1980 3.0L strange overheat issue

[Troubled1]

Finally put my 1980 Starcraft in the water last weekend. Loaded the family and dogs and went up the lake to a nice beach. Just as we got there the engine overheated. We pulled up on shore and waited until things cooled a bit. I pulled the feed hose from the impeller to thermostat housing and started the engine. Had a good flow of water.
Next I took the thermostat cover off and removed and checked the thermostat, it was seized. I left the thermostat out and we headed back to the docks. Boat ran at 125-130° all the way back. New thermostat ordered but slow getting here. Anyway, this weekend (still no thermostat) my wife and I decided to do a short fishing trip. Boat was running 125-130°. I ran full throttle for a couple minutes and even without the thermostat it overheated.
We anchored the boat did a little fishing while things cooled down. Again I pulled the hose from the impeller to verify water flow and again, good flow. I reattached the hose, started the engine and the temperature dropped to 160° right away.
I have a few thoughts but since this is my first sterndrive, the cooling system is a bit of a mystery to me.
1- thermostat needs to be in place to properly direct flow of water.
2- blockage somewhere
3- airlock in cooling system.
Where should I start? End caps on manifold? Water pump? If the cooling passages are blocked or partially blocked, what’s the best way to clean/flush them out? Can it be done with the boat in the water or should it be on the trailer?
Carb has been rebuilt, new cap, points/condenser, rotor and wires. dwell angle 32° timing 4°BTDC. Fresh oil and fresh fuel. Top speed was 33mph according to GPS.
I really like the boat but the overheating issue makes me Leary of using it.

 

[Renken2000Classic]

Ah the wonderful "What's it gonna do this time?" world of driving old boats.

Sorry I can't help. Just commiserating a little.

 

[kenny nunez]

It is acting like the head gasket is leaking just enough pressure to over come the incoming water from the pump. Add a short length of clear hose between the thermostat housing inlet and the incoming water hose. If the head gasket is leaking you will see the water disappear. The only other cause could be the exhaust riser being blocked up from a rust build up in the ports that cool the exhaust gasses. That can be checked by removing the thermostat housing outlet hose to the manifold, then with a garden hose clamped in the hose watch if there is no resistance flowing through the manifold.

 

[itsathepete]

Get your thermostat replaced first. Only then will you know if there are other problems

 

[Troubled1]

I came across an article online regarding manifolds. It stated if an engine runs at normal temperature under light to moderate load but overheats at high speed/load, the most likely culprit is blockage of the manifold. I have also downloaded an older OMC factory service manual which backs this up and states the manifold should be removed and cleaned/soaked in muriatic acid every 350-400hrs of use.
Hopefully the new thermostat shows up soon and I can get that installed and see if anything changes. I was going to order a manifold gasket and valve cover gasket but instead ordered a complete engine gasket set. I’ll do a compression test to rule out the head gasket next weekend when I go back to the lake.

 

[kenny nunez]

When overheating only occurs above 2500 rpm with the manifold clear and the water pump in good condition a cylinder head gasket leak will not show with a compression test unless it is completely blown out.

 

[Troubled1]

I ju

When overheating only occurs above 2500 rpm with the manifold clear and the water pump in good condition a cylinder head gasket leak will not show with a compression test unless it is completely blown out.

I just find it strange that overheating occurs with the thermostat removed. The engine runs strong and there’s no associated head gasket issues ie; rough idle, hard starting due to water intrusion, water in oil etc. That said, a complete gasket set is on the way so I’ll probably just take the time to pull the manifold and clean it out, pull the head and replace the gasket, adjust the valve lash and install the new thermostat. I’ll have all the tools on hand and it really isn’t much more work anyway.

 

[itsathepete]

Overheating can occur when no thermostat is present. Water flows via pressure, resistance, and gravity. The engine block is lower than the water inlet and outlet so there's no natural flow through it. The circulating pump circulates the water through the engine but without a thermostat most of the freshwater flows straight out of the manifold without entering engine circulation. The thermostat forces incoming water into the engine and when it opens, still provides some resistance to flow so that fresh water still enters the engine. The impeller flows a lot of water, but if it's not going where it is needed, will not cool the engine. It's a good idea to pull and inspect your manifold. Clean it up and make sure you don't have corrosion that could cause water to pass between the exhaust stream and water jacket.

 

[itsathepete]

Had a 1969 2.5 mercruiser back in the day. Got the boat for free and was broke so I just put a big stainless washer I scrounged up in place of the thermostat. The hole was big enough to allow flow but enough resistance to force water into the engine. Never got hot but also never got up to normal temp unless running long and hard.

 

[southkogs]

Have you greased your swivel on the outdrive? Another long shot - and Kenny is the brain on the old OMCs - but, sometimes you can have exhaust gasses in that housing encourage overheating. Greasing the swivel, tends to fend that effect off.

