4.3 Merc I/O has starting problems Knock on start

[DanDeltona]

I have a 1997 Mariah 21' bowrider with a 4.3 mercruiser I/O. I had it out on the water and all was fine mostly kept at moderate speed and also chugged along in the no wake zones. Everything ran and worked fine. When I stopped the boat at the dock and went to restart the motor made a loud knock starter stopped and it actually broke a starter bolt in the motor. Since this incident I have extracted the broken bolt, replaced the starter, replaced the plugs and oil. There were no signs of water intrusion in oil. After that, I started the boat and it started and ran fine on the trailer with the ear muffs. After it warmed up it would occasionally make the knock when starting and the starter wouldn't spin. After letting it cool down it would start right up and run fine again on the trailer. I went to the water to test out the boat started and ran fine until I went to give throttle and it wouldn't accelerate past 2,300 rpm. Didn't sound bad or run rough. Just wouldn't go beyond 2,300. Brought the boat back home and starter clanked when trying to start on trailer and then wouldn't engage. I put a new starter and new bolts as they were slightly bent and didn't want to risk breaking off again. Then, boat started and ran fine on trailer and went up to temperature slowly and was fine at idle and would rev right up. Started daily like this for 5 days straight. When I went to start this morning it started fine and I ran it for 3 minutes. Then I decided to try to start again after 2-3 minutes and it made a loud knock and now the starter won't turn. What could be causing this problem/these problems?

 

[Bondo]

There were no signs of water intrusion in oil.

Ayuh,.... Welcome Aboard,.... Are there any signs of water in the Cylinders,..??..??

Bad exhaust manifolds/ risers is My 1st guess,....

 

[DanDeltona]

I pulled the plugs this morning and cranked it over by hand after removing the starter. There was quite a lot of water in the cylinders. I sprayed fogging in cylinders and turned over until nothing more came out. Installed new plugs and reinstalled starter. I am planning on replacing impeller in outdrive. I started without ear muffs and it fired right up and ran great. I only ran for about 30 seconds and shut off then started again after 1 minute and it fired up. Where should I look for the water entering the cylinders? I am sure it is somewhere in the cooling system. Just not sure where to look.

 

[alldodge]

There was quite a lot of water in the cylinders

If it had water on both sides I would think it would be the intake manifold. Otherwise as Bondo mentioned, remove the exhaust manifolds and check for cracks.

 

[DanDeltona]

As an update most of the water appeared to be in the right bank of cylinders with very little water in the left bank of cylinders. This is looking at the motor from the front.

 

[thumpar]

The intake manifold gaskets are a weak point on those. I had 2 go on 4.3l's I have had. It is most likely either that or the risers/manifolds.

 

[DanDeltona]

I will pull the exhaust risers and check to see what I find. I will update when I get them pulled. Thanks for the help

 

[DanDeltona]

I believe I will start with the intake manifold as there was some water in both sides although most of the water appeared to be in the right bank. I will pull it and inspect. I will take some pictures of the assembly once removed and check for a bad gasket. I appreciate all your help and responses.

 

[alldodge]

Look forward to seeing what you find

 

[DanDeltona]

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