My take out 525 Merc started for the first time in boat. No compression #8 cylinder. Took a look with one of the fiber optic deals found the exhaust valve not sealing. So off the heads come and yep it is bent. The computer on this engine shows 35 hours most at low rpm. It is being fixed (got a great mechanic and machine shop) but I need to figure out what happened> The valve did touch the piston but this was dressed down as it was a tiny mark. I hear about the CMI headers leaking on the 525s even with low hours so my guy is going to pressure test the headers.

Could it have been detonation on first start up. I believe it was there when I bought the engine from a reputable dealer. I am a big boy and realize no guarantees on take out engines. I don't want it to happen again.
Any of you guys have an idea or had this problem?

Thanks
Stu

A friend of mine had the headers fail on all 3 525s in his Fountain, always leaked into the 7 or 8 cylinder. Eventually all engines had to be worked on because of compression loss in those cylinders. What may have happened on yours is the valve got corrosion on the stem and it stuck open, until the piston dinged it and knocked it loose.

Wonder if it came out of a boat with Capt Call exhaust. According to Merc Racing, the 525 efi is the most susceptible engine they build for water reversion. Throw in any external condition such as back pressure from modified exhaust, bad fuel, old fuel with ethanol, corroded spark plug, solenoid issue, or any issue related to a cylinder, and reversion can deffinitely occur and cause valve stem corrosion. Happened to several down here on Texas Gulf coast. The fix for some down here was to increase idle rpm so reversion wouldn't occur when idleing in gear.

Sounds like it could be a reversion problem. It will happen on the 7 & 8 more because the water has to travel a lot less far then on the 1 or 2 cylinder. I would do a pressure test on the headers.

I would take a real careful look at the valve guide while you have the head off. It's possible the valve stuck momentarily.

Sounds like it could be a reversion problem. if not look at the vale spring .they are to weak . the presure of the spring needs to be more . that happen to one of my motors

Not an expert at all, but I can tell you I lost 2 525's in my last boat. The last motor failed due to the mechanical Fuel Pump failed. Once the motor was started and warm it would not restart till it cooled down. Just some food for thought

Hey Thanks I will let you know what the situation is and if I change the valve springs for more pressure.