My engine quit running while idling in driveway. Heard a squeal then a shudder. It restarted and would only run with throttle Dies at idle. Will not turn until it sets a few minutes. Oil pressure was 80 psi after restart which is normal. It is a merc420 block, and heads with B&M 174.

Pulled drive and all belts. Blower and all accessories and drive not seized.

With kill switch pulled will crank then like a seize and clnk or clang. Will not crank over again until set 10 minutes. Acts like a bearing seizing since cooling lets it turn again.

Pulled oil and cut into filter. All is clean no metal flakes or particles.

Any recommendation on what to check next or Antony have similar experience with a 454 mark IV?

What would be best and worst case in cost if I pulled engine and had a builder fix? Engine has 230 hours.

Considering a new 540 but if I was not into too much money I would still be happy with output of the 525SC.

Thanks

Pull the spark plugs and make sure there is no water in the cylinders.

I have not run it since August. Will there be rust on plugs or some other damage if reversion issue?

Pull the drive and make sure you don't have a u-joint seizing up. Since you have the belts off anyhow there is no risk of damaging the water pump go ahead and start it up without the drive on.

It still binds with drive off cranking it. Not U joints.

I just had a 7.4l do the same thing. Spun a main in my case.

pull the engine,remove the pan and check for spun bearing.please report what you find.

I pulled exhaust manifolds and saw some evidence in 6. Same dpot you could see motor shaking and wher squealing noise emanated. The carbon in 6 EMI exhaust manifold disturbed snd some rust in iron head. The valve looks to be not shattered. Have to see what I can when i pull the valve cover.

The manifolds were SS +3" riser but fairly short exhaust pipe. I had ran them 3 years. I am going to run water in manifold look for drip to number six. If it is a valve spring any chance of simple fix? What should I look for. Tuliped valve or crack in head.

I ordered a new 670 hp 540 and its too late to back out. Would parting out the 525 SC clone or fixing it and then selling be more advantagous if I sell it?

really depends on what it would take to fix it..... there is a 525sc, no carb, exhaust or acces. on swapshop that has been listed for a long time at 4k and not sold

Leak check of manifold shows 6 does not have manifold leak. The super Seal Mr Gasket was only allowing hairline contact since scalloped back from motor head flange at 6 . Rust was there like hairline exhaust leak. I think probable reversion cause with my low idle.

Anybody have a favored gasket with EMIs or get direct from EMI? I don't want a repeat with 540.