mercruiser 140 low compression & cracked exhaust manifold

[shuckerdaddy]

Just wanted to say thank you to this forum - I bought my first project boat last year and wouldn't have her running without all the great threads here. About the boat - unsure of prior owner's history of winterizing...Also shame on me for not doing a compression test. Bring on the comments.

1969 Mercruiser 140 - GM 4 cyl inline, serial #2582xxx

Runs weak with a chuffing sound, and has a cracked exhaust manifold which leaks a bit of water into bilge.
Compression test results from #1-#4 cylinders: 60, 90, 110, 125
Plugs look cleaner (less carbon) on the cylinders with lower compression.

1. Where should I go from here? I'm afraid of #1 and #2 cylinders being a head gasket or worse.
2. Wondering if cracked exhaust manifold (or gasket) would bring compression down at all? Is it worth replacing exhaust manifold before I pull the head?
3. How bad is 60 psi? Could it be anything to do with valves not seating properly?

Thanks! Turn sound on for video below (sounds like an exhaust manifold leak right?)

https://imgur.com/kr5GwY5

 

[Lou C]

It's a 1969, do you know if it still has the original engine? Sounds like a bad exhaust leak. Exhaust manifolds crack because people don't winterize their engines. What are your plans for this boat? It looks to be in good shape besides the engine. If you want it to be reliable I'd be thinking about a repower, unless all it needs is a top end overhaul. You did a compression test, now do a cooling system pressure test and leak down test to find where the problems are. Normal compression test results should be around 150 psi. These are all low, but the 60 is very low, could be a stuck valve that's not closing all the way, worn rings/piston, etc. You have to do more diagnosis but there's a bit of work to be done here.

 

[shuckerdaddy]

It's a 1969, do you know if it still has the original engine? Sounds like a bad exhaust leak. Exhaust manifolds crack because people don't winterize their engines. What are your plans for this boat? It looks to be in good shape besides the engine. If you want it to be reliable I'd be thinking about a repower, unless all it needs is a top end overhaul. You did a compression test, now do a cooling system pressure test and leak down test to find where the problems are. Normal compression test results should be around 150 psi. These are all low, but the 60 is very low, could be a stuck valve that's not closing all the way, worn rings/piston, etc. You have to do more diagnosis but there's a bit of work to be done here.

Thanks for the quick reply - I really appreciate it.

Definitely original engine from 1969 - I found the crack in the exhaust manifold on the underside (see photo).

I don't think a quick JB Weld job would cut it for this crack - I'm probably going to look into a replacing the manifold entirely.

Sounds like my next move should be the leak down/cooling system tests.

I'd like to repower only if necessary. I agree she's in great condition otherwise and is fun day cruising out on the lake. Plans consist of trying to restore the engine to a well running state.

 

[Scott Danforth]

buy a new motor. suggest buying a 1992 or later 3.0 with an Alpha 1 Gen 2 drive and replace the whole long tail. that way you get a much better motor and a much better drive than what you have for very little money.

your motor needs a full rebuild. the cost just to rebuild your motor is more than a late-model bob-tail out of a rotten bayliner

to top that off, the picture above means the motor suffered freeze damage at least once

 

[Grub54891]

I have to add, that looks like saltwater corrosion on top of the crack. I'd replace the motor before attempting to fix/spend money on that one. Just be sure the hull isn't rotted first.

 

[Lou C]

Having repaired my original old motor (1988) successfully, I know these jobs can be done, but truthfully if you want to be able to boat without anxiety and inconvenient break downs, with a drivetrain that old, replacing with new or late model remanufactured makes a lot of sense. It's hard to enjoy boating if you always have that doubt in the back of your mind. Even after fixing mine, 4 seasons later, I still have that doubt, I don't take it out as far as I did before I had the blown head gaskets, etc. I used to take it out in the middle of Long Island Sound but have not taken out out of the local bay since. No problems but just don't have that confidence. Yes its more money but if you want the boat to be used, and useful, and enjoyable in retrospect that is the better plan. As long as the boat is structurally OK, no serious rot etc.

 

[shuckerdaddy]

Update February 16th (originally posted: Jan 22nd)

Didn't have access to an air compressor and decided to forego the leak-down test. Ended up buying an inexpensive borescope.

Was able to identify a blown head gasket using the scope inside the cylinder itself.


I successfully removed the head and confirmed the blown head gasket. Taking it to a machine shop north of Seattle for magnifluxing + resurfacing + valve work.



