Compression decision

[jimrockireland]

It's late in the New Hampshire season. I've been working for several weeks on this engine and have to make a decision. It's a 1983 Mercruiser 305/5L MCM228R Serial#6275628. My compression readings are
AFT
140 120
140 90
150 120
150 140
Stern
Obviously there is one weak cylinder but I'm thinking it is good enough to salvage what's left of this year and maybe do some more extensive work on it during the off-season. Since this will be the first time I have this boat in the water I might shake out some more problems that I could work on during the winter. Any opinions on this decision?

 

[Bt Doctur]

Re: Compression decision

number look reasonable . If that is how the cylinders are numbered left side 1,3,5,7 and right side 2,4,6,8 I would do remove the plugs in 2,4,6 inject air into #4 and listen in #1 and #6 for escapeing air indicating a headgasket leaking.If no air is heard, some light oil into the ctlinder and retest.higher reading could indicate a ring problem.

 

[jimrockireland]

Re: Compression decision

When doing the compression test I had the exhaust manifolds off and all spark plugs out.I tried turning over the engine to close the valves in #4 - injected air into #4 and tried to listen for escaping air in the #4 exhaust port. I didn't hear anything obvious which I think means the valves are seating properly. When I do the test you describe above do I need to have the valves in #4 closed?

 

[Bt Doctur]

Re: Compression decision

yes, both closed otherwise you would hear it in the intake or exaust.commonly called a leakdown test ,it check rings and valves for leakage.bad rings would be head in the crankcase.

 

[tpenfield]

Re: Compression decision

Looks like the starboard side of the engine has problems. 2,4,6 are not good, #4 being the worst as noted.

you could probably finish up the season and then do some more diagnostic testing (leak test, cooling system pressure test, etc) before you tear the engine apart. Do you know if this boat/engine was run in salt water for part or all of its life? The reason I ask is that with an older engine in salt water, rust through issues can be a concern. The cooling system pressure test make shake that out.

 

[jimrockireland]

Re: Compression decision

I'm not sure if the boat was in salt water but I'm guessing not since I bought it in Whitehall, NY which is pretty far from salt water. However the real answer is I don't know.

 

[Volphin]

Re: Compression decision

I'd run it for the rest of the season and tear into it this winter. There are two ways to do compression testing. Wet and dry. Squirt some oil into the cyls, weak one first and recheck. Add these numbers to your dry compression check and divide by 2. This is a more accurate reading. How does she run?

 

[Bondo]

Re: Compression decision

I'm not sure if the boat was in salt water but I'm guessing not since I bought it in Whitehall, NY which is pretty far from salt water. However the real answer is I don't know.

Ayuh,.... Is this the ole 305 yer takin' out, or the ole 305 you've been askin' 'bout droppin' In,..??

 

[jimrockireland]

Re: Compression decision

The compression numbers are on the 305 I'm dropping in. It was originally a late 70's 898 Mercruiser motor. I swapped over components from a 1983 Mercruiser 305 MCM 228R with a cracked block. So in effect the 898 block motor has been morphed into a 228R motor.
I haven't really got the "new" engine running because I still have the exhaust manifolds off. I am planning on getting these and the risers machined flat before I re-install them which will be sometime toward the end of next week.
I will re-test the compression on the bad cylinder with the oil method suggested by Volphin.

 

[Walt T]

Re: Compression decision

I'd do a running compression test, it might actually build up higher. I dunno, 90 isn't all that bad. You can buy an inexpensive borescope at Harbor Freight and check that cylinder. A borescope is a great tool in my mind.

If money is an issue start putting it aside maybe buy your parts a few at a time as you can, find out who and how much the machine work will be, and if need be run it next summer as it is and by the time you're ready you are ready to do it right. Oh and buy new manifolds and risers before running a new engine. I for one will not warranty any engines without them. It's silly not to buy them.

 

[jimrockireland]

Re: Compression decision

I just tested the compression on cylinder #4 dry and then again after injecting some oil into the cylinder. The compression did not change so I am assuming the ring must be OK and maybe its the head gasket.

 

[Bondo]

Re: Compression decision

I just tested the compression on cylinder #4 dry and then again after injecting some oil into the cylinder. The compression did not change so I am assuming the ring must be OK and maybe its the head gasket.

Ayuh,.... More likely burnt valves,...

 

[888]

Re: Compression decision

Time for a leakdown test