Loud exhaust noise, what can it be?

[Carlos Barberis]

My exhaust sound has changed, sounds like air is being expelled with the water and exhaust gases, the noise is loud.
Sequence of events: when this exhaust noise started , on looking around I found a leak on the water lifter on the exhaust. My mechanic repaired the leak. The noise went away briefly and later on reappeared. My mechanic removed the cylinder head,(because of the exhaust noise) found the cylinder head gasket in disrepair and a tiny hole in the cylinder head.
The cylinder head was replaced with an used one from Moyer, plus the gastket and struts. My mechanic told me the engine was still not working well and needed all valve replaced, he believes that intake valve in cylinder 3, and exhaust valve in cylinder 4 are faulty.
Today I went to check the boat, starts great and runs smoothly, with not much power, as it has been now for some time. The noise in the exhaust was still there. Suddenly it was gone and remained gone for some time, when the engine is turned on again, the strange (and loud) sound in the exhaust is again present.
Current conclusions: the exhaust noise is not related to the engine itself, a friend suggestion: it has to be related to something before the water pump, (air been sucked in)a hole in a hose or something in the water intake in the hull.
The lack of engine power, could be a couple of faulty valves as suggested by my mechanic.
Additional: I wonder what cylinder is working or not. With the engine running if I unplug the cable to cylinder 1, keeps running unchanged, also if I remove the wire to cylinder 3, have not tried on the other two cylinder.
Questions:
1) what is cause of this exhaust noise or how to find its source.
2) how to find out if the valves are not working well.

[msmith10]

With a louder or unusual exhaust noise along with power loss I'd suspect a burned or stuck open exhaust valve.
You need to do a compression test. If you don't have a compression gauge, remove the spark plugs, 1 at a time, and try to hold your thumb down on the spark plug hole while turning the engine over with the starter (make sure water intake is closed). If you can hold your thumb over the hole with moderate pressure while doing this without it being forced off, you probably have a bad or stuck valve.
You can tell if the valve is stuck by removing the valve cover (the plate on the side of the block behind the carburetor and fuel pump) and turning the engine over by hand. Watch the valves and they should go up and down following the lifters. If you do the compression test first, you'll know which valves to keep an eye on.
It could also be valve clearance way off holding an exhaust valve open all the time- you can check valve clearance after removing valve cover- but the comment your mechanic made makes me suspect a bad valve.
So:
1. check compression
2. If low, remove valve cover plate and make sure valves are not stuck or clearance is way off
3. If valve not stuck and clearance OK, needs valve job.

Blocked exhaust could give different exhaust sound and low power, but shouldn't give louder exhaust sound.

[ndutton]

Carlos,

Mark's advice is sound. I'm not too sure about the mechanic though.

He's already removed the head because of the noise which didn't solve the problem and after that exercise it's running poorly. Now he wants to remove the head again to replace ALL the valves. What's next, piston rings? Then what?

I recommend following Mark's diagnosis procedures before letting this guy anywhere near the engine again. It may turn out that he (the mechanic) is right but it sure looks like he's guessing on your dime to me.

I repeat my story of the yard mechanic who diagnosed a terrible A-4 noise as a "shot transmission" that turned out to be a lazy ignition switch intermittently engaging the starter while the engine was running.

[Dave Neptune]

Exhaust noise ??

Carlos, how old is the exhaist? If a hose is colapsing and it is close the the exit at the trasom it can emit a hissing sort of sound. It could also not colapse all the time but should get progressively worse. Might be wortha check. I have seen to many exhausts replaced at the hot end and not further down stream, those hoses can break loose on the inside and colapse.
If it starts and runs smooth it may not be the valves it could be you need a good tune up and the timing set, this can make a considerable difference in power.
Food for thought.

Dave Neptune

[jpian0923]

Are you mechanically inclined? Are you physically capable of doing anything that has been suggested or are you completely at the mercy of the mechanic?

I ask this because all that has been suggested requires some tools, some dexterity, some ability to be a contortionist at times and a willingness to get dirty.

In any case, get some video of the water exiting at the transom (with engine running) and of the engine running when you hear the noise (and when you don't hear it) and try to focus on where you think the noise is coming from. Post it to youtube and link it to a future "reply" on this forum and I'm sure there are plenty of guys here that will help you (or the mechanic) fix it.

p.s. Is your engine running hot when you hear the noise?
p.p.s I'm not trying to be a wiseguy but around here nobody uses a mechanic, "we" are our mechanics.

