383 Rod knock?(video)

[baconbiscut]

I have a repowered 25ft cruiser with a Mercruiser 383 Mag MPI engine. It was installed about a year and half ago. Well i was running a 3 blade SS 19p prop on it and it performed great. however I was taking more and more people out on the boat and as you would guess the slower and slower it was out of the hull.

Well, my buddy had a 4 blade 18p aluminum prop so i figured I would give it a try. Well I did and the boat shot on plane and everything was great. So i brought it up to wot (4800-5200rpm) and i was only getting 4500-4600 rpm so I brought it back down and I got a tic-tic-tic. which got worse and now is a more like a nock. anyways here's the video.
http://www.youtube.com/watch?v=-KXiiNayIPE&feature=channel_page
whats am I looking at doing if I do have a rod knock.

Thanks, Dan

 

[superpop]

Re: 383 Rod knock?(video)

If its a knock then it is a main bearing, does it go away under load. If it knocks on rpm drops then it is definitely a rod or bearing. Ticking could be lifters but if it is a knock then that is not good. I suppose it could also be a loose connecting rod. whatever it is, if it is knocking and getting worse, do not run it any more, when something breaks free your repair bill will go up a lot. You need to pull the motor and drop the oil pan and look for anything fishy. With the motor on a stand and flipped upside down with the pan off you should be able to diagnose the problem pretty quick. I would also check all tolerances and torques for the whole motor while you have it out.

 

[a70eliminator]

Re: 383 Rod knock?(video)

Well yea thats a bad knock you got there, when I first heard the engine crank over something didn't sound right, it sounds like there is a valve timing issue you know it's just that off beat you hear when the engine cranks without starting.
First thing pull the plugs and look at them, do a compression test, even if it's just a quick put your thumb over the hole, If you have good compression then that will eliminate a valve problem.

 

[Dunaruna]

Re: 383 Rod knock?(video)

Sounds to me like it is firing on 7, maybe even 6. A comp test and a leak down test would be a wise first move. Maybe pull the rocker covers and check for a loose rocker bolt.

 

[baconbiscut]

Re: 383 Rod knock?(video)

Ok, I just dropped the boat off at the boat shop. The guy there said it sounds like a main bearing. He's gonna get to it this week and take the boat out for a test ride. I'll keep ya psoted

Dan

 

[Tail_Gunner]

Re: 383 Rod knock?(video)

Throw a timing light on it and check your timing. I suspect the timing chain has jumped time from the hard starting and the way it sounds.

way to much advance.. id bet

 

[CharlieB]

Re: 383 Rod knock?(video)

Sorry Dan,

That engine has GOTTO come apart

Lets hope that you can get by with a new set of bearings BUT.............

Adding a 4th blade and only dropping pitch 1 inch is a BIG NO NO, excessive engine loading has wiped a bearing out, hopefully you got lucky and that is all.

Pull it out and tear it down, inspect EVERYTHING, pistons and ring lands, rod and main bearings.

Keep your fingers crossed.

 

[baconbiscut]

Re: 383 Rod knock?(video)

Ok, just got word from the mechanic. he said that i have 0 compression in my # six cylinder. I know this mechanic pretty well and I'm actually gonna be working at his shop over the summer when school gets out but i'm started to think this has been an on going problem.

When the boat was repowered and it had the 19p SS 3-blade prop I could run it out and get 5300rpm. And no i didn't usually run this boat hard. I usually ran it 3500-3800rpm. However after a while I only got 5000rpm out of it. I didn't think much of it. and, whenever i would take off at the start of the day it would hesitate. but, the mechanic said it probably wasn't warmed up and that's what I thought to but it didn't do that when i first got the engine. also the engine stopped feeling as smooth as it did new. could i have already had this problem from the beginning and the 4blade just pushed it over the edge.

Dan

 

[hemidoc]

Re: 383 Rod knock?(video)

A rod knock will have a hollow metallic sound that gets louder as you aply power. When you free rev to about 2k rpm and feather the gas it will be obvious. A spun rod or main bearing will not give you low compression. A dropped valve or most likely a broken valve spring. A broken vavle spring will make a MPI engine difficult to start 'cause the MAP sensor gets unusual readings. I've seen this on MPI engines many times(03-05 Dodge Hemis would break valve springs and do just what you describe also you may have had a rocker 'back off'). Have your mechanic check the top end first since a bearing won't make it hard to start or low comp unless you broke the connecting rod. Really a SB chevy will turn well over 6K rpm without spinning a bearing unless it runs dry of oil.

 

[baconbiscut]

Re: 383 Rod knock?(video)

Our mechanic pulled both heads. On the #6 cylinder I sucked in the intake valve. It looks like an umbrella in a rain storm. Anyways he recommends that I replace all the valves with stainless steel, or I could do them all with OEM ore i could replace just the one. Anyways what your opinion. Any suggestions.

Thanks, Dan

 

[J JACKSON]

Re: 383 Rod knock?(video)

what kind of valves did you have in there? are you running stock springs? was it lean? really need more info before buying parts

 

[baconbiscut]

Re: 383 Rod knock?(video)

all stock from mercruiser

 

[a70eliminator]

Re: 383 Rod knock?(video)

And it didn't beat up the piston? how many times have the valves been ground? you can tell if they need replaced by how thick they are, if they look sorta sunk in the seats chances are they're done. Stainless valves aren't necessary.
There must have been a problem with the keepers, either not seated properly or the wrong ones were used, not enough piston clearance, or a broken spring, something happened they don't just drop for no reason.

 

[FreeBeeTony]

Re: 383 Rod knock?(video)

Sounds like a "tuliped" valve..............possibly as a result of being overpropped?

 

[baconbiscut]

Re: 383 Rod knock?(video)

And it didn't beat up the piston? how many times have the valves been ground? you can tell if they need replaced by how thick they are, if they look sorta sunk in the seats chances are they're done. Stainless valves aren't necessary.
There must have been a problem with the keepers, either not seated properly or the wrong ones were used, not enough piston clearance, or a broken spring, something happened they don't just drop for no reason.

This motor has not been touched internally since i got it from Mercruiser. FBT yes that is what the mechanic said. I'm gonna go take a look at it tomorrow. I just wanted to know that since I lugged the engine could i have fatigued the other valves and pay the extra to have all the valves done to be on the safe side.

 

[hemidoc]

Re: 383 Rod knock?(video)

Our mechanic pulled both heads. On the #6 cylinder I sucked in the intake valve. It looks like an umbrella in a rain storm. Anyways he recommends that I replace all the valves with stainless steel, or I could do them all with OEM ore i could replace just the one. Anyways what your opinion. Any suggestions.

Thanks, Dan

The stock valves shouldn't have had a problem until 6000+ rpm(I've run them in cars well over 6K) Reasoning that cylinder may have been lean causing a high combustion chamber temp and caused the valve and or seat to melt and fail. What I'm saying is these things(valves) don't normaly fail on thier own. An outside force acted upon them.
The stainless valves is throwing $$ away unless your racing the thing and in that case you'll be overhauling it after every Sunday anyway.
I've fixed many engines that have had valves mar the piston some and they were fine. The biggest concern is the way the flamefront travels across the piston. If the piston is smoothed out with no high spots that creat a hotspot then no predetonation should happen and it's fine

PS You could have had a rocker nut back-off or a bad spring cause this

 

[baconbiscut]

Re: 383 Rod knock?(video)

I'll check with the mechanic he said I did this from lugging the engine. However i pulled up some imformation. mercury put stainless valves in this engine already so that's what i want to replace then with.

Dan