Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?!?

[Timboat]

I have a 1991 Bayliner Ciera with small block v8 Mercruiser 5.7L engine (with 1050 hours) and Alpha One Gen II outdrive with freshwater cooling (heat exchanger).

During a rare wintertime boat outing I was making my return as ran the boat at normal operating temp @4300rpm for 20 minutes back to port, then reduced the engine to 1400rpm. As I came off plane I noticed a new a very distinct “knocking/metal clanking” sound coming from the engine. Other than the new noise coming from the engine, everything else was fine: speed, power, engine temp, oil pressure, voltage. When I got back home I did more research and am leaning toward this being an issue with the engine, more specifically, something along the lines of a loose bearing, rod knock, valve train noise, stuck rod, rocker arm, push rod, etc
I started my diagnosis this evening and took some video. I’m hoping somebody can identify the problem based on the few clues that I collected, especially the sound. Here’s what I got so far:

The noise is a “knocking”, “clacking” type sound that is matched to the rpm (it goes faster when I increase thottle).
It happens at all rpms, idle, at 1500rpm, and at 3000rpm. Also, in neutral, fwd or reverse.

I can’t tell if it knocking sound happens once per crank revolution or twice. I hear that is an important distinction for what part of the engine is making the noise.

The noise is concentrated on the port side of the engine directly at the #1 cylinder.

There is oil in the engine 10w 40 (right in the middle of the dipstick lines) and it was 45F outside that day. The oil pressure in the boat was normal, between 30-75 on the inside pressure guage. I also change oil/filter regularly. The boat has not been exposed to freezing temps. I have heaters and it’s been mild winter so far.

I checked the exhaust manifold to see if it was rattling or loose, and it was fastened to the engine block securely.

I pulled the spark plugs, they all looked normal with little carbon deposits and no physical damage.

I removed the port side valve cover to attempt to pinpoint the source of noise and noticed oil flowing from each rocker arm and pooling appropriately (and evenly).

I then attempted to adjust the “lash” of the two rocker arms for the #1 cylinder. As I turned the farthest forward rocker arm’s stud nut ? turn the knocking sound disappeared! But for only 5 seconds, then it returned. I did this 3 times, but the the sound kept coming back after each turn. Now I am weary to over tighten it this adjustment. (see 2nd video for this) I tried tightening the other rocker arm on #1 and nothing happen.



I find it very interesting that turning the stud nut temporarily solves the knocking issue. But why does the sound come back in 5 seconds?
Any help would be appreciated, I fear the issue is at the main bearing as I would have a hard time fixing and accessing with the boat in the water, but perhaps I could fix a push rod or valve related issue as that only means taking off the top half off the engine. Either way, I don’t want to perform a major engine overhaul. So any suggestions would be welcomed.
Also understand it is an old engine, so advice on how to most cheaply replace or rebuild would be appreciated as well.



Video of the noise at different RPMs:
http://youtu.be/k0JGb8cTLuM


Video of the rocker arm stud nut being adjusted on the cylinder 1 to temporarily quiet the noise
http://youtu.be/tcIGa49KWNo


Thanks!

Tim

 

[Silvertip]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

The noise seems to occur on the power stroke and sounds like an exhaust leak. When you turn the adjusting nut you momentarily force a valve open so wherever the leak is, compression escapes through the momentarily open valve. Check around the exhaust manifold for that cylinder by spraying a soapy water mix at the junction for that cylinder. I think you have a simple fix. This is not a rod, push rod, rocker arm, problem. A bottom end problem shows up as a low oil pressure reading at idle and it definitely does not show up at the top of the engine like a valve train problem would. I think this is simply an exhaust manifold leak.

 

[PBarrett]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

It sounds like you have either a tappet that has gone bad or a cam with a bad lobe (going flat). The other possibility is a rocker stud that is pulling out of the head. In any event, that head has to come off for further inspection. With my experience with SBC, I'd guess it's the cam. With that many hours on the clock, I'd probably pull the engine and look at a partial rebuild.

Good luck,
PB

 

[Pete104]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

Feel under the manifold for an exhaust leak. Silvertip's right, there is a pretty simple fix here. There isn't anything bent or spun. Try to get that valve adjusted properly again.

 

[littlebookworm]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

I don't agree with PB. IMHO the likelihood of a bad cam lobe is low. The more likely scenario is a collapsed lifter or a pulled rocker stud. I'm leaning more to the collapsed lifter as a cause. When you tighten down the lifter, you decrease the space inside. The little oil that's there acts as a shock absorber, quieting the lifter. The oil then leaks out and your lifter is noisy again. To change the lifter you need to pull the intake manifold, no big deal. Soak the new lifter isn some engine oil overnight before you install it. All in all, it's a simple repair. Hy

 

[Bondo]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

During a rare wintertime boat outing I was making my return as ran the boat at normal operating temp @4300rpm for 20 minutes back to port, then reduced the engine to 1400rpm. As I came off plane I noticed a new a very distinct “knocking/metal clanking” sound coming from the engine.

