Mercruiser MIE 340 454 won't turn over when hot.

[JeffT65]

I have a 1984 searay ec390 with twin 454 mercruisers. When cold the engines fire right up and run perfectly. Once the motors are warm/hot, if I shut them off I can barely turn them over. It acts like the timing is way off but it's fine and set at 8 degrees btdc. I recently replaced my exhaust manifolds because there was no water flow and they were packed with scale. I did not replace the risers and extensions. I cleaned up the risers and did not see any cracks. The only thing I can think is that maybe I could be leaking some water somehow back into the exhaust portion of the manifold and hydro locking the motors that way??? I'm really not sure. Oil is fine no "chocolate milkshake" so I don't think it is a head. Any help is really appreciated.

 

[Bondo]

I have a 1984 searay ec390 with twin 454 mercruisers. When cold the engines fire right up and run perfectly. Once the motors are warm/hot, if I shut them off I can barely turn them over. It acts like the timing is way off but it's fine and set at 8 degrees btdc. The only thing I can think is that maybe I could be leaking some water somehow back into the exhaust portion of the manifold and hydro locking the motors that way??? I'm really not sure. Oil is fine no "chocolate milkshake" so I don't think it is a head. Any help is really appreciated.

Ayuh,..... Welcome Aboard,..... Induce the conditions, 'n pull some aft spark plugs, crank it over, 'n look for water,.....

 

[JeffT65]

Ayuh,..... Welcome Aboard,..... Induce the conditions, 'n pull some aft spark plugs, crank it over, 'n look for water,.....

Thanks! It will be happening first thing tomorrow morning. I’ll let you know what I find.

 

[JeffT65]

Ayuh,..... Welcome Aboard,..... Induce the conditions, 'n pull some aft spark plugs, crank it over, 'n look for water,.....

Bondo
I didn’t mention that I replaced all four exhaust manifolds but did not replace the risers and elbows and that’s why I was thinking that I’m somehow getting water from a cracked riser or something. It is happening to both motors. I’m heading down now to follow your suggestion and I’ll report back later. Was just wondering if I should just order new risers and elbows since the stuff is almost 40 yrs old. Thanks

 

[Bondo]

Bondo
I didn’t mention that I replaced all four exhaust manifolds but did not replace the risers and elbows and that’s why I was thinking that I’m somehow getting water from a cracked riser or something. It is happening to both motors. I’m heading down now to follow your suggestion and I’ll report back later. Was just wondering if I should just order new risers and elbows since the stuff is almost 40 yrs old. Thanks

Ayuh,..... Diagnose 1st, then order parts,....

 

[JeffT65]

Ayuh,..... Diagnose 1st, then order parts,....

Warmed up the engines and let them sit for 15 minutes. both locked and wouldn't turn over. pulled #8 plug and saw a droplet of water on the anode. Pulled #6 and probably 5 shot glasses of water poured out. Holy crap! I pulled the elbow and riser and was careful to disconnect the hose that goes into the elbow. there was standing water in the exhaust manifold exhaust ports. I pulled the elbow and inspect and the exhaust port was soaked. I plugged the water ports and taped the exhaust/water port to the muffler and filled the exhaust port with water. Water seeped out of the water inlet port telling me the the riser elbow is bad and dumping water into the exhaust. I suppose when the engine is running the exhaust blows the water out of the system and the motors seem to run fine. Once the motors are shut off, the water bleeds down into the cylinders... Does that theory make any sense. After I put the new exhaust manifolds on a couple weeks ago, I used new graphite metallic gaskets for the riser and elbow. The gaskets are now disintegrated and when I disassembled the riser they literally fell apart. Is that normal fro that type of gasket? I'm hoping the engines are damaged and intend on replacing all of the risers and elbows and changing the oil. Anything else I should do? I really cant believe after seeing the amount of water that came out that I was ever able to start them even after a day of sitting. Let me know if you think this makes sense or if I should be looking for something else as well. Thanks

 

[Bondo]

Ayuh,..... Makes perfect sense,..... Now is the time to order parts,.....

 

[wshekar]

I would probably run a compression check and makes sure the valves are ok after all that hydrolock.

Are the valves showing signs of rust?

 

[JeffT65]

I would probably run a compression check and makes sure the valves are ok after all that hydrolock.

Are the valves showing signs of rust?

definitely. a compression check is in order. The valves looked ok when I had the exhaust manifolds off but I am nervous for sure and would be kind of shocked if there is no other damage.

 

[JeffT65]

Dumb questions here…I have a fresh water cooling system (no closed loop). I understand this gasket if for non closed loop cooling systems. My question is since I have the 3” riser spacers should I have two of these or is it okay to have the bottom one (between manifold and riser extension) be the unrestricted gasket and this one between the riser extension and the riser elbow? What purpose does it serve? Slowing the water down for more heat transfer? Thanks guys.

 

[Bondo]

Dumb questions here…I have a fresh water cooling system (no closed loop). I understand this gasket if for non closed loop cooling systems. My question is since I have the 3” riser spacers should I have two of these or is it okay to have the bottom one (between manifold and riser extension) be the unrestricted gasket and this one between the riser extension and the riser elbow? What purpose does it serve? Slowing the water down for more heat transfer? Thanks guys.

Ayuh,...... Just one of that gasket at the top, a full open under it,....

Ya, it slows the water,....

 

[JeffT65]

Ayuh,..... Makes perfect sense,..... Now is the time to order parts,.....

Hi Bondo,

I wanted to ask you if I were to have reversed the gaskets when I put the old risers on (restricted one on the bottom of the riser extension and unrestricted on the riser elbow) would that cause a situation where water could somehow back flow into the exhaust portion of the manifold. My initial response is no but I wanted to ask because I saw a youtube video where he said that could happen but I would think that couldn't unless the gasket failed. Am I missing something? thanks

 

[Bondo]

Ayuh,..... The water ports in the elbows are well aft of the hump, so I'm thinkin, No,.....
But, I don't mess with boats that run riser blocks,.......