5.7L V8 Engine lock/clunk problem

[18XLT]

Hi Guys - first question here so hope someone can help. I have a MerCruiser 5.7L MIE GM 350 V-8 1988 rebuilt recently. Dipped and new rings, etc. Starts and runs beautifully, except on a number of occasions if you turn off the motor on the water, particularly after a hard run, the motor feels locked with a ‘clunk’ sound. It always restarts if you leave it for a short while to cool down. If we do not turn off the engine at idle, we never have a problem - but what a nuisance.
Couple points - water cooled open system fed from a jet. I have new starter motor and eliminated any hydrolock or vapour lock cause.
My guess is this - I’m getting a hot spot in one of the pistons (for whatever reason) and although the engine cooling is working perfectly - the piston is getting stuck until it cools sufficiently??
The engine gauges never show overheat (140-150F deg , and engine block doesn’t get above 200F even under heavy load.
The temp gauge goes up obviously when it is shut down.
Any thoughts? Prefer not to have to rip the motor down . Is there any possibility there is not enough pressure through the block given its open cooling. Pressure from jet doesn’t get above 2-3PSI at manifold so I’m thinking a bypass supplementary 12V water recirculating pump might just help?
Sorry long winded - tried to provide as much info straight up. Much appreciated.
Tim

 

[alldodge]

The engine gauges never show overheat (140-150F deg , and engine block doesn’t get above 200F even under heavy load.

Explain further; where is the 200* reading being seen?

 

[18XLT]

I have a digital sensor bolted directly to a head bolt on the engine

 

[18XLT]

I have a digital sensor bolted directly to a head bolt on the engine

(Middle of intake manifold bolt to block opposite site to temp sensor)

 

[alldodge]

Thanks
Have you check the temp of the thermostat housing with a IR temp gun?
How much does the temp go up when motor is turned OFF?

If the motor is getting to hot (gauge showing good) then this could be the issue. Did you pull plugs to verify no hydrolock?

 

[18XLT]

Thanks. No IR gun but I have a mechanical gauge into one of the thermostat sensor holes and that remains often under 140F.
The max the temp has risen on OFF is 167F
Pulled all plugs, no water, spark colour looks good and oil clean (not milky).
We’ve been up the river skiing for a week and it has happened 3 times. Only when the motor is stopped and left for say 1-2 minutes. It doesn’t lock up if you restart immediately or have sat 5-10 minutes.

 

[alldodge]

When it does this does it start to turn a slow real fast or just stop?
If it slows real fast and starter grunts hard, it could be your timing is off a bit and/or compression is the issue. If your running higher compression, the fuel may start to burn before the spark hits it due to heat increase from compression

 

[18XLT]

The starter doesn’t turn. It’s like it tries too, but just clunks like its stopped or restricted, the t makes no sound after that. Release the key, and try again, same clunk. Starts sometimes
If you hold the key longer and forces a start (like more Amos are drawn). Engine runs off two heavy marine batteries so it’s not that.

 

[alldodge]

Might not be locked
Maybe the Slave relay (not starter solenoid) or bad connection. Many times its a bad ground connection. The cables look goos, but if they are inspected on both ends under the insulation, might find corrosion

Next time it does it, grab the POS and NEG cables and try to start. Do either of the cables get warm/hot?

Also locate the slave relay and jump across the 2 main top terminals to see if this causes it to crank

 

[18XLT]

That’s a great idea! Thank you, I will try that next time it happens. Sort of makes sense with what you have described. Really appreciate it. Fingers crossed.

 

[Scott Danforth]

Pull your plugs and crank it over. Bet water shoots out of 2 or 3 cylinders

 

[18XLT]

Pull your plugs and crank it over. Bet water shoots out of 2 or 3 cylinders

Doesn’t at all. It’s not Hydrolocked, already tested

 

[mr 88]

Did you find the problem ? If not what i would do , ( not sure what position you are in financially ) , is buy another starter . At this point in my life I have a lot of back up parts , dont like waiting or searching around on a weekend or whenever I feel like heading out. Blowers , pumps , alternator, starter motor , ignition module ,coils,filters ,caps.rotors ,plugs, belts etc. To name a few items on the shelves in the garage. Or pull it and have a rebuilder go through it.

 

[18XLT]

I’m the same! Do I already did a new starter on the rebuild.
So I think I have some progress.
- First of all I added a mini 12V circ pump in the jet water pump line. I found at idle, the jet doesn’t push much water through and the theory being such low pressure in the block it just wasn’t filling all the voids, bubbles and causing maybe small hot spots, despite temp gauges being ok. Well this has improved flow by a few psi and the problem hasn’t happened since!
Second backup is to change that slave relay as kindly suggested (on order) and check connections. I think we might have sorted!
Thanks for the response

 

[tank1949]

Doesn’t at all. It’s not Hydrolocked, already tested

Check your timing. At least that will solve problem or direct you to something else, maybe....

 

[18XLT]

Cheers, will do