Failing cylinder heads valves- WHY???

wakeup 19

Member
Nov 14, 2008
671
(Medford)Boston Harbor to Cape Ann to Cape Cod and
Boat Info
1992 300 Sundancer, Quicksilver ding w/ 6hp merc 4 stroke, anchorlift windlass, extended swim platfo
Engines
t/ 350 merc's (carburated) alpha drives gen II
About a month ago on my way to Aquapalooza I throttled up 92, 350 i/o 300 da and heard rapping.
went back to dock took Valve cover off and pressed in stud was lifting up and rocker came loose. fixed temporarily for the day.
Took head off and found bent exhaust valve on #5 cylinder. Replaced with re-built head as mechanic saw some wear and thought that was best long term course of action.
put the engine back together tuned and timed it up. went out with friend for shakedown and ran great for 1 mile then hit no wake zone for 20 min and gassed back up again to hear hard knocking and no power.
limped back to dock. at low rpm you wouldn't suspect a problem ran quiet.

took other Valve cover off to find bent and jammed pushrod on #8 other side of engine. it was jammed down still on rocker but off valve. (exhaust valve again) this caused lifter to collapse as well.
I put brand new manifolds and risers on at the beginning of the season for preventative maintenance changed oil after new head, no signs of water intrusion in oil.
pulling what little hair I have left to figure out what the issue causing this is.
I have $800 into port side and going to pull other head this weekend.
compression was good on port before and after head replacement.
spoke to many mechanics and know one can seem to tell me why this is happening?
help appreciated.
 
Were both incidents with the Port engine? Regardless....the piston is coming in contact with the valve. The most likely culprit is the valve guides which are sticking to the valve stem and allowing it to remain open until the piston hits it again. If these are the original guides, they and/or the valve stems have issues. Sometimes it is rust (if you have steel guides) or debris finds its way into the valve guide causing it to stick. At this point, I would pull both engines heads and have them rebuilt. You have successfully dodged two bullets. If a valve head comes off......you will be buying a new engine.

John
 
When you saw the condition of the head you replaced, didn't
you think the other one was in about the same condition? Heads usually get done in pairs.
They were probably compromised before you did the last manifold replacement.
I don't think anything is causing this, replace the 2nd head
 
yes both issues were on same engine. I know I probably should have done both heads but$$$ kid in college.
Wierd coincidence that other head on same engine had head problem 1 hour after one head was installed. that is what is frustrating. I was hoping to get the rest of the season out of it with just one.
this engine is 9 yr old re-built with approx 600hours.
When I pulled exhaust manifolds in spring they both looked good but replaced them for maint.
pulling other head this tomorrow.
 
I know it is a pain but once you do it you will have piece of mind for another 6 years. Rebuilt small block heads are relatively inexpensive.

John
 
When you change the oil at the end of the season add a pint of Marvel Mystery Oil to each engine. Helps prevent stinking valves really well.
 
I would think that problem would occur if I didn't use boat all season but I probably put 50hours on it before this happened. I spoke with another merc mechanic and he seems to think that water in a very small amount must be causing this. I no there are a lot of places where water can be getting into engine and I would like to find out where the weak link is before I put in new engine.
Since the problem shifted from one side to the other I would think ruling out risers.
intake looked good when I removed it to change head but I will take a closer look at it tonight when I get down there.
Any other possible points to inspect while I am working on it would be helpful
I had no overheating or any other problems with this engine all season.
 
Is there water in your oil? If the answer is no, then it came in from manifolds and risers. Did you own the boat when they were last replaced? A lot can be told by just looking at the exhaust valve with the manifolds off.

What was the condition of the guides and valves on the head you just replaced. The condition will tell you the story on why it failed.
 
no water in oil, I ahve owned the boat for 10 years and put new risers and manifolds in the spring as preventitive maint. old one looked pretty good.
the new head I just put on engine is rebuilt from factory and seems fine.
old head that was replaced bent the valve and showed signs of wear. hence replaced
 
You should have looked it over closely. Valves don't bend because there old, if anything the guides wear and they get looser. You should take this opportunity to have your machine shop disassemble the old head and get his opinion, before history repeats
 
Last edited:
You should have looked it over closely. Valves don't bend because there old, if anything the guides wear and they get looser. You should take this opportunity to have your machine shop disassemble the old head and get his opinion, before history repeats

I agree. guides don't stick if they are loose. Now if water get's in there they stick pushrods bend all kinds of stuff happens. Ask my how I know!
 
