Replace Head Gasket or Replace Engine

[justthisguyyaknow]

Let me preface this post by explaining I'm not particularly mechanically inclined, and make no claims that I am. I'll be the first to admit by ignorance about boat drivetrains.

The problem is on a Mercruiser 120 4-cyl in a 1973 Starcraft. Ran just fine until, quite suddenly, a loss of power. I shut off the engine while awaiting the readiness of a kneeboarder, and when I restarted it, a noticible loss of power.

The temperature gauge read quite low (and I've since found that the wire to the sender is loose). Therefore, it may have overheated (with a faulty temp gauge, isn't that fun).

So, I shut the engine off as quickly as I could get out of traffic, and towed it to the trailer. I may not know much, but I knew I didn't want to cause any more damage than what was/may have already done.

Took the boat to the mechanic (one listed on the Mercruiser site, and good reviews). I asked the mechanic to check the impeller, and find the cause of the loss of power. Mechanic performed compression test (I hate these, it's like getting a dreaded report card). 115-30-30-115. Darn!

So, I think, blown head gasket. This would cause a loss of power. I'm thinking check impeller (and rest of cooling system and replace head gasket).

Mechanic than does a cursory inspection (through the spark plug holes) of the #2 & #3 cylinders. He finds that there is very noticeable damage to the cylinder wall of at least one of the cylinders. Due to this, he feels my best bet (I'm on a budget) would be to get another engine and have it checked out before installing.

This seems reasonable, given that there is the pronounced damage to the cylinder wall(s). There is also oil on the plugs, so I'd hate to just put a new head gasket on to an engine with lower-end damage (throwing good money after bad).

Obviously, the cooling system needs to be checked out either way (and the temp sender mended).

Now, my apologies for being long-winded. So, questions:

1. Am I right in assuming it unwise to ignore the lower problems in the engine, and simply replace the gasket? I don't mind it using a little oil, but I don't know how long it would really last.

2. What type of tests should I be sure the mechanic performs on my (yet to be located) "new" engine to be sure I don't end up in the same boat (pun intended) with the new engine as with the current engine?

3. Any ideas on where to LOOK for a 120/140 engine?

 

[Maclin]

Re: Replace Head Gasket or Replace Engine

I would have to remove the head and look for myself, unless the mechanic used one of those snake eye cameras and got pics of the damage. If I did not find a holed piston or severe cylinder wall scoring I would probably spend some dough to have the head looked over and freshened up and re-install with a new head gasket then check compression again.

If you cannot do any of that type of work then I would just follow his advice. He probably knows you and engines and is trying to get you a longer term fix and in the end save you both time and money.

 

[stonyloam]

Re: Replace Head Gasket or Replace Engine

being mechanically inclined is sometimes just a matter of giving it a try. Grab a manual and socket set and yank that head off. You can't hurt anything at this point and you may find that you are more "Mechanically inclined" than you think. Good luck.

 

[45Auto]

Re: Replace Head Gasket or Replace Engine

Where are you located at? You can easily have the head off a 120 in about half an hour. A new head gasket is about $75. Probably be worth giving it a try replacing it, at least you'll get a close look at the condition of the cylinders. Might be somebody close enough willing to help you out for a couple of hours for a six-pack or so .....

 

[JustJason]

Re: Replace Head Gasket or Replace Engine

I agree with pulling the head.... Looking at the combustion chambers and the underside of the valves will tell you a bit more than a boroscope can. If the cylinders are deeply scratched and your missing chunks of the piston crown your looking a new motor.
If the pistons look used but okay, if the scarring is light, if the combustion chambers look uniformly carboned.... you may get out of it with a head gasket.
Unfortuneately it may cost you 2 to 3 hundred to find that out.....

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

stonyloam: Good point, I can't do any more damage at this point.

45auto: I'll bet I'll FIND a way to take way more than 30 minutes to pull the head, but...

I'll go ahead and pull the head and see what I can see.

 

[45Auto]

Re: Replace Head Gasket or Replace Engine

Don't pull the intake/exhaust manifold off the head.

Take off the rubber coupler at the back of the exhaust riser, disconnect the carb linkage and fuel line, take off the valve cover, take out the headbolts. Get a buddy to help you lift the whole thing out.

Good luck!

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

45auto: Thanks for the tip. I did NOT remove the manifolds, and lifted (ouch) the whole top right off (with some help). Saves a few $'s on gaskets right there.

Gotta get some sleep, but how about that head gasket?!

Once I get back at it, I'll inspect the head and cylinder walls and see if I may be able to just re-gasket and bolt back up. Fingers crossed.

