5.0 MPI bending No 4 exhaust valve

[rmlsll]

I have a 2007 240 sundeck with a 5.0 merc MPI / Bravo 3. After replacing both heads, exhaust manifolds, risers and adding closed cooling experienced a bent exhaust valve on No 4 cylinder of the new head. Replaced new head with another new head and it happened again. Bent exhaust valve No 4 cylinder. Any ideas ? We are thinking wrong / weak spring, but why same valve twice on two different heads. Runs great at idle. experienced the problem at around 3500 rpm both times.

 

[BillK2632]

I would be looking at the manifold at this point. Sounds like water leaking down through the open exhaust valve. Pull the #4 plug before the next start to see my f it is wet.

 

[scoflaw]

What part of the valve is bent ? Is it hitting the piston, sticking in the guide ?
How many hours did it take to happen ?

 

[rmlsll]

The stem is bent just inside the exhaust port. There is a small dent where the valve hit / touched the top of the piston. Valve spring compressed but not broke and keepers are in place. We did not remove keepers to see if valve is stuck to guide as to not void manufacture warranty. Both times it happened (so far) was within a couple hrs of replacement and only after running on plane +/- 3500 rpm for a few minutes. Runs fine idling / running at no wake speeds.

 

[scoflaw]

Did you replace your timing chain when you put the first set of heads on? Your valve timing may be off letting the valve get too close to the piston.

Or possibly the rod bearing is shot and letting the piston come up too high. See if it is tight in the guide , chances of 2 tight guides on the same valve with different heads are slim to none.

What's the history on why you put the 1st set of heads on ? How the originals failed ? The company your buying and returning heads to aren't going to be too happy with you having the same issue / same valve.

Start there, take a good look at everything .

 

[rmlsll]

Thanks for the input. Original heads, exh manifolds replaced because of salt water corrosion / pin hole No 1 cylinder above exhaust valve. Engine runs very smooth / no lower end bearing knock. If it were timing chain issue we would have problems with entire valve train. I was thinking a bad spring letting valve float until it happened again to the same valve. Might be a guide to stem clearance thing.

 

[scoflaw]

The cam, head and block are all cast parts and none of them are perfect. Maybe the other valves are only missing the pistons by a couple of thou.

Tight valve in guide has about the same odds as a weak spring, on a completely different head. How about head gasket thickness ?

you may want to check piston to valve clearance before final assembly. How stretched is your timing chain ? Every crank degree of slop will get your exhaust valve .010 closer to your piston. Maybe your replacement heads have had .020 cut off them.

Just giving you some things to think about.

 

[Jetgod]

I was wondering if you figured out your bent valve issue?

 

[rmlsll]

nope, still working thru the issue

 

[rmlsll]

Hopefully final update, replaced aftermarket "sierra" cylinder head with brand new GM head. After 8 hrs of run time happy to report engine is running fine. Have been told of a similar problem that when the head was cut in half showed no waterjacket cast around / behind the sticking exhaust valve. Sounds like I had one of those.

 

[scoflaw]

You mean 2 bad heads with the exact same problem on the same valve ??

 

[rmlsll]

nope, just one.

 

[rmlsll]

Ya, I reread my original post. now i'm confused

 

[rmlsll]

Sorry, my mistake, The faulty head went to the machine shop after both the first and second times valve bent