 

[Troubled1]

Have you greased your swivel on the outdrive? Another long shot - and Kenny is the brain on the old OMCs - but, sometimes you can have exhaust gasses in that housing encourage overheating. Greasing the swivel, tends to fend that effect off.

I assume you are referring to the swivel that permits steering. This is my first ever stern drive so it’s a bit of a learning curve.
One question. While searching around the net, I’ve heard talk of exhaust flappers. Would this drive setup incorporate these?

 

[Troubled1]

Had a 1969 2.5 mercruiser back in the day. Got the boat for free and was broke so I just put a big stainless washer I scrounged up in place of the thermostat. The hole was big enough to allow flow but enough resistance to force water into the engine. Never got hot but also never got up to normal temp unless running long and hard.

That I believe is the issue. I can run all day at about 1/2 throttle and temp stays around 125-135° Once up to 3/4 and beyond the temperature climbs fast.
Thank you for this.

 

[kenny nunez]

Do not forget to add the section of clear hose next time you test the boat out. Have someone to watch the water flow when the engine starts to overheat.

 

[southkogs]

You've got an OMC Stringer stern drive. It is basically an antique: OMC stopped making that kind of drive in 1987 and it's rather different than the Mercruisers of it's day or any stern drive of today.

Swivel greases from a point behind a weird lookin' red plug on the front of the stern drive. It is what the drive rotates on when you turn, but it also keeps the exhaust gasses from backing up.

I know you said it was feeding water up from the lake to the thermostat, but did you actually change the impeller (or know it was changed) when you bought it? An old or damaged impeller could monkey with you in terms of overheating "sometimes" too.

 

[Troubled1]

Thermostat installed. Same results. Anything over 3/4 throttle results in overheating. I have an engine gasket set but have decided that I’ll just run the boat below the overheat point until I’m ready to put it away for the season. I figure it’s better to be able to run it lightly than to risk taking it apart and running into time consuming issues that keep me off the lake.

 

[Troubled1]

Finally back at this issue. I have the manifold off and took off the front cap. Quite a bit of rust buildup. I flipped it end for end and took off the cap on the riser end. I found this.


That’s a 1 1/2lb ball pien for comparison. There’s still more to scrape out. Needless to say, this is most likely causing the overheating issue.

I’m going to take a piece of winch cable, fray one end a chuck the other end in my drill and clean out as much scale as I can. After that I’ll cap the riser opening and the the rear end and fill it with muriatic acid to clean it even more. After that I’ll pressure test for leaks.

I still plan on pulling the head off and replacing the gasket and check the cooling passages in it and the block.

 

[Gary H NC]

Wow! A ton of rust! Maybe time to hunt a new manifold..
I know they make them for the old OMC Cobra but not sure on the Stringer drive..
What engine is it?

Did some looking and they do make a replacement for the 3.0 Stringer manifold.

 

[Troubled1]

Wow! A ton of rust! Maybe time to hunt a new manifold..
I know they make them for the old OMC Cobra but not sure on the Stringer drive..
What engine is it?

Did some looking and they do make a replacement for the 3.0 Stringer manifold.

I’ve found a few places that have the 3.0L replacement which is what I have. I’m am however going to run it with this manifold for now to make sure this is actually the problem. I’m still going to pull the head and water pump and check all the cooling passages to be sure they are fine.

This manifold cleaned up nicely inside as did the riser. I installed caps and pressure tested to 10psi and no leaks.

 

[Gary H NC]

I’ve found a few places that have the 3.0L replacement which is what I have. I’m am however going to run it with this manifold for now to make sure this is actually the problem. I’m still going to pull the head and water pump and check all the cooling passages to be sure they are fine.

This manifold cleaned up nicely inside as did the riser. I installed caps and pressure tested to 10psi and no leaks.

That is good! I hope it solves the over heating problem...save you about 400 bucks!

 

[Troubled1]

Ran into another problem. I pulled the cylinder head to clean out the cooling passages as well as check the block passages. I did find the small passages on the head where cooling water comes from the block were partially to fully blocked. They have been cleared and flushed out. The small passage from the block to the thermostat housing bypass and the bypass in the thermostat housing was also blocked up. Aside from the above, all the passages in the block and head are clear

I removed the valves to re-lap them and found the exhaust valve on #2 cylinder is cracked. I’m debating whether to order a single exhaust valve or just bring the head to an engine rebuilder and have whatever work needed taken care of. I was surprised when I removed the valve cover at how clean everything inside was/is. There was no sludge, no signs of water intrusion and no noticeable cylinder ridge.

Next I’ll pull the distributor out and check for wear in the bushings and clean everything up and make sure everything is good. I’m thinking that while everything is apart, it’s probably a good time to replace the engine water pump.