 

[45Auto]

Re: Replace Head Gasket or Replace Engine

That's pretty bad! Hopefully replacing it will get you back on the water. Check the block and head closely where it was blown and make sure there's no "flame erosion" of the metal. If they're gouged up from the leak it'll just blow again. Shouldn't be any problem since you didn't keep running it for weeks once you noticed the problem (unlike some people I know ....). Check the head closely while you have it off to make sure it's not warped. If it's out of flatness specs have it surfaced (VERY light machine cut so it's perfectly flat) while you have it off. Hopefully everything's fine mechanically, and the head gasket just failed from "old age" (too many cycles).

I've got a similar problem on the starboard engine of a 1987 30' Sea Ray. 350 Mercruiser, original engine, 600 hours. 2 center cylinders on the starboard bank are showing low compression. Not as bad as yours, 100 PSI vs 150 PSI on all others. I'm in the process of pulling the head now, hoping I can just replace the gasket and that'll cure it.

Good Luck!

 

[Maclin]

Re: Replace Head Gasket or Replace Engine

Good job so far! That gasket explains the compression numbers you gave. Now the condition of the head (warping and/or needing valve job) and block and cylinders can be determined. You may get out of this with just gaskets and labor and some knuckle gouges, good luck!

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

The head is in very good shape, actually. I'm fortunate.

I did notice a bit of scaring in a very small area between #2 and #3 cylinder, but it's too narrow to insert a gauge. I did put a flashlight behind a straight edge and was able to see a bit of light getting through, though. Other than that, the head is perfect.

The valves seem in very good shape as well. Combine this knowledge with my observation that the head bolts were SURPRISINGLY easy to remove, and I'd say this engine (at least the top half) was worked on not too long ago.

After putting this back together, it will be on to figuring out how to replace my impeller. I have a suspicion that the impeller may be weak, which may have contributed to the blown gasket. Either way, I'll sleep better knowing it's a fresh impeller.

Is there anything else I should be checking while I've got the head/manifold removed?

 

[Bifflefan]

Re: Replace Head Gasket or Replace Engine

When you put the head back on, read, and then reread the proper torque sequence. and follow it. Toqure the bolts in 3 steps. 1/3 or max, then 2/3 of max, then the max rating for the head bolts. start in the middle and work your way out doing all the bolts at one torque then moving on the next higher torque.

Its not so bad finding that you have hidden skills

 

[soaringhiggy]

Re: Replace Head Gasket or Replace Engine

I hope that you kept all of the push rods in the order and orientation that they came out of. If you replace them other wise the ware will most likely be accelerated.

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

soaringhiggy: Yes, I did keep the pushrods in order and orientation. Wasn't sure it was necessary, but to play it safe, it did.

bifflefan: I shall torque the bolt set in phases as you mention. Wouldn't want to warp the head at this point.

I have a question about the exhaust flapper. Google and forum searches reveal that many people recommend AGAINST putting in a flapper, since it can disintegrate and partially clog up the exhaust passages. Others seem to feel that it's a good thing to have.

Removing the head reveals to me that my flapper is not present. (I'm assuming the exhaust flapper hinged on the piece in the picture shown).

Before bolting the head back on, now would be the easiest time to replace it. Any opinions on whether the exhaust flapper is better replaced or left out entirely?

 

[fishrdan]

Re: Replace Head Gasket or Replace Engine

Replace the shutters, they will keep water from swamping the exhaust (and cylinders) if you take a wave up the stern. You need to drill out 2 of the rivets and replace them with SS screws and nuts.

Any pics of the cylinders?

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

Yes, here are some shots. Pistons are up in C1 and C4, since I was most concerned with C2 and C3 (the initial problem was low compression on 2&3).

Cylinder #2 (piston down) shots:

Cylinder #3 (piston down) shots:

My Canon is perhaps not the best camera in the world to take these shots.

 

[stonyloam]

Re: Replace Head Gasket or Replace Engine

Congratulations, wasn't so bad was it? Only one other suggestion (only because you said that you are inexperienced with this), get yourself a good "modern" torque wrench, one that you set the torque with the handle and it "pops" when you hit the proper torque. Much easier to use. You can probably borrow one from Autozone if you don't want to buy one. BTW those cylinder walls look just fine. It will sound extra sweet when you start it again.

 

[justthisguyyaknow]

Re: Replace Head Gasket or Replace Engine

Yea, I got the click type of torque wrench. Much easier that way.

I did waste a head gasket, though. Didn't get the head straight on (was trying to put it on by hand with a helper), and ended up crushing the edge of the first gasket. (I used a winch the next time).

I've got my outdrive (lower section) off at the moment since I replaced the impeller. Once it's back on, I look forward to firing it up. The impeller was toast. Probably moved a small amount of water, but that's it.

Thanks to everyone for the tips along the way.

Was a fairly painless process.

 

[Ritchie555]

Re: Replace Head Gasket or Replace Engine

Hey, the block looks like it has erosion between the 2/3 piston bores. If the block is not "skimmed" back to a flat surface, your new gasket will blow through at that spot again.