Engine knock

[jes.112]

Hi
New to the forum..and saltwater boating.
we picked up an older boat with twin 5.7l merc inboards. have done required maintenance including heat exchangers, plugs, etc. Port motor began acting up by overheating (was shut down immediately upon notice) so had mechanic change both exhaust manifolds (both had blown and freeze plug had come out as well). He installed 160F thermostat. When he restarted the motor after install, he forgot to open the water intake and after approximately 3-4 minutes noticed the risers getting red hot. steam was coming out of the exhaust as well. we shut the motor down and opened the valve. He started the engine again, risers cooled immediately, steam stopped. Motor now has a noticeable knock, temperature isn't well controlled and oil pressure sits around 40. knock seems to settle as rpms increase but returns upon decrease. he feels this may be rod related. The knocking was not noticed prior to this work being done.
we have spent a fair amount on this motor and may be looking to replace rather than continue with the $ pit. Could this knocking be related to the overheat with the valve closed? Trying to work out what next step is.
thanks for any info.

 

[Jamie Dundee]

For me the next step would be for the mechanic to rectify the damage he has done ?

 

[volvopaul]

I doubt it unless it partially seized before you opened the intake valve .
what I would have been worried about was the sudden influx of cold water on to the hot metal.

 

[jes.112]

I doubt it unless it partially seized before you opened the intake valve .
what I would have been worried about was the sudden influx of cold water on to the hot metal.

and the result of the cold water on hot metal could be....?

 

[jes.112]

For me the next step would be for the mechanic to rectify the damage he has done ?

yes..that's why I'm looking to gather info on possible issues so I can have an informed discussion with him.

 

[jon and michie]

cold water on hot metal = cracks especially if its cast iron

 

[volvopaul]

For me the next step would be for the mechanic to rectify the damage he has done ?

The engine would need to be stripped Down , the damage found , then the recent damage proven it didn’t exist before the engineer did the work .

A highly unlikely case as it’s clear that the OP had overheated the engine prior to the engineer ever being on board , that’s the reason why he was employed to rectify the over heat .

I've never been in that position as I avoid Petrol engines that are direct cooled especially V6 and V 8s , the manifolds and risers have around a 4 year lifespan in salt water unless they are a trail or dry stack boat where you can flush with fresh water after every flush .

Ive always found that when V 8s need manifolds the inlet manifold gasket has also let water into the middle of the V , the manifolds corrode away letting water in .

 

[spannerman]

I would remove the plug leads one at a time to see if the knock alters then you have an idea which cylinder/ rod / bearing its on.

 

[Freebee]

It is unlikely that you will get any share of culpability from your engineer ........as you probably wont be able to prove how the damage was incurred before or during or after the event, if its an engine with high running hours then it could have been quite worn already. On an older boat with big engines they probably had quite a hard life already before you purchased and you would not know it..

 

[jes.112]

thanks for the responses. I appreciate the info.
the mechanic has been onboard for a few weeks now doing other work so he is familiar with how it was operating prior to this issue. i will have another mechanic provide a 2nd opinion. best case scenario..we sucked up a plastic bag. worst case... crate vs long block

 

[jes.112]

I would remove the plug leads one at a time to see if the knock alters then you have an idea which cylinder/ rod / bearing its on.

thank you

 

[jes.112]

have a lead on a vortec long block with 60hrs on it. any thoughts on what would be a fair price ? thanks.