Ayuh,.... With it runnin', pull the sparkplug wire off that cylinder....
Does the knock go away,..??

Unlike the other ideas mentioned,...
I'm thinkin' ya ran the snot outa it long enough, 'n it was detonating the whole time...
I'm guessin' ya beat a rod bearing out of it, or wallowed out the wrist pin...

Tightening the lifter causes that cylinder to loose compression, 'n the knock goes away, til the lifter recovers...

 

[tpenfield]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

I have not looked at the videos, but will do tonight. I'm in the bad valve lifter camp based on what you described. If so, the fix would only require the intake manifold to come off. Not so bad.

I'm not at all in the bad bearing camp. (unless it is the muffler bearing )

 

[bcasurvey]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

my vote bent exhaust valve

 

[scoflaw]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

Doesn't sound metallic. Sounds like an exhaust leak, and big enough to find easily

 

[trevtt600]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

Sounds like an exhaust leak to me... Check the exhaust manifold for a crack. You said you took it out for a winter time ride. Was it winterized properly?

 

[greg82255]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

It could be an exhaust leak or a spun bearing, but would such a noise go away when you adjust the rocker?

While you have that valve cover off, hold a straight edge across the top of all the rocker studs. The studs are pressed in, and it's entirely possible that one of those studs could have pulled out slightly after 1050 hours repetitive forces on them. If it pulled out, the rocker will sit slightly higher than it should, making a clanking noise every time it moves up and down, but once you adjust the rocker the sound would go away because it would return it back to zero lash. It's at least worth a try while the valve cover is off.

 

[dollarten]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

It could be an exhaust leak or a spun bearing, but would such a noise go away when you adjust the rocker?

While you have that valve cover off, hold a straight edge across the top of all the rocker studs. The studs are pressed in, and it's entirely possible that one of those studs could have pulled out slightly after 1050 hours repetitive forces on them. If it pulled out, the rocker will sit slightly higher than it should, making a clanking noise every time it moves up and down, but once you adjust the rocker the sound would go away because it would return it back to zero lash. It's at least worth a try while the valve cover is off.

A lot of good responses here . In order to rule out any type of bearing, cam lobe and /or lifter wear I would pull an oil sample from the engine and get it analyzed . This would detect if there is any microscopic metal particles in the oil . Any oil distributor in your area would be able to do this for you

 

[rte592]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

WOW
no one mentioned the garden hose as a stethoscope trick for finding exhaust leaks or valve train noise.

The CHA CHA CHA does sound like an ehaust leak,not a solid thump thump thump

get 3 foot of garden hose to use as a stethoscope Run the motor at idle and check around to find out where the noise is the loudest.......watch out for the MOVING BELTS

Easy

check the valve covers for valve train noise
and exhaust for bad gaskets/or cracks

 

[tpenfield]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

Tim-

I watched the videos . . . you make a nice video, very clear.

IMHO - next steps would be compression test and more preferably a cylinder leak test. Hopefully that will shed some more light on the situation.

Based on what the tests reveal, you could also remove the intake manifold and/or the exhaust manifold for a look at the valve, push rods, etc. Basically, looking for an issue in any of the following pieces - valve lifter, push rod, valve.

I would avoid tearing the cylinder head off until you are very sure of what it is.

Keep us posted.

UPDATE - Also check the valve spring . . . maybe it broke towards one of the ends.

 

[netting one]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train ...What is this SOUND?

Had a simaliar thing happen to my 1996 5.7 the porcaline of the spark plug cracked and had a exhaust leak. I was a little quieter but on the same line of sound. ( i think exhaust leak ) good luck maybe just blew the exhaust gasket out?

 

[Timboat]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

First off, thank you all for your suggestions! This forum is amazingly helpful!

It appears the two most common diagnoses are either the stuck/collapsed lifter OR perhaps an exhaust leak in the cylinder.

I tried to visually and audibly identify an exhaust leak near cylinder #1 and was unable to prove this during my inspection (admittedly I didn't do a compression check or leak down), so I moved on to removing the intake manifold to inspect the lifters and rods.
I figured if it was an exhaust leak I would need to take off the intake manifolds THEN the cylinder heads anyway so checking the lifters would be a natural place to start either way.

Currently, I'm only half way removing all the components off the top of the intake manifold (heat exchanger, coil, carb, distributor, electrical wires, etc). I will have it off by this weekend and post video updates of the status. Anything else special I should know about this intake manifold removal??

Also, as far as the cruising at 4300 RPM, I agree it is high. The boat is a 25ft Bayliner fiberglass fly bridge, I routinely have 8 people onboard, and even with their bikini bodies the boat barely has enough power to get on a plane. If I back it off to less than 4000rpm the boat falls off plane. The prop is a 3 blade mercury aluminum 15.25 x 15. I have a 4 blade aluminum that is 15 x ?; I simply can't get the boat on a plane with at all.
Is there a prop that can get my heavy underpowered boat on a plane, yet keep me at 3500rpm??
Less girls is not an option

Thanks again!

