1979 Cobalt 18DV w/ MerCruiser 260 and Alpha 1 - No Raw water to engine

[tobinmarch]

HIN: FGE8V302M79J
Sterndrive Serial: 5345649 1.50R (second digit might be an 8)
Engine: MerCruiser 260 (Alpha One Gen I)
Model: 1979 Cobalt 18DV


This boat has been garaged for the last 20 years. I just inherited it from the original owner—189 engine hours, winterized and untouched since the early 2000s.


I proactively replaced the impeller in the lower unit. I expected it to be crumbling, but surprisingly, it looked brand new.


After reassembly, I ran the engine on muffs. Within 2–3 minutes, temp spiked to 200°F. Waited awhile, tried again with better muffs and a larger hose—same result.


Next, I launched it to test for transom leaks and see if submersion helped. No leaks, but still overheating.


Pulled the lower unit again and found I’d missed the bushing under the guide tube during impeller install (rookie mistake—watched too many YouTube videos).
Anyone have a good illustrated guide for rebuilding the impeller on an Alpha One Gen I?


Reinstalled the lower unit properly. Disconnected the raw water hose feeding the thermostat housing to check flow—nothing after 10 seconds.


Removed the inlet hose to the oil cooler to eliminate that as a restriction—still no water.


Backflushed with a garden hose into the raw water inlet tube—no flow out the lower grates.


Pulled the lower unit again. Water pickup tube is clean, cavity above it looks pristine.


With the engine-side hose still disconnected, I blew compressed air up the water tube in the upper unit—no airflow. Seems like a firm blockage between the upper drive and the transom fitting.


I can't access the transom hose from inside the boat (engine’s in the way), and I’m new to boats but comfortable with light mechanical work.

Any ideas on what to do next?

Attached pictures for illustration of hose disconnected and identified raw water source.

 

[alldodge]

Welcome
Remove the drive and back flush. If water flows it's in the drive, if it doesn't it's between cooler and transom assembly

 

[Bt Doctur]

isn`t that feed hose on the wrong side of the T housing?Water inlets are normally on the port side

 

[alldodge]

I looked it up in manual 3 and it appears to have 2 different thermostat housing

 

[tobinmarch]

I am certain I found the issue and am on the road to full recovery.

I did as @alldodge recommended and pulled the drive (upper part of the sterndrive). I found a bunch of hardened chemicals, like real hard toothpaste material blocking the water passthrough on both sides of the drive unit.

Cleaned them out and water and air passes freely.

Went to South Austin Marine today and got a gasket and o-ring kit.

I have the upper unit on and will do the lower when I can bear the heat once more, later today.

Here are some pictures of the obstruction and after some preliminary cleaning. Looked better after more cleaning and pre installation.

 

[alldodge]

Yep that be a slight it clogged

 

[tobinmarch]

I got it back together and ran it on muffs.

Has a new 140degree thermostat. i was running it at idle till about 160F then ran it at 2000rpm and the temp went up past 180. moved back to idle and hit 200. ended test run.

any ideas, should I not run it on muffs except at idle?

new to boats.

thanks.

 

[alldodge]

It still has a restriction somewhere or air is getting in
Put a short piece of clear hose inline with intake to thermostat hosing, your looking for air bubbles

 

[Lou C]

Are the exhaust elbows getting hot? Did you make sure all the passages in the thermostat housing are open? Power steering cooler not clogged?
Sometimes the water tube grommets in the outdrive get hard & don’t seal or it’s possible moves out of place while replacing the impeller?

 

[nola mike]

I got it back together and ran it on muffs.

Has a new 140degree thermostat. i was running it at idle till about 160F then ran it at 2000rpm and the temp went up past 180. moved back to idle and hit 200. ended test run.

any ideas, should I not run it on muffs except at idle?

new to boats.

thanks.

2k on muffs might be enough to overheat. Pull the line to the stat and see if you're getting flow

 

[tobinmarch]

What about this idea...

Since the water passage between the upper sterndrive and bell housing were severely clogged there may be similar gunk in other parts of the waterway restricting waterflow.

I am considering removing the lower unit and attaching a submersible pump in a container large enough to hold several gallons of fluid, also capable of catch the fluid exiting the exhaust, then pump the fluid into the water tube in the upper creating a closed loop. remove thermostat for full flow.

Use a chemical like Rydlyme Marine in a 1:1 ratio with H2O and cycle the system for a few hrs.?

THoughts on this?

 

[Lou C]

That might work, here in salt water land I/O boats moored in the water often start to run hot due to marine growth in the water intakes. I have to clean em each season sometimes 2x.

 

[tobinmarch]

Ran it just now for 30 sec with the RAW water hose off the thermostat housing and into a bucket. Here is the flow with a good size garden hose on at about 3/4 flow.

The flow rate looks terrible. See videos.

Tomorrow I plan to get a full rebuild kit for the impeller. I purchased an Alpha One rebuild kit from Amazon and only did gaskets and up. I plan to remove the lower part of the impeller housing and replace all impeller parts.

I did purchase an Alpha One impeller kit and just recently, per this forum, discovered I have a pre Alpha. (Sterndrive MC-1 sn:5345649 1.50R)

Anyone know if the details above are a glaring issue?

Welcome all thoughts.

 

[alldodge]

Oh yes your clogged up

 

[Lou C]

that, or the muffs do not fit tight enough
the true test would be does the flow look the same if the boat is in the water
I like these muffs, they work the best of all that I have tried...
Also, for impellers and drive shaft bellows, use OEM! Amazon parts can be OK or just junk, those are 2 parts that should be OEM as long as you can get it. Failure of either will be expensive!

 

[Scott06]

Ran it just now for 30 sec with the RAW water hose off the thermostat housing and into a bucket. Here is the flow with a good size garden hose on at about 3/4 flow.

The flow rate looks terrible. See videos.

Tomorrow I plan to get a full rebuild kit for the impeller. I purchased an Alpha One rebuild kit from Amazon and only did gaskets and up. I plan to remove the lower part of the impeller housing and replace all impeller parts.

I did purchase an Alpha One impeller kit and just recently, per this forum, discovered I have a pre Alpha. (Sterndrive MC-1 sn:5345649 1.50R)

Anyone know if the details above are a glaring issue?

Welcome all thoughts.

Not enough water way way too little. Get a OEM water pump kit

this should be the correct parts breakdown for your lower https://www.marineengine.com/parts/...0-thru-6225576/gear-housing-assembly-complete

 

[tpenfield]

I am wondering if there are additional clogs further along the intake line (hose). If you press a garden hose turned on about 1/2 way into the bell housing fitting, I am wondering if you will get the same trickle of flow up to the t-stat housing?

You mentioned an oil cooler . . . I am wondering if you will get good flow before the cooler, but not after the cooler by disconnecting those hoses.