Mercruiser rebuild low compression on cylinders 2 and 3

[Binderboat]

Finally looking for some fresh advice after seeing several archives -about my 1988 Mercruiser 120 hp 4 cylinder engine/project boat. The story...bought it as is-running with no reported engine problems, needing a lower shift cable replaced. It had not been used for a year and a half. When I got it home it barely ran with lots of black smoke, oil or gas residues in the exhaust water and then the engine oil turned to "chocolate milk" So in talking with a mechanic and online forums I presumed I had a cracked block. So took off the head, but it looked good and compression tests of block and manifold did not reveal any leakage. New head gasket and rebuild and it still ran horribly rich with quick fouling of the plugs until I bought a rebuilt carburetor and then it ran perfectly and all cylinders when tested had good compression 120-150. Water did not get into the oil. The second time out on the water I blew the head gasket between cylinders 2 and 3 running at WOT. ( I had not retorqued the head bolts and also had used the original bolts nor did I use feeler gauges to check for head warpage during the rebuild); (valves, piston/ rings also seems to look ok but were not disturbed). So I just replaced the head gasket, and used new head bolts- compression testing was good120+ in each cylinder- started right up but had a metallic ping with each revolution. I ran it only 10 minutes or less. I jump to the (?mistaken) conclusion that it was a loose valve lifter and took off the valve cover and to my surprise nearly all the valves seemed to be out of adjustment and close to zero lash- so I tightened them all as per the manual again and tried to restart only to have it spit and backfire without starting. I played with the valve adjustment and confusingly noted that now cylinders 2 and 3 again had no compression as if the head gasket was blown again in the same spot- a thin strip between the cylinders. The metallic pinging went after tightening lifters. Also cylinder 4 seem to loose compression when the valve lifters were tightened to 3/4-1 turn past 0 lash. Now wondering what to do next and looking for help realizing I probably ned to pull the head again and look more carefully for a problem- perhaps another blown head gasket, a cracked block or head, a stuck or cracked valve head, ?broken valve ring (Still no water in the oil pan) Any ideas would be helpful. Also realize I may need a mechanic to take over- but the 1st head rebuild seemed to work so easily initially.
-Tom

 

[s.hadley81]

When you say zero lash are saying the up and down movement of the push rod or twisting it with your fingers?

 

[NHGuy]

You may also need to be certain your valves are closed before adjustment.
Best way is to remove spark plugs and bar over the engine by hand with a crank adapter. Don't use the crank bolt. They snap off!
Here is a great way to do you valve adjustments.

It's called EOIC, which stands for exhaust opening intake closing.

Turn over the engine til the exhaust valve just starts to move, at this point adjust the intake valve. That's the exhaust opening point adjustment.
Turn it over more til the intake opens and is nearly closed, at this point adjust the exhaust valve. That's the intake closing point adjustment.

Do this for all your valves.

Before setting valves you first find the point where there is no lash. It's done by feel. Very gently adjust the lash with one hand while raising and lowering the pushrod slightly with the other hand. At the point where the pushrod has contact at the bottom and top but there's no force on the valve you are at zero lash. Then you do your adjustment. Most Mercruiser engines are one full turn from this point. (3/4 ain't bad BTW) If overadjusted there's poor sealing because the valve is held off it's seat.

 

[Binderboat]

Zero leash for me was when the push rod would just spin when twisted with fingers I can try the EOIC method of valve adjustment. Any thoughts on how to know if a valve is not closing completely of when the valve springs are weak?

 

[achris]

Spinning the push rod can lead to false results. I have done the test. I adjusted a valve using the up-down method and confirmed I had zero-lash. I then was able to turn/spin the pushrod for another 1/2 of a turn on the adjustment nut. I guess if you were really aggressive you could probably spin the pushrod all the way until the lifter bottomed out. That is why the 'spin' method doesn't work, and people end up with over tight valves...

... Any thoughts on how to know if a valve is not closing completely when the valve springs are weak?

Leakdown test. There is also a special tool for checking valve springs in situ. You could also pull all the valves, and have the spring tensions checked.

Chris..........

 

[s.hadley81]

I've tried twisting the pushrods to establish zero lash with poor results. I would try to set your lash feeling the up and down movement of the pushrods and then set the rocker nut with 3/4 to 1 turn. I'm thinking maybe your rockers are too tight and the valves are staying open. As achris said if you adjust your rockers using up and down to set lash and your compression is still off than its time for a leak down test!

 

[Binderboat]

Pulled the head again- and the head gasket was blown between cylinders 2 and 3 again! I wonder if the last gasket was faulty in that it looked like the metallic edge in one spot was encroaching into the piston chamber. Also considering whether the head is warped/ or there is a slight depression between the 2nd and 3rd cylinders. It looks pretty flat - guess I will sand it with a straight edge surface and see. Turns out the valves where not the real problem although at one point they were over tightened. Their seats and springs logo good. Valve adjustment advice has been helpful- thank you

 

[harleyman1975]

Did you have the head done? Is the block true? Is the head true? Rebuild means different things to different people. To me it is a complete major overhaul with block decked and cylinders bored, new pistons, crank turned and head shaved valve guides replaced and valves ground.

 

[Bondo]

and the head gasket was blown between cylinders 2 and 3 again!

Ayuh,.... Welcome Aboard,...... I'd sooner think the head or block is cracked or worn 'tween the cylinders where the gasket is blowin' out,.....

 

[airshot]

When I had issues with my 3.0, I was told the heads are prone to warp so I just sent mine out to get redone. Don't waste your time checking valve springs as they are only a couple bucks each for new ones. My head was tested for cracks, shaved/straightened, all new valve springs, new valve seals and valves ground and seated all for less than 300 bucks. Was recommended by the good people on this forum to use a different head gasket besides the metal mercruiser ones which do have a habit of blowing thru. She is now purring like a kitten.