is saying , plus:

Is the drive unit all the way down when running the engine?

There is a distinct "ticking" sound of a lifter making noise vs: a bearing or piston "knock" (or rattling noise). Kind of difficult to diagnose the sound thru the a web site. There are a host of reasons that cause a knocking noise.
1. The drive input shaft / u-joints can make a racket if engine is run while drive trimmed up to an upper / raised position and the joints need attention.
2. Generally, if there is a "tick" that is constantly there at most any RPM, it's a lifter.
3. Generally, If there's a "knock" that's there and gets louder as soon as you quickly remove throttle, then you may be looking at a cracked piston / wrist pin / connecting rod problem. Usually worse under light to no load.
4. Generally, If there's a "knock" that is there that get's louder when the trottle is applied, then you may be looking at a rod or main bearing. Usually worse under moderate to heavy load.

If it is indeed a bearing, with the 40 PSI of oil pressure, there is a good chance the damage isn't too severe "yet" and you haven't completely destroyed the motor "at this point", but motor would need to come out and worst case, a reground / new crank and bearings. Wrist pin / piston is a whole different bag of worms, usually a full rebuild.. Best to have someone farmilliar with the different noises take a look at it before it grenades.

 

[MSRTBUDWISER]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Thanks to everyone for the help... I took it to two local mechanics and they both said either a rod is coming loose or I have a broken piston skirt. They both said either way that the motor needs to come out. The price I got was $950 for a full rebuild at a reputable shop.

As for the responses:

@ Bond-o; The oil looked brand new on the dipstick and just a hair under the full line. I ran a compression test and got about 170-190 in all cylinders. Spark plugs all looked great, beautiful color, no fouling or metal.

@ 04fxdwgi; Although I have never heard what a lifter "tick" sounds like, I can say that this is definitely a "knock". It literally sounds like with every revolution, someone is smacking the side of the engine with a 5 lb hammer.

Either way, thanks for the advice guys, I appreciate it... Ill be pulling the motor out and getting it rebuilt this week. I plan to post a video on youtube of it running before the rebuild so that anyone else out there may be able to compare their "knocks or ticks".

Thanks again!

 

[90stingray]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Is the uncle going to step up and split the costs with you? Sorry about your bad situation.... sucks having the boat down during prime boating season.

 

[MSRTBUDWISER]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

I haven't proposed the idea to him yet... I am hoping so but either way I got it for a good price. Even if I have to monkey up and pay the repair costs, Ill still be ahead of the game. And you're exactly right, it definitely sucks having it down. Good news is the motor shop is only going to take a week or two so Ill be up and running soon! Thanks!

 

[Grand Larsony]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Exhaust flappers ??

 

[MSRTBUDWISER]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Hey All,

Thanks again for the input. I pulled it out yesterday and took it over to the engine shop. He pulled everything apart in front of me and it was a spun rod bearing. Fried the rod and the crank. Oh well.. Good news is it will be fixed and we'll be boating by next weekend!!!

 

[bds85466]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

how'd that happen? was/is oil making it everywhere it needs to be? not an expert just curiosity....

 

[MSRTBUDWISER]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Not sure... I had "okay" oil pressure, and he said that it could have been installed wrong if the motor was rebuilt before. The one that spun was #6 so the last one to get oil. He said it could have been because it overheated or lost a little bit of oil pressure. I am going to replace the impeller in the outdrive to be sure it was getting plenty of water. (On the trailer with the muffs on it stays cool, but its also under pressure from the hose, in the water, it might not have been getting enough water). Its getting a new high volume oil pump so I shouldnt have this problem again!

 

[bds85466]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

I had a buddy who's pressure was reading good, high even. He developed an engine knock after his boat was sitting for a winter. checked under the valve cover and oil wasn't even making it/splashing to the side he checked. Turns out he hadn't changed his filter in a long time. Probably completely clogged up (duh). Changed the filter and got oil back to his criticals....and now has a slight knock....

 

[MSRTBUDWISER]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

Yeah, I thought that might have been the problem but when we pulled the valve covers after pulling the motor... there was oil and metal in the in by the valves. I think it was getting oil but the question is how much. With the new oil pump it should be good. I'm going to see how much a new sender is so I can be sure its good. Thanks for the advice though! ~Happy Boating!~

 

[jeffschocker]

Re: Mercruiser 4.3L Loud Knocking at 1200 + RPM

I plan to post a video on youtube of it running before the rebuild so that anyone else out there may be able to compare their "knocks or ticks".

Did you ever post this video? I have a 1992 Maxum, 4.3L V6 with a very similar problem. Haven't dived into it yet but wanted to compare the "ticking" noise I'm hearing with what you encountered.

 

[GA_Boater]

Did you ever post this video? I have a 1992 Maxum, 4.3L V6 with a very similar problem. Haven't dived into it yet but wanted to compare the "ticking" noise I'm hearing with what you encountered.

MSRTBUDWISER has been passed out under his boat since August 1, 2011 - Four years ago. Read the Help Tips on top of the page, especially the part about old threads. Start your own thread. Thanks.