Also had a massive crack in my intake/exhaust manifold - gasket also leaking onto block (rusty). Purchased new exhaust/intake manifold & new elbow/riser.


Excited to have the head machined properly this next week, a new head gasket, and a new exhaust manifold.


I love this boat. I foresee a repower on the horizon, but for now, I'm going to have fun this summer. More updates to come after machine shop is finished with it.

 

[todhunter]

Post a picture of the block where the head gasket blew, without the gasket in place. Make sure there is no damage (fire slot) to the block deck in that spot, or you may just blow another gasket.

 

[shuckerdaddy]

Gasket removed, but not cleaned yet.

 

[kenny nunez]

Be sure that the machine shop mills the exhaust/intake side of the head also. It looks like at one point the engine overheated, be sure to keep a watch on the cooling system. Seal the head bolt threads with either #2 Permatex or a similar product.

 

[todhunter]

View attachment 356005

Gasket removed, but not cleaned yet.

Clean it up good and check it with a straight edge and feeler gauge. I don't know what the service limit is, but I've gotta assume it's in the manual. Can't tell from the pic if only the HG got torched or if it got a little of the deck also.

 

[Scott Danforth]

.004" is the service limit IIRC

 

[Lou C]

also make sure to clean out the head bolt threads with solvent, a thread chaser and compressed air. They tend to get crudded up with raw water cooled engines. Make sure all your head bolts will thread all the way in before installing the head. That will assure proper torque readings.

 

[shuckerdaddy]

Update March 4th:

Machine shop resurfaced head + new valves + hardened seats. They commented the head was in excellent shape internally, but needed the valve job. I had them throw in hardened seats while it was there.

Cleaned up decking on block with razor blades and found a small torched area. Machine shop and I agreed it was far enough away from the cylinder and would not interfere with gasket. I'm not worried.





Used feeler gauges + straightedge to measure the block decking and .002 barely made it underneath in that area. .003 was not able to make it under at all. The whole decking on the block was under .002 everywhere else between cylinders. Confirmed spec is .004 as Mr. Danforth mentioned above.

Using a thread chaser today to clean head bolt holes on block so they can screw in smoothly for torquing later.

What should I be using to install brand new head bolt threads? I bought them new, but they have a dried powdered white substance on some of the threads already...is this dried never seize?

Permatex #2? Any anti-seize?

Thanks!

 

[SternDriveEd]

Update March 4th:

Machine shop resurfaced head + new valves + hardened seats. They commented the head was in excellent shape internally, but needed the valve job. I had them throw in hardened seats while it was there.

Cleaned up decking on block with razor blades and found a small torched area. Machine shop and I agreed it was far enough away from the cylinder and would not interfere with gasket. I'm not worried.

View attachment 356683

View attachment 356684

Used feeler gauges + straightedge to measure the block decking and .002 barely made it underneath in that area. .003 was not able to make it under at all. The whole decking on the block was under .002 everywhere else between cylinders. Confirmed spec is .004 as Mr. Danforth mentioned above.

Using a thread chaser today to clean head bolt holes on block so they can screw in smoothly for torquing later.

What should I be using to install brand new head bolt threads? I bought them new, but they have a dried powdered white substance on some of the threads already...is this dried never seize?

Permatex #2? Any anti-seize?

Thanks!

 

[SternDriveEd]

As for the head bolts, if they go into a wet hole ( coolant) they need sealant on the threads. Those going into a dry hole I like to lightly coat with engine oil. Definitely chase the threads in the block with a thread chaser. I’m concerned about the straight edge you used to check for a warped deck on the block. A craftsman 12 ruler is not the correct tool. I suggest you borrow one from the machine shop doing the head work for you. It’s a precision straight edge that they used to check the head for warp.

 

[Grub54891]

Recheck the surface after you get it cleaned up properly. Looks like gasket residue on there to me.

 

[Lou C]

Agreed I spent a LOT of time getting the block deck & bolt holes clean on mine. Just when I thought I was done I found SOMETHING else! I used a level as straight edge, not orthodox but worked. Even though this engine had a bad overheat it was flat to less than .001”. And the heads were reman units but I checked them too same thing. 4 years later no leaks running as well as it ever did.

 

[Grub54891]

If you can get hold of some old planer blades, they make good straight edges. The backside is usually pretty flat, you should look them over good before use, but it works for me.

 

[Scott Danforth]

I use a machinist straight edge