Scissors-like abnormal noise from 5.7

[Happygolucky]

Have noticed this noise for some time but been busy trying to find the cause for engine dying when idleing/ low rpm..
AI tells me it could be related to mechanical fuelpump or valves.. Any input from a human would be highly appreciated..

 

[Lou C]

Unless that engine is older than the beginning of the Vortec series (96 and up) it likely does not have a mechanical fuel pump. The sound could be:
An exhaust leak
Related to the drive belt or tensioner of it has one
Valvetrain related (lifter or rocker arm)
You can try starting it with the belt off for a minute to rule out # 2.
#s1&3 it helps to have a mechanics stethoscope….

 

[Happygolucky]

1997 with mechanical fuelpump! (Not vortec)
Will test with belt off tomorrow -

Had exh system replaced last fall by Merc authoriest shop .. elbows risers manifolds ..

 

[Lou C]

The exhaust looks new but try snuggling up the mounting bolts/nuts (like 25-27 ft lbs). Never had a mechanical pump make noise. I’d get a stethoscope and listen at the valve covers and also each belt driven accessory. I had a circulating pump make noise once. PS pumps can make noise as well. Nice that you have closed cooling!

 

[IslandExplorer]

X2 on the stethoscope, really is the only way to figure it out sometimes. Can use a mechanic grade shade tree stick against the ol ear lobe in a pinch too! Doesn't always work but often times you suddenly hear it clear as day in one specific area.

Don't forget to give alt a listen, notorious scissor squeakers they are, when brushes and bearings are old.

 

[Happygolucky]

The exhaust looks new but try snuggling up the mounting bolts/nuts (like 25-27 ft lbs). Never had a mechanical pump make noise. I’d get a stethoscope and listen at the valve covers and also each belt driven accessory. I had a circulating pump make noise once. PS pumps can make noise as well. Nice that you have closed cooling!

Ok , will do-Went to ”Biltema” and got this today! (Swedish Harbor Freight)

 

[Lou C]

Ok here’s a story. In 2017 I had to do a top end overhaul on my 1988 4.3 due to previous overheat damage. At the same time I converted the old one piece OMC V6 exhaust to the later 2 piece style. So the re-assembly went well & it started right up. But I heard a faint knocking noise that I couldn’t localize. Well I drove myself crazy checking this and that. FINALLY used the stethoscope and found it was simply an exhaust flapper! The later style exhaust flowed water slightly differently. So it’s good to have that stethoscope!

 

[Happygolucky]

So today I took belt off , however noise is still there. Did tighten the portside exhaust bolts a bit..( I’m getting concerned about the seemingly slopply job the shop did.). no change to the noise though
so now I need to look into the Valvetrain

 

[Happygolucky]

forgot to mention,, when staring engine today I had blue smoke with the exhaust.. related to the suspected valveissue or yet another separate problem !?

 

[Scott06]

Unless that engine is older than the beginning of the Vortec series (96 and up) it likely does not have a mechanical fuel pump. The sound could be:
An exhaust leak
Related to the drive belt or tensioner of it has one
Valvetrain related (lifter or rocker arm)
You can try starting it with the belt off for a minute to rule out # 2.
#s1&3 it helps to have a mechanics stethoscope….

X2 on what Lou recommended. Def sound exhaust like but noises dont do well diagnosis from the keyboard. Pull the belt and start there

 

[Scott06]

forgot to mention,, when staring engine today I had blue smoke with the exhaust.. related to the suspected valveissue or yet another separate problem !?

blue smoke on start is usually indicative of valve guide seals leaking oil down in combustion chambers while it sits. How many hours on it ? Have you done a compression test to see if it is mechanically in good shape. bottom line I wouldn't sweat oil smoke at start up yet, just indicative of some engine wear, not necessarily terminal yet. If spark plugs dont look fouled Id fix other stuff first

Run ability is probably separate as I cant see that noise being a vacuum leak. Dying at idle is probably basic tune up struff - fuel pressure, carb rebuild, ignition timing and component condition.

 

[Scott Danforth]

when was the last time the alignment was done and the u-joints checked?

valve guide seals are common issues on Small Block Chebbies......because GM cheaped out on them in the engine assembly plant.

I wouldnt think twice about that, as its common. takes about 2 hours to change them while the motor is in the boat with one of these.

