7.4L Horizon - Ejected Spark Plug Insulator

Daleo56

Member
Aug 9, 2016
69
Long Beach, CA
Boat Info
1998 Sea Ray 370 AC
Engines
454 Mercruisers
7.4L MAG MPI Horizon Gas Engines / Starboard rebuilt with previous owner in 2015
1998 Sea Ray 370 Aft Cabin

I was out on the ocean yesterday at about 3,000 rpms and heard a loud bang on the starboard engine followed by ticking. I shut down the engine and went into the engine compartment and saw the noise was from the #1 cylinder missing the spark plug insulator. The plug wire was dangling and the insulator was nowhere to be found. We limped back to the marina on one engine and I then found the insulator (shown below) at the rear of the engine compartment. I used a bore-scope to look inside the cylinder this morning. The picture is the second one below from the #1 cylinder looking through the spark plug hole.

I had changed the plugs a few months ago and only had a few hours on the engines since then.


TWO QUESTIONS

PART ONE: With the new spark plug installed the engine started and ran for a minute before dying. I noticed water (not antifreeze) coming from the PCV hose connection (the valve cover PCV) and when I removed the PCV hose a pint of water came out. Fortunately it was not making it to the valve cover because of the one way PCV and there is NO water in the oil. There was water in the plenum based on the routing of the PCV hose though! I then left the PCV hose disconnected. I let the engine sit for a few minutes and started it back up. After about 60 seconds of the running the engine died again and then another pint of water came out of the now disconnected PCV hose. I assume since the hose was spitting out water that the plenum had a lot of water and it. Since the second attempt was new water, I gave up and figure I must have a stuck open intake valve in #1. Before I start pulling the engine apart I was hoping someone had seen this before and could offer some advice.

PART TWO: Does anyone know what the round metal thing (pictured below) on the left side of the inside head? Could that be the intake valve smashed against the valve seat, shattered and deformed?

NOTE: The slight rust is because we got back late and I did not take the picture until this morning.

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The piece of metal you see is the valve seat. The heads have pressed in hardened seats. Those motors have history of reversion, causes the valve seats to rust and pop out head, they usually smash into pieces and get lodged in piston and head if they aren't blown out with the exhaust. looks like yours is hung around valve and jammed into port. Don't ask how I know, brings back bad memories with those horizons.
 
The piece of metal you see is the valve seat. The heads have pressed in hardened seats. Those motors have history of reversion, causes the valve seats to rust and pop out head, they usually smash into pieces and get lodged in piston and head if they aren't blown out with the exhaust. looks like yours is hung around valve and jammed into port. Don't ask how I know, brings back bad memories with those horizons.

Thank you for the quick response. I have new exhaust manifolds to install next month so it looks like I will be replacing the heads too. Ugh.
 
After I thought about it, I drove back down to the boat and began to prep it. I pulled the plugs, turned the engine over a few times and then sprayed Sea Foam lube in the cylinders. I also pulled the plenum off and sprayed down the intakes. I cleaned what I could and organized the parts I removed. I started ordering new parts when I got home. Ugh.

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If you are doing all that work and installing new manifolds you should strongly consider changing out the heat exchanger and going to a FULL fresh water cooled instead of the Half system you have now.

Manifolds should last ten years after swap, instead of the 3 to 5 you will get from the new ones your installing, and the full cooling is much better for those big blocks with the extreme valve overlap they have causing reversion.
 
If you are doing all that work and installing new manifolds you should strongly consider changing out the heat exchanger and going to a FULL fresh water cooled instead of the Half system you have now.

Manifolds should last ten years after swap, instead of the 3 to 5 you will get from the new ones your installing, and the full cooling is much better for those big blocks with the extreme valve overlap they have causing reversion.

I have already bought the Mercruiser OEM exhaust manifolds, elbows and 3" risers for the starboard engine. Can I still use them with the closed system? Is that why the risers have the threaded hole in them? My heat exchanger is original and could use replacing anyway.
 
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google search for a FULL fresh water cooling kit. It will have new exchangers, brackets hoses and fittings needed to change over . It does not include the manifolds, and all the new manifolds and parts you already bought are compatible, the only thing you need different is block off plates/gaskets for under elbow to riser joint
 
google search for a FULL fresh water cooling kit. It will have new exchangers, brackets hoses and fittings needed to change over . It does not include the manifolds, and all the new manifolds and parts you already bought are compatible, the only thing you need different is block off plates/gaskets for under elbow to riser joint

I just finished a conversation with CPP and they said the MC330 kit will not fit the Horizon engine. They also recommended I stick with the factory installed cooling system.
 
They make San Juan full fresh conversion for your motors, I had them for 7years then sold boat with them.
 
Finding heads for your motors might be tough, because they have very few cores in the industry because of the non rebuildable heads that were corroded away.
 
They make San Juan full fresh conversion for your motors, I had them for 7years then sold boat with them.

I talked with Brad at SJM and they do not make a kit for the 7.4L Horizon engines. I will leave the existing system as is and just make sure I flush it often.

Finding heads for your motors might be tough, because they have very few cores in the industry because of the non rebuildable heads that were corroded away.

I ordered new heads from Michigan Motorz.
 
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I tore down it down to the block last weekend and got it cleaned up. I have the new heads and other new parts ready to go at home. I had the brackets all blasted and powdercoated, and purchased all new coolers as well as new hoses. I also bought new sensors, sea water pump, exhaust and even a new heat exchanger while I was at it. I will load them up and install them over the next few weekends. I also have a Neutra-Salt system I am installing on this engine.

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It looks like the top of #1 piston is a little beat. Hopefully I am wrong and it's just the pic. If it is the case, and considering the amount of parts you are throwing at her already, I would yank it and at the least replace that piston. The little dings could lead to hot spots down the road causing detonation. Don't mean to sound negative but that cylinder could also have possible piston skirt and connecting rod issues. Hoping the best for you.
 
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It looks like the top of #1 piston is a little beat. Hopefully I am wrong and it's just the pic. If it is the case, and considering the amount of parts you are throwing at her already, I would yank it and at the least replace that piston. The little dings could lead to hot spots down the road causing detonation. Don't mean to sound negative but that cylinder could also have possible piston skirt and connecting rod issues. Hoping the best for you.

I think it is just the reflection of the oil I sprayed it down with and the angle of the light but I will be sure to check it again before I put on the heads!
 
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All done and put back together. New heads, exhaust, sensors, new exchanger and coolers, hoses, wires and and added Neutra-Salt systems. Compression is around 110 and the engine runs smooth. I still have a slight issue with the throttle and rpms (as I did before) that I need to track down. No codes and I had already replaced the TPS and MAP but it is still very slow to return to idle.

It is always something...

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Hate to the bearer of bad news but .....110psi is not a good number. You should have torn it down a bit further
 
Hate to the bearer of bad news but .....110psi is not a good number. You should have torn it down a bit further

The manual said 100 psi was the minimum and that was the worst cylinder. I could have warmed up the engine more but #97-25 says to check at room temperature.
 
A healthy non vortec engine should be in the 150 range.....Vortec engine 180. I would assume merc uses 100psi to defer warranty claims. Confirm with a leak down test.
 

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