I just find it unusual that one side bends valve and an hour after test drive the pushrod on opposite side of engine bends??
#5 and #8 cylinder. again my mechanical skills limit me to taking things apart to save time for mechanic to inspect and re-assemble.
 
After reading this post the only other thing I can think of would be weak valve springs if the springs are weak they will let the valves float at higher rpm's this could cause the valve to hit the top of the piston causing the stud to pull out & bend the push rod (not sure if your engines have a float plate). I would look closely at the top of the pistons to see if there is any physical contact, it may not be easily visible. You could take the springs to a shop & have them checked to see if there within spec.
 
After reading this post the only other thing I can think of would be weak valve springs if the springs are weak they will let the valves float at higher rpm's this could cause the valve to hit the top of the piston causing the stud to pull out & bend the push rod (not sure if your engines have a float plate). I would look closely at the top of the pistons to see if there is any physical contact, it may not be easily visible. You could take the springs to a shop & have them checked to see if there within spec.
on the first valve that stuck/bent you could def see a mark on piston. I had trouble getting one of the bolts off on the new problem head on #8 and will try to get it off tonight. then i can get it over to the machine shop for testing as well as look at cylinder and check for leaks etc
I don't know what a float plate is? but will ask shop if i have one?
thx
 
on the first valve that stuck/bent you could def see a mark on piston. I had trouble getting one of the bolts off on the new problem head on #8 and will try to get it off tonight. then i can get it over to the machine shop for testing as well as look at cylinder and check for leaks etc
I don't know what a float plate is? but will ask shop if i have one?
thx

It's actually called a lifter restricter plate that goes between each pair of lifters not allowing the lifter to float on roller lifters I think mercruiser also used them on non roller lifters but I might be wrong.

Weak valve springs maybe could be your common problem between both heads.

Only a guess.
Good luck
 
It's actually called a lifter restricter plate that goes between each pair of lifters not allowing the lifter to float on roller lifters I think mercruiser also used them on non roller lifters but I might be wrong.

Weak valve springs maybe could be your common problem between both heads.

Only a guess.
Good luck

It's a retainer plate for the roller lifter and its purpose is to keep the roller square with the cam, so they can't rotate, nothing more. The older style hydraulic lifters are made to rotate in their bores so they have no reason to be retained. Pushrod pressure keeps them riding on the cam, both styles
 
It's a retainer plate for the roller lifter and its purpose is to keep the roller square with the cam, so they can't rotate, nothing more. The older style hydraulic lifters are made to rotate in their bores so they have no reason to be retained. Pushrod pressure keeps them riding on the cam, both styles

Your right about the hydraulic lifter, here's what it states in the merc service manual.

IMPORTANT: Keep push rod and hydraulic valve lifter from each valve together as a matched set and mark them so they can be reinstalled in the same location later.
1. Remove rocker arm covers.
2. Remove intake manifold.
3. Remove lifter restrictor retainer on roller lifter models.
4. Remove rocker arm assemblies and push rods.
NOTE: Engines with roller lifters have additional valve train components shown below.
 
You got to remember that this is General Motors engine with merc bolt ons. Design has been around since 1955.

Also thinking that weak valve springs causing his issue would be a stretch. These motors have a low lift cam, low compression, and only turn low rpm's 4800 or so. So I don't see that happening. I'm sure his machine shop will figure it out
 
Got head off last night and bringing it to shop today. Piston def hit valve. What is causing this on engine that ran perfectly for 3 months then smashed a valve on opposite sides of engine.
do all signs point to water intrusion? from where? again brand new manifolds and risers. intake looks solid. what else should i be looking at.
I removed pushrods last night and did not mark them, I didn't know they had to be marked.
 

Forum statistics

Threads
112,942
Messages
1,422,697
Members
60,927
Latest member
Jaguar65
Back
Top