Exhaust manifold/riser

[Dnewton1]

ok this is my first post hopefully it’s all right.i have a 1990 mercruiser 4.3 freshwater raw water cooled.my issue is the motor gets up to around 215 I’ll have the the water hooked up to the drive idling and it starts to over heat but if I open the block drain plugs and let the water flow out while it’s running with new water coming in it’ll run forever and stay cool.but as soon as I close the drain plugs it’ll over heat in about two minutes the risers stay cool but the manifold gets really hot what does everyone think.

 

[Bondo]

Ayuh,...... Welcome Aboard,...... Have you changed the impeller yet,..??

 

[Chris51280]

Hi Dillan, Glad you can make it.
achris and Rick Stephens . Maybe you can help. You seem very knowledgeable at the 4.3 issues

 

[alldodge]

Have you changed the impeller yet,..??

Agree

 

[Chris51280]

I referred him over here from facebook. He actually had the drive off and had the hose directly hooked up to the inlet on the housing. not using the impeller. The next thing we suggested was the thermostat. He said there was not one in there.

 

[alldodge]

I referred him over here from facebook. He actually had the drive off and had the hose directly hooked up to the inlet on the housing. not using the impeller. The next thing we suggested was the thermostat. He said there was not one in there.

Always good to know the rest of the story

If there is sufficient water coming in from the hose, and water coming coming out the exhaust I would look at the thermostat housing and are the check balls in place? Then is the circulating water pump working, vane not loose or broken

This is V8 but is same for V6

 

[Scott Danforth]

the power steering cooler (item 7) is the best strainer in the world. the tube size is smaller than the inlet casting in the outdrive so if the vanes of the impeller blew up, they would be pushed into the power steering cooler.

any fish, rocks, mud, etc. that get pushed thru the raw water impeller also make it to the power steering cooler. if they get stuck, they plug it up.

overheating is a sign of lack of flow.

like AD stated, start at the thermostat housing and work backwards to find where the blockage is.

also, since this overheated, and you may not have sufficient water flowing thru the exhaust to keep the rubber bits from burning up, you may want to inspect those as well. the rubber bits start to burn at 250, and exhaust temps without water is about 900

 

[Rick Stephens]

Of note, you can pull the inlet line off the thermostat housing. That's the hose coming from transom, through the oil cooler, to the thermostat on the port side. Pull it off the thermostat housing and hook a hose up and back flush the oil cooler and transom fittings.

 

[alldodge]

He actually had the drive off and had the hose directly hooked up to the inlet on the housing.

With all the comments were missing the above post 5

 

[Rick Stephens]

Dnewton1, you might tell us more about the history of the boat and where it has been used - ie: salt water? or brackish? General condition of the inside of the thermostat housing would tell us something about whether the motor is full of rust scale, for instance. And how much you had to probe sand out of the block drain plugs.

My opinion is you need to start by inspecting the thermostat housing by taking each hose off and making sure if it has check balls, they aren't broken or stuck. Install a thermostat before further testing. Get a 160 or 165ƒ one. Often full flow without a stat installed is not efficient at cooling.

If that doesn't get it, then I'd pull the motor circulation pump and see if it is intact inside.


Rick

 

[Scott Danforth]

With all the comments were missing the above post 5

Hey AD...we didnt over look or miss the information that came late in post #5

covered in post #7

dont care if he has a hose in the hole or hooked directly to the oil cooler. the oil cooler is still a better strainer than the rest of the system. it was a problem 40 years ago, its still a problem today.

Lou covered the flushing in post #8

we all can agree...... there is a serious blockage somewhere. my guess its a rock or shell and other debris in the oil cooler from burying the outdrive in the muck as well as stuck check balls

 

[Scott06]

Think the discharge from thermostat to exhaust system is restricted. He must be getting enough water if he can remove drain plugs and keep it cool. Hot water isn’t getting out of the engine. The fact that thermostat was removed means somebody couldn’t solve the issue, maybe it got mis plumbed replacing a manifold etc. would be nice to see a pic of which of the many setups merc put out he has.

 

[Dnewton1]

The motor is ran only in fresh water I was messing around with it today if I take the hose off that runs from the top of the thermostat to the exhaust riser it will run cool.i stuffed the water hose into the exhaust riser hose and it will flow great out the back off the transom on both sides.it seems like it doesent have enough power to push it from the thermostat through the the hoses and though the risers and out the back.but as soon as I open the block drain plugs with water running to the engine it will run cool at about 125 degrees but as soon as I shut the block drain plugs it’ll start to over heat in about 5 minutes

 

[harringtondav]

"4.3 freshwater raw water cooled." ...sounds like raw/sea water cooling.

"but if I open the block drain plugs and let the water flow out while it’s running with new water coming in it’ll run forever and stay cool."

Per OP's comments I'm going with a bad thermostat. Engine fills with water, gets hot, thermo opens, fresh water enters, engine cools. Unless the thermo doesn't open.

Open the drain plugs, fresh water enters, engine cools down.

That's my theory, and I'm sticking to it,...until someone stomps on it.

 

[Rick Stephens]

"4.3 freshwater raw water cooled." ...sounds like raw/sea water cooling.

"but if I open the block drain plugs and let the water flow out while it’s running with new water coming in it’ll run forever and stay cool."

Per OP's comments I'm going with a bad thermostat. Engine fills with water, gets hot, thermo opens, fresh water enters, engine cools. Unless the thermo doesn't open.

Open the drain plugs, fresh water enters, engine cools down.

That's my theory, and I'm sticking to it,...until someone stomps on it.

He said it had no thermostat.

 

[achris]

.... Unless the thermo doesn't open....

That's my theory, and I'm sticking to it,...until someone stomps on it.

Stated, no thermostat in the engine (post #5)... Your theory, stomped.

 

[achris]

If someone else has been messing with the engine, it's quite possible that the wrong gaskets have been put between the manifolds and elbows. If the elbows are always cool, and the manifolds get hot, block off gaskets would do that.

The elbows need to be pulled off anyway to check for blockages in the manifolds anyway, so watch the gaskets.......

Chris.........
(BTW, your posts are REALLY hard to read. Would you consider putting in some full stops and capital letters? Thanks)

 

[Dnewton1]

Ya sorry about punctuation in a hurry.but correct the manifolds get real hot while the risers stay cool.i didn’t know for sure if you understood yesterday.if I take the the two hoses of the thermostat that connects to the risers and let the water flow out of the thermostat it will run at 125 degrees and not get up any higher at all.i also have run the water hoses through the riser hose to make sure there was no blockages through the transom assembly on both sides.and the water flows out like a river.but as soon as I hook everything back up and start the motor it just spits out of the transom assembly.its not a good steady flow.

 

[alldodge]

Remove the belt and large hose on the circulating water pump. Look up inside and use a screw driver of stick and place against the pump vanes inside the pump. Now try to rotate the pulley. Trying to find out if the vanes are connected to pump shaft or if there broke

You just don't want to use Capitols