RPM issues, now 20-30 compression on a cylinder

[dgyank70]

Mercruiser v6 4.3l with TKS

Serial #: 1A335597

August 2008 4.3 TKS

Family 8M9XM 04.32VC

7
8
9

Thunderbolt V ignition

Hello.

Sad story commences here.
Key points in bold.
Trying to reclaim boat performance with above engine.

Old (prior to this year) injuries include

1. TKS failed
   1. a few years back. We've left it disengaged (i.e., never adding fuel), as it had failed where it was adding fuel all the time, killing plugs.
2. Occasional stalling out and such; left idle up higher than it should be (closer to 1100 than 650)
3. Water on engine
   1. Engine was abused by a boat oddity; above the engine, the lid had a round circle cut in it for some reason, below the cushion. The cushion started letting rain through, and so we kept finding moisture on top of the engine (on the breather) and inside the distributor cap, when we had trouble starting the boat reliably. Once we realized there was a hole (never had noticed it), we plugged it

This year
-boat was not reliably starting, and also often stumbling / stalling in low speed maneuvers (idle still too high at 1100, were trying to help keep it from stalling out)
-but seemed to run ok at higher RPM, though just barely getting to around 3800-4100, when it should be 4600-4800

1. replaced distributor / rotor (4 weeks ago)
   1. That seemed to improve the starting / reliability, but the top RPM was still poor, sometimes even as low as 3500.
2. replaced all 6 plugs (2 weeks ago)
   1. That didn't seem to change the top RPM significantly either, though more like 3800 typically
   2. was hearing a little 'chuh chuh' sound at acceleration between 1500 - 2500 or so, then it was mostly not audible
3. changed oil / gear oil / cleaned the hull at the marina (1 week ago)
   1. Upon leaving the Marina, the first time I accelerated, it was smooth, quick, on plane quickly, and got to about 4300 or so; markedly better than it has in years. I initially attributed that (surprisingly) to the removal of the significant buildup of algae on the hull.
   2. After about 3 minutes at high speed, I slowed to idle, and then went to test the acceleration again, in glee
   3. In horror, I found it had fallen back to hesitation on throttle, took a long time to get on plane, and the 'chuh chuh' sound was almost sound like something was breaking / scraping
   4. So I mostly stayed slow home, though if I accelerated very slowly, it did eventually get up to 3500 or so (but not close to the 4300)
4. In the past week / today,
   1. we confirmed the fuel delivery system seems to be working fine (i.e., pumping fuel as expected, when throttle applied, etc)
   2. But today we found a cylinder not firing
   3. Compression test seems to show 20 at idle, 30 with some throttle applied (EEK)
   4. water seems to have been in that cylinder as well; came out with the gauge, plug seemed to have some on it (EEK)

We are going to double check the compression gauge used to be sure it's reading right.

But on the assumption it's correct, it would appear that perhaps this cylinder has been losing compression over time, reflecting the drop from ~4600 to ~4100 and then ~3500, and the corresponding poor performance at various times.

So- two questions:

1. does any of this raise any 'aha' moments?
   1. (Not as in 'EEK', I got that moment myself- I mean in terms of 'something to check')
2. if I'm correct in the current state, what's your general assessment of
   1. possible causes
   2. troubleshooting effort
   3. repair effort / logic
   4. what I should do

Please note that while this is a 2008, it's probably only got a few hundred hours on it. Unfortunately, it's typically in the lake uncovered by a roof, and therefore things like the rain pouring in certainly did some unpleasant things.
Thanks for any helpful commentary...

 

[Bondo]

Compression test seems to show 20 at idle, 30 with some throttle applied (EEK)

Ayuh,..... Throttle should be wide open,......

Anything under 100 psi, is not enough,......

For the runnin' problem, I'm guessin' a crab rebuild is needed,.....

 

[achris]

Idling at 1100rpm is a surefire way to destroy the drive. You MUST get that idle speed back down to 650rpm.... 700 rpm max.

Chris....

 

[dgyank70]

Thsbks for the feedback fellas...

On the high idle, how do you mean that could / will ruin a drive, may I ask?

And am I not in a really bad way with water in a cylinder already, or could it be water coming back through the exhaust, and not necessarily a head gasket or something horrific?

Coincidentally we had just noticed one side of the engine being hotter than the other (but maybe because the cylinder wasn't firing I guess).

And why would it have been able to run so nicely when I first left the marina, and then suddenly reverted?

Just grasping, I know ...

 

[nola mike]

Gears clunking together at high speed is more wear than gears clunking together at a lower speed.
You can try a wet compression test, but would probably get more info from a leakdown test. 30 is a problem, I'd also pull the valve covers and make sure that you don't have a stuck valve or broken spring or something, but I don't see many causes that wouldn't require you removing the head. Did I miss the other compression numbers? Water coming into the cylinder is maybe what's causing the problem, but the numbers mean damage has already been done.

 

[achris]

..On the high idle, how do you mean that could / will ruin a drive, may I ask? ...

