Would a stuck closed exhaust flapper cause knock?

[OldNBold51]

I've been trying to locate my engine knock on my 7.4L with 58 hrs on rebuild. With the engine out, so far the only thing I've seen obvious was a stuck exhaust flapper that needed a hit with a hammer to dislodge. When I last sea tested, while loading boat on trailer, there were good size rollers hitting the back of boat. The flappers have needed new rubber since I bought this boat. The boat was briefly overheated when impeller grenaded. Fixed impeller and after 2 hrs of run time at avg 2500 rpms, the knocking started.

I have yet to find anything glaring at me with motor on the engine stand. Very little metal shavings in pan, no more than you expect from a new break in.

 

[scoflaw]

Re: Would a stuck closed exhaust flapper cause knock?

Knocking is usually a bearing. Take your pan off and pull the caps for a look see.

 

[zbnutcase]

Re: Would a stuck closed exhaust flapper cause knock?

Really shouldn't be any visible shavings in the pan, even at break in, as all should be small enough to end up in the oil filter. If you got metal too big to fit through pickup screen on pump, I would be looking MUCH harder at the engine. And I can't even imagine a flapper making a knock.

 

[coheej]

Re: Would a stuck closed exhaust flapper cause knock?

I agree. There should be no metal in the pan unless it was a dirty rebuild.
Cut the oil filter open. If there's grey, silver metal or copper (even worse), then you most likely lost a bearing. With it on a stand, by all means, take the pan off and look at every rod & main bearing

 

[Bondo]

Re: Would a stuck closed exhaust flapper cause knock?

Ayuh,.... If she's already on the stand, roll 'er over,+ Look at the bearings,...

I agree, you'll probably Not find good news....

 

[OldNBold51]

Re: Would a stuck closed exhaust flapper cause knock?

I agree. There should be no metal in the pan unless it was a dirty rebuild.
Cut the oil filter open. If there's grey, silver metal or copper (even worse), then you most likely lost a bearing. With it on a stand, by all means, take the pan off and look at every rod & main bearing

No metal in the cut open filter. The pan's off and that's where I found the small amounts of metal flakes. By small amount, I'd estimate it was 1/8 teaspoon or less. I get more than that when I break in my dirt bike.

I stopped disassembly when I pulled out the "suspected broken" piston and found nothing wrong. I'd done a leakdown test and had big leakdown #'s on cyl #5. I was tired and thoroughly disgusted at that point. I'll keep looking at the bearings and cam lobes and lifters. The stuck exhaust flapper, so far, has been the only obvious thing I've come across.

 

[Gary H NC]

Re: Would a stuck closed exhaust flapper cause knock?

At my friends shop we had a 7.4 that has been rebuilt twice now due to defective Comp cams...It keeps wiping the cam out at about the 50 hour mark.
Comp cams knows of this problem but was not very helpful..
The last rebuild we switched to rollers and it has been fine now.

Does it sound like valve train or a bottom end knock?

 

[OldNBold51]

Re: Would a stuck closed exhaust flapper cause knock?

Does it sound like valve train or a bottom end knock?

It sounds more bottom end as in a deeper pitch knock. However: with a stethescope, I cannot find a definitive spot where it's loud and deep sounding. I can hear lifter chatter, rocker arm chatter , exhaust flapper rattle, but not the sound of the knock with the stethescope. Maybe i just suck at identifying the source.

I'll get around to pulling the caps off in a week or so.

Thanks for everyone's help.