Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

[Silly Seville]

Hi Guys,

A quick question with backstory.

What is the most likely culprit if I have good compression readings on cylinders 1 & 2 (135psi) but only 65 & 75 on 3 & 4 (respectively)?

I'm thinking blown head gasket between the cylinders, or possibly sticking/stuck valves on both?

I bought the engine (1983 Merc 120 2.5L 4 banger) in unknown condition. It was stuck from sitting for three years. I got it free and it rotates fine with no unusual noise. The only thing I know for sure about its history is the PO had hit an obstruction and broke the lower unit wide open. The boat then got parked. I'm thinking they didn't shut down the engine, and with a failed outdrive, the engine suffered from a lack of water and overheated. My other thought is simply, the valves are sticking from non-use/rust, as that's why the engine was stuck. (Rings rusted to cylinder walls)

One other possibility but I hope not the case...when I got the engine, it still had water in it! They had not removed the drains. Before I bought it, I got on my back/belly and searched everywhere on the block for signs of freeze cracks. I found none. Strange because the engine had spent at least two Indiana winters outdoors. The (brass) core plugs had not budged. Do you think that the cylinders are cracked in between?

Your thoughts before I tear it apart?

 

[Bondo]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

Hi Guys,

A quick question with backstory.

What is the most likely culprit if I have good compression readings on cylinders 1 & 2 (135psi) but only 65 & 75 on 3 & 4 (respectively)?

I'm thinking blown head gasket between the cylinders, or possibly sticking/stuck valves on both?

I bought the engine (1983 Merc 120 2.5L 4 banger) in unknown condition. It was stuck from sitting for three years. I got it free and it rotates fine with no unusual noise. The only thing I know for sure about its history is the PO had hit an obstruction and broke the lower unit wide open. The boat then got parked. I'm thinking they didn't shut down the engine, and with a failed outdrive, the engine suffered from a lack of water and overheated. My other thought is simply, the valves are sticking from non-use/rust, as that's why the engine was stuck. (Rings rusted to cylinder walls)

One other possibility but I hope not the case...when I got the engine, it still had water in it! They had not removed the drains. Before I bought it, I got on my back/belly and searched everywhere on the block for signs of freeze cracks. I found none. Strange because the engine had spent at least two Indiana winters outdoors. The (brass) core plugs had not budged. Do you think that the cylinders are cracked in between?

Your thoughts before I tear it apart?

Ayuh,.... Pull the head off, 'n Look,....

My guess is the whole motor needs to be rebuilt.... if it's salvageable, at All...

 

[dingdongs]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

put some oil into bore of each cylinder and retest.if it goes up the rings will be the culprit.the head may have cracked but what are the plugs like as in rusty internally or once manifolds off you may be able to do a leakdown test to confirm suspicions.

 

[Silly Seville]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

Ayuh,.... Pull the head off, 'n Look...

That was my next step, Bondo...just didn't want to do it in the rain. Waiting for dry weather and ideas.

Plugs were missing when I got the engine. That's how the rings got rusty. And there's a whole 'nother aspect to this engine that I was going to get to in another thread about the plugs, or really about the head itself. I think this is a Frankenstein engine. The numbers just don't add up...

The block says HECHO EN MEXICO. I didn't think they were making the blocks in Mexico back in '83!
And to make matters more interesting, the head is painted red underneath the black paint that is peeling off! AND, it will not take the MR43T plugs that are supposed to be in that head. It will only take the LTS version, which is a longer threaded body. I'm getting the casting numbers now.

 

[Bondo]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

That was my next step, Bondo...just didn't want to do it in the rain. Waiting for dry weather and ideas.

Plugs were missing when I got the engine. That's how the rings got rusty. And there's a whole 'nother aspect to this engine that I was going to get to in another thread about the plugs, or really about the head itself. I think this is a Frankenstein engine. The numbers just don't add up...

The block says HECHO EN MEXICO. I didn't think they were making the blocks in Mexico back in '83!
And to make matters more interesting, the head is painted red underneath the black paint that is peeling off! AND, it will not take the MR43T plugs that are supposed to be in that head. It will only take the LTS version, which is a longer threaded body. I'm getting the casting numbers now.

Huh,..??

 

[Silly Seville]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

Huh,..??

That's exactly what I said!

The threaded part of the spark plug hole starts about 3/4 inch down in the head. The standard plug will not reach the threads. But the plugs out of my 350 will fit. They are AC Delco MR43LTS. But only HALF of the threads on the longer plug actual make contact with the threads in the head. I'm stumped. I wonder if the head has had heli-coils put in it?

 

[Silly Seville]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

Head casting # is 3905998. Date code is J229. Does that mean October 22 of '89 or '99?

 

[ENSIGN]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

It's time to pull it apart,Sounds like you bought some body's problem.

 

[Silly Seville]

Re: Low compression on 2 of 4 cylinders. Head gasket or stuck valves?

I'm not terribly sad about it...I paid scrap iron price for it.

I'll let you guys know what I find though, just for future knuckleheads who buy salvage engines.