 

[PBarrett]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Tim, I'm guessing that you have the same boat as me, a Bayliner 2556. That boat is a 6000 lb. boat, with a 9 1/2' wide beam, stern heavy and terribly underpowered. Add eight people and related gear and you have a severely overloaded boat. I know that I can stay on plane at 3600 RPM's running about 24 mph with 1/2 a tank of fuel(50 gal) and four people, anything more than that and I have to move heavy items to the bow. In order to accomplish this, the engine must be in proper tune, a clean hull with no growth or slime and the use of trim tabs. Even the slightest amount of bottom growth will knock 5-7 mph out of my boat. Currently I am running a 16" x 13 pitch three blade prop with an Alpha I Gen2 drive. My WOT is 4500 RPM, so your running at 4300 is the same as running at WOT for extended periods. I would never try something like that with an engine with 1000+ hours.

I see that some have suggested an exhaust leak, but I think that is wishful thinking, actually, I think that, unfortunately, Bondo probably got it right. In my opinion, Bayliner did a dis-service to their customers by even offering this powerplant in this boat, the bare minimum should have been a 7.4 (454). I love everything about my boat, except that it's underpowered. This summer my plans call for a more powerful replacement, either a Mag 350 or a 383.

Good luck
PB

 

[Timboat]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

ThanksPB. Yeah I have the 2556 with the Alpha I gen2. It's a fantastic boat except for the underpower issue.
When I get the motor issues smoothed out, I'll pickup a new prop for a more realistic cruising speed. Would you recommend your 3 blade aluminum 16x13?
Also how much are we talking to have a mechanic install a more powerful Mag 350 or a 383? and will all my exhaust risers and components still fit on the 383?

 

[Silvertip]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

A lot of diagnosing could have been done "before" you bagan tearing it down. Chances are "looking " at the lifters will prove nothing. You can only see about 1/4 inch of them sticking out of the bore. Unless you have a leakdown fixture you can't even test them. As for definitions, a collapsed lifter is one where the plunger is inoperable and sitting on the base of the lifter. A leaky lifter is one that simply has a high leakdown rate and may present noise at varying rpm. If this were a collapsed lifter, short adjustments would have no effect until such time that all clearance is taken out of the valve train (e.g., functioning as a solid lifter)

1) A flat cam lobe results in a rocker arm that doesn't move as far as one working on a good lobe. Measure the lift at the pushrod and compare with other rockers. Cost = Zero (or at most -- a cheap dial caliper). Obviously do this with the engine off. I've dealt with lots of flat cam lobes -- none of them ever made this type of noise and that issue doesn't pop up in a second or two of coming off throttle.

2) A rocker stud pulling out would have pulled out to the point of being very noticeable since the nut has been already turned down a fair bit. Again -- measure the height of the stud as suggested using a straight edge across the tips. They won't all be exact but a partially pulled stud will be significantly different.

3) A spun bearing does not make a knock that high on the engine. Certainly possible but I wouldn't begin pulling an engine apart before exhausting (no pun intended) all other checks. A wrist pin or collapsed piston skirt makes a dull wrapping sound under load as pointed out. But rocker nuts can be turned down only a little before the plunger limit is reached and the engine begins to miss. Turning it out too far causes the clattery rocker. These are very different noises.

4) As suggested, pull the spark plug wire and start the engine. Noise still there? Yes - or No.

5) What did you see when you pulled the exhaust manifold? Did you check it from top to bottom -- with a magnifying glass. Did you check the mainifold/head surface with a straight edge for flatness. A hot manifold will expand exposing a crack that you might not see when cold.


6) I've been around small block Chebbies since the little bugger hit the market in the 1955 model year. That simply does not sound like a collapsed lifter, pulled stud or leaky valve. Leaky valves always leak so adjusting the rockers doesn't fix that, even briefly (unless of course they were too tight to begin with in which case loosening the rocker fixes the problem).

While the microphone on the camera may be muiffling the sound, a valve train problem makes a much sharper metallic click rather than the rather dull/muffled sound my ears detect. Of course I could be totally off base. I have been wrong before -- once I think.

But since the tear down has started, I'm curious to know what you are really looking for?

7) If this engine has ever had head work done on it where a valve seat had been replaced, it is certainly possible that the seat has come loose and that's what rattles. Although remote, this happens frequently on aluminum head engines where steel valve seats are used.. Good luck in your search.

 

[fishrdan]

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Re: Engine Knock or Clanking in Mercruiser V8 5.7L Valve train .. UPDATE

Wished I would have seen this thread earlier. A mechanics stethoscope would have helped diagnose the problem. Touch it onto the engine in different areas so you can pinpoint where the noise is coming from. Tube trick works too, but not as accurate for locating noises inside the engine.