[hanleyclifford]

Why not install a vacuum gauge and an exhaust back pressure gauge? With these instruments you can gain a wealth of information about how your engine is running and how it is being treated by the exhaust system.

[rickinnj]

When I was a pup, working at a local 'Precision Tune', I would use a continuity checker (not the kind with the batteries in it) and short out the cylinders to check for even reduction in RPM's. I would just clamp onto the block with one end and poke through the spark plug wire boot with the other end. A zero reduction in RPM's would indicate the offending cylinder and give a good starting point for further diagnosis. Before that, I would just pull the ignition wires one at a time...but with the advent of HEI, I got tired of getting zapped!

Just my $0.02.

[JOHN COOKSON]

Is it possible a valve is stuck intermittently?
If so a compression test may be normal only to have the valve stick sometime later.
Not much of an expert in this area - only my $.02 worth.

TRUE GRIT

[Kurt]

I'll put my 2 cents in here, although it is hard to tell what is going on with this information. Some items to consider:
(1) Fire your mechanic - terrible things he has recommended and done. He removed the head and it probably didn't need to be removed. Found a head gasket problem - well, anytime you remove the head you are probably going to damage the head gasket. I have trouble understanding why a change in the sound of your exhaust would be related to things other than, well, the exhaust system. If this were my boat, I would thoroughly inspect the exhaust system prior to diving into the valves, etc.
(2) You state that the mechanic repaired a hole in the water lift muffler and the sound was temporarily fixed ----- you should replace your water lift muffler.
(3) A member here stated that you should look at your wet exhaust hose. Good recommendation. Well known to forum members is the fact that these hoses can collapse internally and cause exhaust back pressure problems. Excessive exhaust back pressure will cause the reduced power symptoms you are experiencing. In fact, reduced power is often related to excessive exhaust back pressure due to restriction(s) in your exhaust system. Again, your exhaust system needs to be taken apart and inspected from the hot section all the way to the transom exit. Eliminate this as the cause of your issues prior to tearing apart the engine. I had an engine that ran terrible with very little power. A "mechanic" told me that the distributor was bad - I replaced it. He told me the carb was bad - I replaced it. No change. I then turned to this forum and diagnosed the issue - restriction in the hot portion of my exhaust where the cooling water enters. I replaced my ENTIRE exhaust system - from the hot section all the way back - all the wet exhaust hoses were original from 1976! This exercise solved my issues.

[lat 64]

Hi,
I like the idea that your friend had about a air leak in the coolant hose before the pump. Jpian seems to be thinking along those lines too. I know my exhaust note gets much louder than usual when I forget to open the seacock for the coolant water. The water really does help muffle the noise.

Don't rule out the possibility that two or more problems have occurred at the same time. The exhaust leak your mechanic fixed may have been disguised by good water flow and then it showed up and made noise when you lost the muffling effect of the water. I'm just trying to give an example of a chain reaction of more than one problem all showing up at once.

Regarding the valves: all the advice is good from the above.
Any good mechanic can do an efficiency evaluation to any kind of engine.
That is, a compression test properly done, a spark test(like mentioned), and other stuff like the vacuum testing.
This testing can get very close to full diagnosis without tearing things apart. That's why the guys responding to you are asking so many more questions. In this way we can zero-in on the problem.
Engine diagnosis is like March madness; we don't know know the winner until we eliminate the lower-seated possibilities.(Duke?)

So anyway, you can do much of this testing by yourself, and then you have good data to confirm a mechanics opinion. Even if you don't want to get upside down in the engine bay and sweat blood while you fix it, you'll be a much more informed captian for it.

The compression tester and other tools are a reasonable cost compared to a mechanic's time.


cheers,

Russ

[roadnsky]

[B]...With the engine running if I unplug the cable to cylinder 1, keeps running unchanged, also if I remove the wire to cylinder 3, have not tried on the other two cylinder.

Pulling plug leads with no change?
Classic exhaust back pressure symptoms?

Today I went to check the boat, starts great and runs smoothly, with not much power

Would it start and run smooth with bad valves?

I like Russ' thought of multi-issues too...

[anglosax]

Yes It will start and run with bad valves - I had one running fine at idle but progressivly rougher at any speed and no power at all - bad valve clearance [ who me??]