 

[IslandExplorer]

Nice new gadget doc!
Be sure to schedule appt with your engine then make it wait 3 plus hrs before you listen to it
How do you like the Biltema brand? Never heard of them before. Swedish harbor freaight sounds interesting.

 

[Happygolucky]

blue smoke on start is usually indicative of valve guide seals leaking oil down in combustion chambers while it sits. How many hours on it ? Have you done a compression test to see if it is mechanically in good shape. bottom line I wouldn't sweat oil smoke at start up yet, just indicative of some engine wear, not necessarily terminal yet. If spark plugs dont look fouled Id fix other stuff first

Run ability is probably separate as I cant see that noise being a vacuum leak. Dying at idle is probably basic tune up struff - fuel pressure, carb rebuild, ignition timing and component condition.

No idea of hours I’m third owner- (for sure the one adding the least hours to it)
looking back on my previous posts I realize I’ve done nothing but chasing various issues last few seasons
I did a compression test last year which to my knowledge showed the engine to be in ok condition 175 lowest- 190 highest.
The blue smoke thing has been seen last few seasons.. usually as the boat’s been sitting for awhile.. so the valve guide seals sounds to be a likely cause!

Along with the replacement of exhaust system last year also the carb was rebuilt by the Merc shop ( they replaced impeller as well since there was this overheating of elbow (though I doubt it was the culprit as It was only one season on it / very few hours)

So if engine dying on low rpm is related to carb that would mean their expert on carbs did a substandard job ?!

As there still is an intermittent issue with fuel delivery I cannot avoid suspecting the mech. fuelpump - Yesterday before pulling the belt, Engine started without any hesitations - idling fine a couple minutes only to die..
I need to put a clear hose in the fuelline watch it as a hawk

Another funny thing is that the buzzer/alarm doesn’t sound even though key is in run position ..
Oil pressure switch was replaced last season which then seemed to solve that issue but obviously not..
couple of days ago I went through the harness under the helm - found the crimped threeway splicing of the purple wire -soldered it as an extra measure and got the alarm to go off lounder than ever..
until Yesterday when I again find it to be silent on Key on.. Oh I have tried to reseat and clean the pins in the 10 pin multi contact .. what’s next?

 

[Happygolucky]

Nice new gadget doc!
Be sure to schedule appt with your engine then make it wait 3 plus hrs before you listen to it
How do you like the Biltema brand? Never heard of them before. Swedish harbor freaight sounds interesting.

You get what you pay for.. usually ”good enough ” ..check it out

https://www.biltema.se/en-se

 

[Happygolucky]

when was the last time the alignment was done and the u-joints checked?

valve guide seals are common issues on Small Block Chebbies......because GM cheaped out on them in the engine assembly plant.

I wouldnt think twice about that, as its common. takes about 2 hours to change them while the motor is in the boat with one of these. View attachment 418526

2 h. .. Hmm ..I guess for someone who's never been under the valve covers of any engine I fear it will take slightly longer .

 

[Scott Danforth]

Its not the mechanical fuel pump. they either work, or they dont - its pretty much that simple. do your troubleshooting vs firing the parts cannon

So if engine dying on low rpm is related to carb that would mean their expert on carbs did a substandard job ?!

my guess is you have crud in your fuel system, which is now in the carb or fuel pump or anti-siphon valve, or pickup tube strainer if its dying at idle. that has nothing to do with the guy that rebuilt your carb, and more to do with the guy maintaining the boat

regarding electrical, remember, these boats were slapped together to make it thru the warranty period. look for bad connections starting at the battery, and work your way thru every connection to the bow light. specifically look at the grounds.

back to your noise. when was the last time the drive was pulled and the u-joints and gimbal and alignment checked. remember, this is an every 50 hours, or yearly thing

 

[jimmbo]

I have found power steering pumps can make a variety of sounds when they are being starved of Fluid

 

[Lou C]

Ok , will do-Went to ”Biltema” and got this today! (Swedish Harbor Freight)

In the USA some refer to Harbor Freight as Horrible Freight or Horror Fright lol.
Yet, we all go there for deals!

 

[Lou C]

I have found power steering pumps can make a variety of sounds when they are being starved of Fluid

And power steering actuators do commonly leak but the leak can be hard to see. I changed one of those, not a fun job.