The drive uses a dog clutch, not a friction clutch like your car. It has 2 states, in gear and out of gear. It's all or nothing, no in between. When the idle is high and you select a gear, the speed of the engine is transferred fully and immediately to the drive gears. A sudden high speed load will destroy them!

Merc even released a service bulletin about it. See if I can find it.

Chris.........

 

[Scott Danforth]

think of it this way, imagine taking a hammer and slowly bringing it to your forehead, that is shifting below 650 RPM

now swing that hammer as fast as you can against your forehead (without any reduction in speed), that is shifting at 1100 RPM

the same thing is happening to the dog clutch in your drive

 

[dgyank70]

OK- I definitely got it on the idle... Thanks for clarifying, and we will work on that for sure. But I think my bigger concern is currently around the water in the cylinder thread... Any thoughts on that?

Am I not in a really bad way with water in a cylinder already, or could it be water coming back through the exhaust, and not necessarily a head gasket or something horrific?

Coincidentally we had just noticed one side of the engine being hotter than the other (but maybe because the cylinder wasn't firing I guess).

And why would it have been able to run so nicely when I first left the marina, and then suddenly reverted?

 

[FishnFool77]

I once had a 4.3 Merc with water in one cylinder bank. Was coming from a rusted out exhaust manifold letting water in thru the exhaust valve. Heard it was a common issue. The exhaust manifold has water chambers built in to reduce exhaust temp. When those chambers rust out, they can let water into the exhaust side & in to the exhaust valves. Was to the point of hydro-locking (starter having a tough time during initial cranking). A rusty exhaust valve can affect your compression. If that is your problem, I would suggest to initially change the exhaust manifold only. Then drive it awhile, run several serious treatments of sea-foaming the carb, trying to burnoff any buildup on the valves. The type where you inject it direct into the carb while running, letting it sit 5 min & having it smoke horribly. (Directions on can). Might be wasting $10, but it's a whole lot cheaper than a tear down.

 

[dgyank70]

Thanks so much Fool! So it's possible that there isn't already permanent damage done? And the exhaust thread I've been looking at yes, because
a) it has been starting with some difficulty (replacing distributor helped)
b) it has been making a 'chuh chuh' sound, almost like a blockage or something that made us wonder about the exhaust
c) it's only in freshwater, but sits in the water nearly year round unfortunately
d) we actually noticed one side of the engine quite a bit warmer than the other (though that might have been due to the cylinder not firing, and therefore creating less heat; dunno which of the two sides was 'normal' temperature, just that they were different)

The oddity of the timing is that it had one good run leaving the marina, then suddenly reverted to 'normal' ('bad'), at which point we verified water and poor compression.

So- I think definitely it'll be worth reviewing the exhaust and also the water inlets just in case...

 

[achris]

Pull the manifold and pressure test the water jacket. I recently had a 350MPI fill #6 and #8 with water. Ended up being a hair-line crack in the exhaust manifold jacket. Pressure test by making a block-off plate for the elbow surface and using plumbing fittings to block the hose fittings. I pumped it up to about 10psi and then squirted it with soapy water. The crack was so obvious, bubbles aplenty.

Chris......

 

[dgyank70]

Thanks Chris. I guess in your case though, you still had compression in those cylinders... Maybe also with Fool...
I'm still trying to get a handle on how bad my situation is, not really sure...

 

[nola mike]

See post #5.Valve covers, leak down will let you know the damage. I still see head removal in your future.

 

[dgyank70]

Thank you- this is so hard to navigate. Marina is booked for weeks, turns out these Nauticstars are really bad for engine access (knew that from oil / plug changes), no idea how to tackle any of this without pulling the engine just for access reasons... But I also don't know how to do things like the leakdown (I'm an ex software guy- hardware is beyond my brain capacity).
Argh.

 

[achris]

... (I'm an ex software guy- hardware is beyond my brain capacity).
Argh.

I'm an ex hardware guy ('core' memory and programming in octal). We know that software guys... :lol: :lol: :lol:

Chris....

 

[nola mike]

Thank you- this is so hard to navigate. Marina is booked for weeks, turns out these Nauticstars are really bad for engine access (knew that from oil / plug changes), no idea how to tackle any of this without pulling the engine just for access reasons... But I also don't know how to do things like the leakdown (I'm an ex software guy- hardware is beyond my brain capacity).
Argh.

It's not too hard. You just need a leak down tester and a compressor and Google (or iboats). I got a cheap harbor freight version, works fine. As far as the valve covers, well, as a software guy can you at least open a computer case? Once the covers are off, a stuck valve, broken spring, bent pushrod etc is pretty obvious.

 

[dgyank70]

Chris, as you're a moderator, I'll let you have that one, ha!

But yeah, definitely a true thing. I'm also afflicted with general spatial navigation disorder.

It's a wonder I can even keep track of which light goes on what side of the boat...

 

[achris]

Chris, as you're a moderator, I'll let you have that one, ha!

But yeah, definitely a true thing. I'm also afflicted with general spatial navigation disorder.

It's a wonder I can even keep track of which light goes on what side of the boat...

"Is there any red port wine left?"

My mother's offering....