Stuck valve question

[deputydawg]

2 weeks ago I started the boat to get it ready for the summer. Before doing this I pulled all of the plugs and spun it to make sure all of the winter oil was clear out of the cylinders. Checked the plugs and checked the gap on all of them. Put them back in and started the engine. It started after about 3 seconds of cranking, but was missing. I re-checked the firing order, plug wires, plug gaps, points, and cap and rotor. All were in good shape. Started it again, still missed. Started pulling plug wires, found it was missing on cylinders 4, 5, and 6. It was getting good spark but I pulled the plug wires and it did not change the idle. I opened the carb and checked the float, it was good. I checked all fuel filters they were good. (new last fall) So not finding anything wrong, I took the valve cover off the top. I found a rocker arm was off of the exhaust valve on cylinder 5. Closer inspection I noted the valve was stuck open slightly. I tapped it with a plastic hammer and it stuck farther open. So I removed the head and pulled that valve. By the way this is on a 1976 Mercruiser 165hp inline 6.<br />This exhaust valve had a small bit of rust or corrosion on the stem. This head has less than 30 hours on it. I think closer to 15 hours on it. Took it to the machine shop where they found nothing wrong. The valve was not bent, the guide was good, everything checked out. <br />My question is this, if the exhaust valve is stuck open slightly, enough to lose compression but not enough to strike the piston, would this cause all 3 cylinders to miss? And if this exhaust valve was stuck open would that allow water into the cylinder? <br />I found a slight milky film on the top of the valve cover and a slight bit around the rocker arms. BUT the oil on the dipstick is nice golden brown with no sign of water. The oil drained from the pan is nice golden brown just like the day it was poured into the engine. No sign of water intrusion.

 

[QC]

Re: Stuck valve question

A stuck exhaust valve should not have any impact on the other cylinders. A stuck intake might, but not exhaust. It will definitely not introduce any water into the equation unless it gets jammed through the head and breaks into a water passage. You can get condensation under the valve cover that would explain the milkiness up there.

 

[deputydawg]

Re: Stuck valve question

Thanks for the input. The amount of water on the top of the valve cover looks like condensation, but I wanted to check and get some opinions. The reason I wondered is I have read that if an engine diesels after shutdown it could suck water in. This made me wonder if a valve open would have the same effect during the intake stroke.

 

[newport dave]

Re: Stuck valve question

Maybe this will help. The following is taken from Mercruiser service bulletin 01-13 "Gasoline Engines and Water Intrusion".<br /><br />

<br /><br />Dave

 

[QC]

Re: Stuck valve question

I can't think of why dieseling would bring water in either. Same rotation.<br /><br />Running backwards would bring in water, but that is pretty rare in Marine engines. Pretty common with wheeled vehicles as they can roll backwards in a forward gear or visa versa. It can happen to a marine engine with a partial bump of the starter that ended up kicking itself backwards due to compression or if the engine is turned backwards by putting it into reverse at a high enough speed that it actually stopped the engine and then rotated it backwards. The latter would be pretty rare as it would usually just stall, more likely with a very large diameter propeller. Like the Titanic's props . . .<br /><br />Has anybody else with more marine experience than me seen them run backwards?

 

[TilliamWe]

Re: Stuck valve question

DD, number 3 & 4 is what I was going to caution you about. Sounds like you have avoided it, so whew, lucky!

 

[QC]

Re: Stuck valve question

Boy, I guess I shanked that one. It is starting to make sense to me as it will actually suck from the exhaust side during what would have been the power stroke. Still not sure how a fouled plug would cause that though as the exhaust valve would never be open while the piston was traveling down. Weird.

 

[deputydawg]

Re: Stuck valve question

Word from the machine shop....the intake valve on that same cylinder was not seating good. So I should be back in good shape soon. Had to order the head gasket so will be next week before I can get it back together. Hopefully this will get power back on the back 3 cylinders.<br />Thanks everyone for the advice.