200 HP Mercury started knocking

[CardsFan84]

I have a 93 Mercury 2.5L 200HP carburated outboard on a 90 Ranger 391V. The motor was running fine for three months then two weeks ago while running to a spot in a bass tourny the motor started to knock. I shut it down immediately only starting it one more time to try to detect where the noise was coming from. After two weeks of having people look at it and looking at it myself here is what I have found. The compression of cylinder number four is 0psi. The other five cylinders held between 110 and 120 psi. The flywheel was turned with the plug out and I could see the piston moving (does this eliminate a broken rod?). I am trying to determine all I can before paying 350-450 bucks to have the motor torn down. Is there anything else to check and what hope do I have of spending less than $1000 on this motor?

 

[CardsFan84]

Re: 200 HP Mercury started knocking

I know the questions from the original post were centered toward cost, but I would really like to figure out what is wrong with this motor regardless of cost. The motor has a "knocking sound" but could it be from a broken reed? I am confused on what could cause the lack of compression in the cylinder with the piston moving. Is it possible to break a reed and have this "knocking sound" without damaging the cylinder sleeve, rod, or any bearings? Is there any adivice for pulling and inspecting the reeds?

 

[Maxz695]

Re: 200 HP Mercury started knocking

O compression points more to a broken or frozen ring. Try spaying some oil gas mixture into the cylinder the recheck compression. If this doesn,t work try shooting PB Blaster into the plug hole and let sit a day or so and then recheck the compression. If all fails I think you need to tear it down.

 

[Maxz695]

Re: 200 HP Mercury started knocking

From what I have read tonight while searching my own issues, I read that an over sized piston weighs the same as a standard piston. 1 Cylinder can be bored out without boring the rest of the cylinders being bore (They may get tired but never bored). This is good news! Therefore if the crankshaft has suffered no damage yet and I say yet! then your chances of having this fixed at minimal costs are good. If you can do most of the work yourself like removing the power head and break it down for boring then your gonna save a ton of money. Of course your gonna have to have the crank mic ed for specs and the rod. Replace the bearing is recommended as well. Good luck

 

[Chris1956]

Re: 200 HP Mercury started knocking

Knocking is usually not a piston/ring problem. Obviously, zero compression is a ring/piston problem. I would therefore expect you will need a bore job and new oversize piston/ring set, plus some new bearings. These replacement parts still need to be identifed. They could be main bearings, connecting rod bearings (likely) or wrist pin bearings. the only way to know is to dissassemble the powerhead and inspect the bearings and piston/cylinder/rings.

if you do the work yourself you can get by for less than $1K. If not, you can look into a rebuild powerhead you install yourself for $2500 or so, or have yours fixed for more $$.

One more thing. Normally there is an underlying cause to powerhead failure. In may be a clogged carb, overheat, incorrect timing or some combination. This needs to be figured out and fixed.

 

[CardsFan84]

Re: 200 HP Mercury started knocking

I have disassembled the cylinder head on the side with the damaged piston. The pictures I took show some serious damage with metal chunks everywhere.


I am planning to do the work myself to keep the cost to a minimum. I have the service manual but this will be my first attempt at a major overhaul. The cylinders in this engine are sleeved, do I have to take this head to a machine shop to have a new sleeve pressed in?

 

[Chris1956]

Re: 200 HP Mercury started knocking

Normally you bore the cylinder oversize. There is a max boresize, and if the damage is too severe, you cannot bore them enough. You need to check it out.

I do not know if sleeves are replaceable in that motor.

 

[Faztbullet]

Re: 200 HP Mercury started knocking

The piston damage from a broken skirt will cause a knock sound(slap) and yes that block can be sleeved but by a marine machine shop....

 

[CardsFan84]

Re: 200 HP Mercury started knocking

Well, I have torn the motor down as far as I can while it is still attached to the hull. I am having a little trouble getting the lower cowl off. I can't seem to get a socket on the small bolt above the power tilt/trim. The lower unit is already off and I'm thinking of just pulling the rest of in one piece. I am assuming there is no way to get the powerhead off without removing this lower cowling? I will post more pictures tomorrow, but I found a lot of metal in the crank housing. I hope the crankshaft is not damaged as I would really like to salvage this motor, and I think a damaged crank might make this a little too pricey.

On a side note, I have an opportunity to get a 98 Mercury 150 EFI for around $1500. The only issue for me is that it has sat for at least a year and I haven't seen it run. I am going to do a compression check and try one more time to see it run, but I would like some thoughts on this option.

 

[CardsFan84]

Re: 200 HP Mercury started knocking



Here are some pictures of the motor. I am borrowing a cherry picker to pull the motor tomorrow so I should be able to inspect the crank in the next couple days. Anything specific I should be looking for other than obvious damage?

 

[Faztbullet]

Re: 200 HP Mercury started knocking

Rod or wrist pin bearing failed......