68 Mercury 200 (20hp) engine rebuild

[Arkman]

I changed the title to reflect what this post is actually about.
I just figured that it would make more sense when people searched it for relevant info.

I was out on the lake a couple days ago and was cursing for about 2 min (if that) and then my motor lost power then died. I looked over everything and it all seemed ok. I started it up and it started a bit hard then ran for a second and I noticed no water was coming out the hole. I shut it down instantly. Trolled back to the landing and loaded up.

Today I got to taking apart the lower unit and it looks like the metal collar separated from the rubber fins on the impeller. The rubber part of the impeller just spins around that fin. I'm assuming that is my loss of the tell-tale stream (is that what the pee stream is actually called).

My big concern is damage to the engine itself. It wasn't running very long without water but certainly long enough to hurt it. How can I tell if I did any damage to it? I replaced the impeller at the end of last season (maybe that was a mistake, should I wait till after winter?) and everything looked ok, but now look at one of the plastic tubes that comes out of the impeller. Its all melted.

I'm considering replacing the entire water pump but I don't want to do that unless I'm some what sure the actual engine is ok. The plastic that makes up the water pump is becoming cracked and damaged so I figure I should replace it.

 

[Hackamore]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Your loss of power and stopping was most likely from the engine getting to hot and seizing up. This usually creates damage to the rings and, or the pistons. You can pull the heads with out to much problem, it is a simple job. This way you can look at the piston tops and tell if the rings are bad. Also look for scored marks in the cylinder walls. If the pistons are hurt, put it back to gethere and rebuild or replace. If they are not hurt then you can buy head gaskets and reassemble. May not have the same power but might run for awhile for you. Good luck

 

[Jacket4life]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

You can do a compression test without removing anything. If you have piston damage, it will show up there.

 

[emckelvy]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

There is no cylinder head to pull on this motor, the cylinders are integrally cast with the block. Compression test as the O.P. said will tell you if you did serious damage to the motor. It doesn't sound like it seized-up at least, so maybe there's hope.

However, it did get plenty hot enough to melt the plastic water tube guide sleeve. It's gonna be real hard to get the water supply tube back in the pump, without that guide sleeve to catch it.

Check out the stainless steel 'cup' in the water pump cover, and also the stainless pump plate, for damage. If they're not too scored or otherwise damaged you can re-use them.

Here's a parts diagram so you can pick out the parts you'll need:

http://www.boats.net/parts/search/M...R HOUSING ASSEMBLY, COMPLETE(CONT)/parts.html

HTH & G'luck...........ed

 

[Jacket4life]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Oh, and I said you didn't have to remove anything, but you will have to remove the spark plugs.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Thanks everyone. I'll do some research on how to do a compression test. Maybe I'll ask one of the shops in town how much it would be to do it real quick. I don't know if I have the right tools for that. What should I look for as far as compression to see if there is damage or not?

This motor is not really worth all that much but has a lot of sentimental value so I'd really like to fix it.

I checked the metal plate between the two parts of the water pump and that seems fine. No visible damage. I also inspected the metal cup the impeller sits in and that seems fine too. I think they are both ok to reuse.

When you replace the impeller do you always have to do the two gaskets between the two plastic parts of the water pump. Or am I doing something wrong that is wrecking the gaskets.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Is there a special tool for compression testing a boat motor or will any compression tester work. I can rent one from the auto parts store, I just want to make sure it will work.

 

[Jacket4life]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

There is a thread on here about how to do it properly. As far as what you are looking for, the two main things are: Compression should be in the 115-150 psi range, and you should have NO MORE than 10% difference in your highest and lowest.
Ex: Mine this year were:
#1: 125 psi
#2: 120 psi
#3: 115 psi
#4: 125 psi

My #3 will be the one that bites the dust, I'm guessing. Also, it is very close to the 10% rule. I rebuilt the carbs this year (may be the first time ever, judging from the condition!) I baby it along, hoping I can hold it off for another year then sell the boat and get another one to mess with!! Although, it may have improved since the rebuild and other things I've done this spring.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Well I got a compression tester and I think its bad news. I rented one from Autozone to test. I'm not sure that I did it right or the tester was working properly.

Normally does the tester keep your compression reading? The needle would shoot up to about 60 psi then go back down after I pulled. It would just jump up and back. On the bottom cylinder I got up as high as 90 but only once. The top cylinder the needle didn't move.

I think I'm going to try to get a different tester just to see, but it doesn't look good.

