Compression test results 4.3L V6 2005 what do You think?

[Lou C]

If perhaps water is getting to that cyl from a leaky exhaust manifold to elbow gasket the exhaust valve in that cyl might not be sealing 100% due to rust. Might want to do a leak down test. If it shows leakage past the valves esp the exhaust valve I’d remove the exhaust manifold to take a look. This is a 2005 Merc? Should have dry joint exhaust then; not so likely to leak as the old design. If in salt water though could be corrosion in the manifold.

 

[tpenfield]

Will do, any ideas Why only this one cylinder is weaker? I don’t know the weak points this V6 blocks but would like to know

The #5 plug looks like it has a spot of rust on the isolator - so perhaps you've had some water in the exhaust manifold. A leak test will tell you where you are losing pressure on that cylinder. Sometimes the valve seat gets pitted if you are getting water where it shouldn't be. Maybe it's time to check the manifold/riser gaskets and mating surfaces.

Also, a 16 year old engine could have some carbon build-up and sometimes bits of carbon can be trapped on the valve seat causing some leakage. A de-carb of the engine could improve the compression.

A 10% loss of compression is not terrible - but you are doing a good thing by being mindful of it.

 

[Kubaat3lover]

I really like work on my boat, im still learning but its fun, also im from Europe and its VERY VERY difficult for a good mechanic in my area...
Do You think its rust? For me its just more bright brown colour only (??) doesnt look like rust.
I did use Power Tune from Quicksilver and sprayed into the carb throat but dont know if it works or not.
I have to change my valve cover gasket and will remove the risers and exhaust manifolds to check it, I havent touched it since I own a boat since 2017 and also it looks like is OEM from 2005 (bolts are untouched by the wrench).
I was wondering maybe to put some snake oil into the cylinder for 1 night and blow it off by compressed air, what do You think??

And question regarding leak test, to what PSI i have to blow the cylinder? And the cylinder #5 have to be in TDC position ? how to set it by hand??
take care

 

[salvageyard saviour]

About the snake oil, I won't say do or don't.
My own experience with an inline 4, it was impossible to get it all out the next day. The vapor lock when I started it broke the long insulators of all 4 new plugs ! Was lucky to not blow the head gasket or bend a rod.

 

[Lou C]

Question:
When you did the comp test was the engine warmed up and throttle valve all the way open? If not the results might not be as accurate as they could be.
Also, I’m not convinced that the difference you saw is really enough to be concerned about on a 16 year old engine. I would definitely make sure the exhaust is not leaking back into the cyls ESP if you are in salt water. Leaky manifolds are the main killer of inboard marine engines. The Merc dry joint exhaust is not prone to leaking at the joint because the exhaust gas and water passages are separate but they can still corrode internally and leak into a cyl.
If you remove the elbow & manifold you will clearly see rust trails and you’ll be able to see the exhaust valves to inspect.
Since I’ve had this boat (19 years) I’ve changed the exhaust 3 times the last in 2017, and those I had off recently to check.

 

[Kubaat3lover]

Yes engine was warmed up @ fast idle in marina, WOT during this test.
Ok probably next week after winterization I will remove the exhausts and let You know.
Any advice when removing exhaust manifolds ??

 

[Scott06]

Question:
When you did the comp test was the engine warmed up and throttle valve all the way open? If not the results might not be as accurate as they could be.
Also, I’m not convinced that the difference you saw is really enough to be concerned about on a 16 year old engine. I would definitely make sure the exhaust is not leaking back into the cyls ESP if you are in salt water. Leaky manifolds are the main killer of inboard marine engines. The Merc dry joint exhaust is not prone to leaking at the joint because the exhaust gas and water passages are separate but they can still corrode internally and leak into a cyl.
If you remove the elbow & manifold you will clearly see rust trails and you’ll be able to see the exhaust valves to inspect.
Since I’ve had it (19 years) I’ve changed the exhaust 3 times the last in 2017, and those I had off recently to check.

this is good advice, especially if you have to remove the manifold to do he gaskets look for streaks of water or rust isnide it and exhaust ports

 

[Kubaat3lover]

How about some bolts? Ive read somewhere but could find this thread that long bolts are usefull while removin exhaust ?? or just unscrew and pull it back ??

 

[alldodge]

How about some bolts? Ive read somewhere but could find this thread that long bolts are usefull while removin exhaust ?? or just unscrew and pull it back ??

Get some 3/8 x 16 all thread and cut so they are a inch or so longer then needed.
Remove the bolts on the ends of the manifold. Screw in the all thread, then remove the rest of the bolts from the man. Slide the man off the all thread.

This way you don't have to hold the man

 

[Scott Danforth]

How about some bolts? Ive read somewhere but could find this thread that long bolts are usefull while removin exhaust ?? or just unscrew and pull it back ??

or simply use an electric impact. crack all the bolts loose, then with your dominant hand, grab the manifold, with your non-dominant hand, use the electric impact to zip off all the bolts

to assemble, just line up the two outer bolts, and drop the gasket in place (the outer holes are slotted to do just this)

using the longer bolts does help some people, however not required.

 

[Lou C]

Yes engine was warmed up @ fast idle in marina, WOT during this test.
Ok probably next week after winterization I will remove the exhausts and let You know.
Any advice when removing exhaust manifolds ??

Well you’ll need:
A pair of manifold to cyl head gaskets (regular automotive Chevy 4.3 ones are fine)
Merc OE manifold to elbow gaskets
Razor scraper to scrape off old gaskets
Mill bastard file to clean up sealing surfaces if reusing the manifolds & elbows. Now if you’re in freshwater you may be able to reuse them but in salt if they have not been changed in the last 7/8 seasons they are past due.
I like to take a manifold bolt to the hardware store, get 2 longer bolts of the same thread pitch & cut the heads off with a hacksaw. Then cut a slot in the stub for a screwdriver. Now thread these into the first & last holes in the head. Then slide on your gasket and slide on the manifold. This saves you from having to try to line up the heavy manifold & gasket & bolts. Then install the remaining bolts; remove the two headless studs and install the last 2 bolts. Torque to spec. Install elbow the same way. If using used the sealing surfaces must be very clean & level. After warm up retorque.
If reusing the bolts take the time to clean out the threads and test fit first to make sure they thread in. If the holes in the cyl heads are rusty I’d clean em up with a thread chaser.

 

[Kubaat3lover]

Thanks, Y'aal!

 

[Lou C]

or simply use an electric impact. crack all the bolts loose, then with your dominant hand, grab the manifold, with your non-dominant hand, use the electric impact to zip off all the bolts

to assemble, just line up the two outer bolts, and drop the gasket in place (the outer holes are slotted to do just this)

using the longer bolts does help some people, however not required.

Lol this depends on age and orthopedic difficulty if you’re old enough for Medicare use the bolts!
the V6 manifolds aren’t that bad the V8 ones are probably 45 lbs each.
V6 manifold gaskets are not slotted but you could just cut a slot.