GM 250 Inline 6 - MILKSHAKE!!!!

[Will Fish]

Background - 21' Scatcraft Cuddy w/ 165hp GM 250ci Inline 6cyl. mated to an alpha one drive, salt water cooled. I replaced the gimbal housing, outdrive, reinforced the transom, and installed a new-used motor this spring. Ran the motor on the stand for a good 30 min, made sure had good compression and no signs of issues before installing.

I just got the boat in the water 2 weeks ago. Went out for my initial test run. Motor ran fantastic, until I pulled back on the throttle and the temp started to climb. I shut the motor down the second she hit 200. I pulled the intake hose off the termostat housing, and I had no water. Pulled the boat and the clamp on the water tube broke and the tube came off the outdrive side. $1.19 and 10 minutes later she was pumping great and keeping the motor cool on the muffs.

Back in the water and out for test run #2. once again motor sounded great, nice and smooth, and great power. I get about a 1/2 mile and my oil pressure dropped. I killed the motor immediately and opend the hatch. Milk shake everwhere, coming out of the breather on top of the valve cover.

Got towed in and drained the motor. I took about 3-4 gallons of water out of the sump, before I got to the shake. Drained as much as I could, changed the filter, put fresh oil in, turned her over, and did this 3 times, until I got basically all the water out. Also poured a drop down the carb for the valves.

I checked the riser, manifold, head, and head gasket, and all are good. They are also, ALL BRAND NEW. A friend who has been a professional auto mechanic for 25+years, and boating for just the same amount of time, pressure tested the motor for me, and stated there is NO CRACKS. Now I am completely baffled

The only thing I can think of is a backflow from the exhaust. Is this even possible? It makes sence, only because of the amount of water that was in the motor, but I am not sure if this is possible. When I installed the Alpha One gibmal housing and transom assembly, I had to change my original lower exhaust pipe (like the Y-pipe for V6&V8's), which offset to the port of the motor mounts. Whereas the original pipe was L shaped and went straight up to the riser between the 2 rear motor mounts. Now I have an off-set to the port, around the rear-port mount, then off-set back to starboard in order to connect the dump. Therefore, possably not getting the proper exahust flow?

Any guidance would be great.

 

[Don S]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

You have a cracked block. Probably from improper wintization. You will not and can not get that much water in your oil through the exhaust while the engine is running.

Just drain all the coolant, block off the exhaust system from the block and try to pressurize the block.

 

[Will Fish]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

My friend did the pressure test, and said the block was drained and oil removed. And the block held pressure and could not find any signs of leaks. I am not exactly sure what was done, as I was not there when he did it.

 

[Will Fish]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

2nd pressure test - system completely drained of oil and water. all passages blocked up and block was pressurized at 12 psi. After 2 hours, no pressure drop on the block?

 

[NHGuy]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

Don has been doing this a long time, do a recheck in person and be positive of the test. Your friend may be nice, he might not be saying what you are hearing.
Only reason I say that is the description you give fails the logic of the system.

 

[Will Fish]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

You have a cracked block. Probably from improper wintization. You will not and can not get that much water in your oil through the exhaust while the engine is running.

Just drain all the coolant, block off the exhaust system from the block and try to pressurize the block.

Don - The motor was previously a FWC motor and had anti freeze in it. The manifold, riser and water components were removed down to to the bare block prior to putting the motor together. It was also kept in a garage which maintaines a temp of 55 deg. I wouldn't even think there was an improper winterization since a freezing temp was not reached. Also, the block had anti-freeze in her.

 

[71Cobalt]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

Will Fish - Did you ever resolve this problem? I seem to have a similar issue... I'd love to learn from your experience to avoid running down too many unnecessary dead-ends. Thanks!

 

[Will Fish]

Re: GM 250 Inline 6 - MILKSHAKE!!!!

Will Fish - Did you ever resolve this problem? I seem to have a similar issue... I'd love to learn from your experience to avoid running down too many unnecessary dead-ends. Thanks!

Since I have this boat for many years, I have accumulated a warehouse of good spare parts.

I change the riser, manifold.... still getting water. Pulled the head in hopes of finding a blown gasket.. nope gasket looked good. put my spare head on with a new gasket... still water in the oil.

Don was right. I had a crack in the lower block. The circulating water pump was pumping water straight down into the crank and oil. Time for a motor swap again. I guess thats what I get for putting a used motor in, despite running it and doing a compression test on her before I installed it in the boat.

Found a good rebuild shop that has a few 250's all redone with new pistons, marine cam, brass freeze out plugs, valves springs, etc etc...ALL NEW and a 2 year gaurantee. I don't have the time now to do the motor swap, so it will have to wait until the end of Sept before I can get into her, but it should be a fairly easy swap since everything else is new and lines up. Should be back running after 1 day of work.

71Cobalt - Hope you have better luck than me.