Mercruiser (250 L6) 165 Wet Cylinder

[crazyboutamercury]

First time out this year my 165 temperature went to 220 degrees on the guage. I idled back to the ramp to load up with no issues or what seemed like excessive heat. I removed and replaced a defective impeller and thermostat and put the boat back in the water. The boat seemed to have low speed surge issues but ran out fine never getting over 140 degrees. At idle the normally smooth 6 ran rough so I loaded up and pulled the plugs for a compressiio check. Noticed adrip of water from gasket between intake and head and found water in #6 cylinder with head removed. Cleaned and checked flatness of head. Found intake and exhaust ports of manifold holding water. Drained water, replaced gaskets and ran engine from hose. Startup was fine and sl;owly transitioned to rough idle. #6 plug found wet with water coming from plug hole when cranked.

Is it possible to damage the manifold with low water (bad impeller) and get water to leak from heat exchanger into intake and back cylinder or did I miss a crack in the cylindetr head?

 

[Bt Doctur]

Re: Mercruiser (250 L6) 165 Wet Cylinder

Chevy inline 6, more than likely a cracked manifold . Remove the manifold and test by itself. Almost positive you see it flow out a intake port or exaust port.Mine blew a hole just idling on a hose, same thing with water coming out a spark plug hole.

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

Thanks for response, I noted the boat always has #6 lower during use and in driveway and any water in intake will naturally flow to #6. Compression check at 190-195 all 6 except a wet #6 was 240. I also noticed that I seem to be getting exhaust and water thru the transom area and mostly just water thru the prop now. Is it possible a hot exhaust has damaged something in the outdrive allowing the exhaust to exit before the prop? It may be that I have never noticed this before and looking close now I see new things. The boat did not take on any water during the last lake trip after the impeller was changed.

Thanks again, Looking forward to inspecting the manifold

 

[ziggy]

Re: Mercruiser (250 L6) 165 Wet Cylinder

after about a months ownership of my boat, this is what i found, after i found i had water in the oil.

i removed my manifold and filled up the waterjacket with water. in the pic. you can see the trickle of water running down the inside just to the right of the stovepipe for the auto choke. i left my manifold overnight with water in the waterjacket. by morning, the waterjacket was empty, the exhuast runner was full.

i think my manifold just sat unused for too long of time. (15 years)

if you R&R the manifold, i'd do the riser too. might as well know both are good.

after your overheat. did you go find all the pieces of the impeller that broke off. other things effected by an overheat could be, melted waterpocket cover (DO NOT REMOVE waterpocket cover screws, most likely will break off prior to removal), flappers melted and fell off and could be stuck in your exuhust pipe restricting water and exhaust flow, degraded rubber bellows between riser and exhaust pipe, melted waterpump housing, blowed head gasket. seems like there's more, but i can't think of anything else right now... maybe others will get you hip..

your compression is sure high. spec is 140 i believe... no idea why and the only thing that comes to my feeble mind would be if water was in the cyl. and your trying to compress that.

you have a fwc L6? normal operating temp be between 160-180*. rwc would be 140-160*...

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

The mercruiser thermostat is 140 and the boat seems cold now, ran 180 last year. Impeller was not missing anything only vanes cracked and bent over, Rubber boot between exhaust manifold elbow and something standing upright is in good shape with no heat related issues. There were no obvious water passages found on the old head gasket and no cracks observed in block, cylinder or head after cleaning. I have rebuilt many performance and standard auto engines but this is my first marine unit. The high compression may be related to the head as it was definitly not normal for a production GM car/truck 250. The intake passages had been gasket matched and ported with no sharp edges left from sand cores at production. This may be normal for a marine engine but not automotive. It may be that the head has been decked and ported in the past?????? I believe that i will find a crack somewhere in that manifold has been feeding water to the lowest cylinder and maybe the moisture has effected all cylinders including compression.

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

What is fwc and rwc? This unit is in my 1973 Cobalt with an Alph1 Sterndrive

 

[ziggy]

Re: Mercruiser (250 L6) 165 Wet Cylinder

What is fwc and rwc?

fwc= fresh water cooled.
rwc= raw water cooled

fwc has a heat exchanger, like a radiator but for boats. works like a radiator but instead of air doing the cooling, water from the lake circulates around the hot coolant (antifreeze) cooling it.
rwc has no heat exchanger and lake water is used to circulate thruout the engine, raw and right out of the lake.

reason i asked was,

and get water to leak from heat exchanger into intake

the 140 tstat would be for the rwc setup..