Sad day. I worked so hard last year getting this thing running. It ran great until saturday

 

[Wingedwheel]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

The tester should hold your reading untill you hit the pressure release. Did you get one with the screw-in adapters or the rubber compression cone. I've never had too much luck trying to turn a motor over and hold those for an accurate reading.

 

[cobra 3.0]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

If it's a hand pulled motor, 60 psi is still fine. The tester should keep its reading and only let the pressure out if you press the valve in the tip of the tester (the part that goes into the cylinder hole) Unless this tester has no valve in the tip? (You can get a pressure tester at Walmart for quite cheap.)

No pressure at all for one of the cylinders? Put your finger over the spark plug hole and crank the engine. Do you feel a push on your finger?

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

It had a screw in connector. I pulled and my wife watched the needle. It does have a release valve on the side, it just decided to release on its own or something.

Does it make sense that there is zero compression at all on the one cylinder?

 

[cobra 3.0]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Did you put your thumb over the hole and pull. It should pop your thumb off with the pressure. Your pressure gauge I don't trust.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Cobra- I didnt see your reply right away...we replied at the same time. So I went back out and tried it again. Same result. I got no reading off the top cylinder. I tried holding my finger over it and there was defiantly a push. According to my highly sophisticated compression tester (my wifes finger) she claims it feels really no different between the two. Obviously that means almost nothing as far as how close they are but I would think that if it was truly zero compression you wouldn't feel anything. Also, if there was a drastic difference I'd think you'd be able to maybe feel it at least a little.

I think I need a new tester.

 

[cobra 3.0]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Zero compression does not equal any push whatsoever. I would be surprised if only one cylinder took such a big hit and the other is ok.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

I'm really confused. I got a different gauge I measured the bottom cylinder and pulled 5 or 6 times. Until it stopped going up like I read. It got up to almost 130 psi. Still noting at all on the bottom. This doesn't make sense to me. Could I be doing something wrong? I can see the piston move and I can feel air pushing out when I pull, but the gauge reads nothing. I would think there should be some kind of compression. If I damaged the top cylinder that much wouldn't you think there would be damage to the bottom as well?

 

[cobra 3.0]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

Those are my thoughts too. Why would one blow completely and the other show great pressure readings? Is there something wrong with the spark plug hole? Do you get blow by air when taking your readings somewhere? Maybe the outside engine head is cracked or a gasket is gone on the upper one?

When you put your finger or thumb over the whole, push as hard as you can and let someone pull the cord and see if there's a significant amount of pressure to push your finger off the hole. Or try jabbing a rubber piece or a piece of wood dowel wrapped with electrical tape tightly into the hole and see it it will push it out. Careful, you don't want to lose the piece inside the piston.

If the pressure looks good, I would try to start it and see if it will run even if there is no water in it. Just don't run it long. Just enough to see if it works.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

I will inspect it a bit more tonight. As far as I can tell there is nothing wrong with the hole. It looks just like the other one. Could it blow the upper part of the gasket and do nothing to the lower part? That seems strange to me, but maybe that happens, I don't know. Is there a good way to inspect the gasket to see if it looks like its gone? How about if I cracked the head? Can I inspect for that anyway? If I did crack it can I fix it some how?

I thought about sticking something in there..or putting tape over the hole. Maybe I will try that tonight. I'm hoping to get parts tomorrow for the water pump, maybe I can try to put them in and see how it goes.

 

[Arkman]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

As I'm doing more research while bored at work I have found similar instances where just the head gasket is bad (if understand the other post I'm reading right). Could this be the case?

Does anyone know how hard it is to replace the head gasket? It seems like the head(is that what the part that you screw the plugs into is called?) is almost nothing to take off. Remove a couple parts then unbolt it. Is this correct? I figure there is at least some damage in there that I need to fix so I might as well pull it off and take a look right?

 

[cobra 3.0]

Re: Impeller Failure. 68 Merc 200 (20hp)

Re: Impeller Failure. 68 Merc 200 (20hp)

It's as easy as you say and yes you can only blow the gasket in one little spot that would only affect one cylinder. You need to determine where there is air leakage around the gasket. Put the sparkplug back into that cylinder only and crank away and feel around the upper cylinder area. if the actual head is cracked, there isn't much to do. If it's a gasket, all you need is a new one and know the torque sequence and torque setting (how many foot lbs to apply to each screw. You need a torque wrench.)