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OMC 4.3 cobra 1988 loud deep knocking?

C

Chrisnomates

Member
Engine was great yesterday but today I headed off shore with revs just under 3000. About 3 miles out I gave her a fast run full throttle for about 30sec.
When I took her back down to 3000 I heard a quiet knock deep down.
Worried I turned for shore and began to panic.
Oil pressure was low but all else was ok.
After about 60sec knocking was getting louder so I went to slow her down to 2000rpm and she stopped.
Spent next 2 hours motoring in on 4hp outboard.

During that final 60sec I pulled engin cover off.
Burnng smell from engine and smoke coming out of oil cap.
Oil presure was low but temp was ok.

During that 60sec I suspected big-end issues due to low oil but had no alternative other than to try and reach shore.

Finally back on shore I checked oil level and it was extremly low however yesterday t was full!

Anyone any thoughts on the matter?
My obvious questions are:
Do you thnk it's the big-ends?
Is this an expensive fix?
Is it better to scrap engine now?
Should I give her a re-fill of oil and try starting or get her into a boat shop without trying anything myself?

Had a car that did this once and she was saved by replacing bearings as crank was ok.

Thanks in advance,
Chris.
 
Ok. Will try and do later tonight if I find my brothers tester.
Plugs out, full throttle and a few turns on each I presume.
Engine will obviously be cold
I think I can manage that.

If she did run too low on oil have I wrecked the engine? The knock did sound like the big ends!
 
buy a filter cutter and cut the filter. Look at the debris in the pleats.
Cost of analysis: $5 filter

All the noises and smells you describe could be a gimbal bearing or outdrive upper gearset.

You wouldn't be the first person to strip down a perfectly good engine because of a baffed outdrive.

Verify engine oil pressure at cranking with a bourdon tube gauge.

oil_filter_cutter.jpg
 
Ok Test done.
I started right side front to back 1,2,3
Then left side front to back 4,5,6

#1 = 9.4bar
#2 = 9.9bar
#3 = 9.1bar

#4 = 10.3bar
#5 = 8.3bar
#6 = 9.2bar

From other threads I gather this is not to bad if lowest is about 80% of highest.

What next?
 
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Checked dip stick and she appears to be empty.
Could be angle of boat but maybe I should just top it up and start her and see if knocking is gone.
Would this be a bad idea?

Should I cut open the filter now then?

Thanks for all the help so far people.
Don't want to pay boat shop if I don't need to.
 
if you cut it now and it looks like this inside
4904_view.jpg

148_0207_mag_3_z.jpg

then you won't be wasting any more time, or new oil
you can just yank the engine


If the pleats are clean, then you can be 90% sure you did not damage the bearings

But you need the cutter.
If you are going to cut it with a hacksaw, don't bother. You'll learn nothing.
 
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Good news!
1) I got a repair manual from ebay and it will hopefully arive on saturday.
2) My plumber friend says he has a cutter that will crack open oil filter.

Bad news!
I feel silly but where is the oil filter?
I noticed a couple of bean tin white things on the engine.
Is it the one at the front or back?

Sorry if noone has time for me but you guys are the reason guys like me try to learn new stuff and hopefully become one of you guys!! lol

Thanks in advance,
Chris.
 
who don't have time for you ? I had to help my son finish his eagle project for the boy scouts.

Then tomorrow i have to put away all the tools and trailer.

Then clean out the truck to get ready for west point cause i'm going there tomorrow for the weekend on a boy scout trip.

But then again i see your post and many others, and reply when i can.
 
What I was trying to say was I am extremly greatfull for all the help I am getting and sorry if I am a pain and thank you all for taking the time to help me.

Back to the project!
My plumber friend dropped a big pipe cutter over today so hopefully tomorrow after work I can get oil filter cracked open and some results for Mr Hystat. Hopefully the filter condition will give an indication of what I have in front of me. I think he's right that you can tell a lot from the oil condition.
Hopeully manual will drop through mail box tomorrow also.
I am going to take a run to the parts shop tomorrow during lunch. I store boat a few miles from home so can't look now to see what replacement filter I need. Can anyone tell me what to get and also what oil I ask for?
Hopefully when manual arrives I can stop asking these silly questions. :)

Thanks in advance,
Chris
 
Got it cut open tonight.
Should it be spotless?
It has some grit in it but not much.
 

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Some more pictures as it dried out a lillte.
 

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So there we go.
I got the repair manual today, it looks quite good.
I presume this is going to be a big job now.
I sucked the oil out through the dip stick but only got about 2ltr.
I presume there should have been about 4ltr at least.

What do I do next? Replace engine or fix this one?
I will probably have to leave it in boat shop.

I probably would not find another OMC n Ireland so may have to change to Mercruiser. Would that mean outdrive change too?

Oh! My head is starting to get sore thnking of the money ths wil all cost.
Probably more than the boat is worth.
 
there's enough bearing material in that second pic especially to say there's engine damage. However, it's not as bad as one would suspect if a bearing had spun.

Here is how I would proceed:

1. pull the outdrive off
2. check u-joints and gimbal bearing for failure
3. connect cooling water to thermostat line and run engine up to temp with new oil and filter. recut filter.


I still think there's a slight chance the noise might be behind the engine in the gimbal area.


Your engine is a Chevrolet 4.3
It is identical to the 4.3 Mercruiser used and identical to the 4.3 Volvo Penta used in 1988
The core part of the engine where the bearings are is the same as every 1988 Chevrolet car and truck that used a 4.3 V6 - I'm sure there's one in Ireland somewhere.
Otherwise, you can rebuild a 4.3 easily.
You disassemble and look very carefully at the crankshaft. That is the critical part when a bearing fails. It may need replacing.
Aside from that, it's just a rebuild.
 
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Still some hope then!
I think an engine rebuild is beyond my skills though.
But the more I can do the more I should save in the boat shop.

Will get new filters and oil tomorrow if shops are open. (It's a bank holiday over here).

Today I will try and have a look at the gimble etc.

If I have to take it to a boat shop and they take out engine and do a rebuild what is the average number of hours needed to do this?

Just so I can estimate costs.

Thanks so far.
 
I was looking at these!
http://repower-marine.com/marine-engines/new-v6-marine-engines

From a UK supplier.

Have boat booked into a mechanic on wed.

Thought I may as well take the time to restore the inside so gutted it today and found the floor is rotting out of her.

This is going to be an expensive summer.
 

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Gimbal checked and it appears 100% no slap, play, or movement. Turns smooth.
Did notice water inside the tube and no seal round the outdrive.
I presume this is why there was water in there.

I will run engine tomorrow when I get more oil and filters. (Shops closed today).

Thanks again,
Chris
 

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Good point!
Interesting story.
The owner BEFORE the guy I got boat from said he sold it because it was leaking and he got it out of water before it sank. He said he had no technical knowledge so sold it cheap knowing it needed work. The guy I got it from said he had fixed it all up and repaired the outdrive and leak problems. But after today and finding no seal round the outdrive I suspect he just fixed it up enough to catch a fool like me.
 
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