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1999 Mercruiser 454 Horizon Valve Seats

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1999 Mercruiser 454 Horizon Valve Seats

Old 10-15-2018, 03:04 PM
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Default 1999 Mercruiser 454 Horizon Valve Seats

I purchased a 1999 Sea Ray 370 EC with 3500 hours on it in July of 2017 knowing the port motor had issues. When I finished the 2017 season, I pulled the motor and brought it to a very reputable motor builder. He went through the entire motor and found the Cam was bad (Wiped Out). The bottom end was perfect, so he honed the cylinders, put new rings on the pistons,New bearing, Cam, new oil pump and did a valve job... I put it back in the boat and it ran great. This year I noticed performance starting to go down throughout the year. I did a compression check and found 125 to 130 across all 8 cylinders. I pulled the motor and brought it to him. Everything inside looked great except for the Intake Valves. All 8 intake valve seats showed a gap on 25% of the seat next to the Exhaust valve. There were NO signs of heat build up anywhere. Nothing was discolored. Nothing smelled when he pulled it apart.
Has anyone ever heard of this? The engine ran about 125 hours with 100 of them at Idle Trolling for Salmon. It never ran over 4000 RPM for more than a few minutes ever. Cruise speed is around 3500 - 3700 RPM. I did use a lot more fuel in that motor throughout the year too..

Thoughts? What are we missing?
3150Sportcraft is offline  
Old 10-15-2018, 03:40 PM
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seems the only thing missing is the contact info for a better machine shop to fix the heads...
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Old 10-15-2018, 06:59 PM
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That PSI indicates a 8.5 CR or so at cranking speed. AND, they are even.
What exactly is this gap? Are you saying there is light getting thru 25% of the valve seat area on all Intakes?
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Old 10-15-2018, 07:17 PM
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Yes, you could see light through 25% of the valve seat on the side closest to the exhaust valve. There was no sign of High heat or burning.. My builder can't explain why this happened.

My starboard motor which has over 3700 hours has 150 - 155 compression on all 8 cylinders. My port motor is the issue
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Old 10-15-2018, 07:20 PM
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That many hours both engines needed rebuilding, 3500 hours your heads are shot. Normal for a 454.
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Old 10-15-2018, 07:23 PM
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Originally Posted by 3150Sportcraft View Post
Yes, you could see light through 25% of the valve seat on the side closest to the exhaust valve. There was no sign of High heat or burning.. My builder can't explain why this happened.

My starboard motor which has over 3700 hours has 150 - 15 compression on all 8 cylinders. My port motor is the issue
125hrs, You can see light thru 25% of the intake valve seats and no signs of burning. I would suggest a poor quality valve job during the rebuild. He himself has no other explanation. He should offer to correct the problem at his expense.
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Old 10-15-2018, 07:28 PM
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I rebuilt the port motor last year and this is the motor I am having valve issues with now. The starboard motor is still running strong, but I know Its just a matter of time.
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Old 10-16-2018, 03:32 PM
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Originally Posted by diver dave View Post
That PSI indicates a 8.5 CR or so at cranking speed. AND, they are even.
What exactly is this gap? Are you saying there is light getting thru 25% of the valve seat area on all Intakes?
There's no correlation between cranking pressure and compression ratio.
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Old 01-11-2019, 09:43 PM
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Sounds like your builder installed the wrong valves and / or valve springs.
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Old 01-12-2019, 07:36 AM
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Most likely worn guides,the worn guide allows the valve to get pushed slightly sideways from the rocker arm pressure.Either replace the heads or have them repaired.
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