This unit is in my 1973 Cobalt with an Alph1 Sterndrive

the original drive for a 73 would have been a 1 drive. pre alpha. the drive could have been replaced though. a alpha 1 will bolt up in place of a 1 drive..

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

Aye, aye captain, pulling the manifold tonight for inspection, if unit is cracked as expected, will order a new mainfold and 180 thermostat for reassembly.

Thanks for the input and advise, I was almost ready to just roll her into the barn until I found a head assembly.

 

[ziggy]

Re: Mercruiser (250 L6) 165 Wet Cylinder

if unit is cracked as expected,

i don't have to be cracked. it can rust through. which is what i feel happened with the manifold in my pic. same thing happens to risers from what i understand too.

and 180 thermostat

why 180*? while we don't know what year L6 you have. i'll suspect that they're all pretty similar throughout the years. i just read in s/m #2 for my rig that the tstat is supposed to be 143* (that would be a correction for what i'd said before obviously) (so guess i'll correct myself and say, generically speaking it's 140 for rwc and 160 for fwc)for fwc and rwc if i read it right. the only note was for the 470 engine which uses a 160* tstat... there were no comments about using a 180* tstat at all that i could find in s/m #2...

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

Found it! Removed intake manifold, took to work bench, leveled and filled water jacket with water. Water poured into the exhaust port used for choke heat riser. Looks like an old patch, possibly JB welded over an old crack, is leaking? The heat tube is bent not straight like yours suggesting additional space was needed to do the shady work. I looked online for a warmer thermostat, 140 and 142 was all I found, may stay with the 140.

Thanks so much for the direction, much appreciated Capn Ziggy!

 

[crazyboutamercury]

Re: Mercruiser (250 L6) 165 Wet Cylinder

I have tried to attach a picture of the patched area. I am hoping that this is the root cause for the water in cylinder 6. If so it looks like I should have evaluated the manifold before removing and replacing the cylinder head with a new gasket.

 

[Fishermark]

Re: Mercruiser (250 L6) 165 Wet Cylinder

You might want to take the opportunity to do the recommended re-routing of the water passage for the exhaust manifold while you are replacing it. Here's how I did it on a recent rebuild:

Here is a diagram:

I believe Mercruiser put out a bulletin outlining the how and why of this conversion. Basically it forces water to go through the manifold from the back to front, that way when the bow is high, water still flows completely through the manifold and offers more even cooling. Otherwise, the front can get hot and when the bow comes down and water rushes back to the front of the manifold it can crack.

You will also need to install a block off plate between the riser and manifold. If you do a search, one of the posters has posted a step by step procedure.

 

[frostywalrus81]

Re: Mercruiser (250 L6) 165 Wet Cylinder

You might want to take the opportunity to do the recommended re-routing of the water passage for the exhaust manifold while you are replacing it. Here's how I did it on a recent rebuild:

Here is a diagram:

I believe Mercruiser put out a bulletin outlining the how and why of this conversion. Basically it forces water to go through the manifold from the back to front, that way when the bow is high, water still flows completely through the manifold and offers more even cooling. Otherwise, the front can get hot and when the bow comes down and water rushes back to the front of the manifold it can crack.

You will also need to install a block off plate between the riser and manifold. If you do a search, one of the posters has posted a step by step procedure.

Geez I would love my motor to look like that. hehe

 

[MikDee]

Re: Mercruiser (250 L6) 165 Wet Cylinder

Aye, aye captain, pulling the manifold tonight for inspection, if unit is cracked as expected, will order a new mainfold and 180 thermostat for reassembly.

Thanks for the input and advise, I was almost ready to just roll her into the barn until I found a head assembly.

Stick with the stock 140* thermostat!,,, it'll never get hot on the garden hose in the driveway, but in use you'll see that temp gauge rise. The reason for a low temp thermo is because boats don't have a fan like a car, and you must keep the motor running cool, or else possibly you'd get preignition under the constant load in a boat (there's no coasting), and blow a hole in a piston.