Mercrusier v8 overheating

[Thomas A Ross]

I Have a 1976 stamas 24ft v. I just replaced replaced the engine from a mercrusier 165hp straight 6 to a self built 400 small block Chevy v8. Now my problem is that the engine over heats under load. The engine was removed from my 1977 Chevy c30 motorhome. I had the local machine shop bore the engine .30 over due to pitting in the cylinder wall. They work with marine engines a lot bored to mercrusier spec .01 extra due to temp marine engine run at. The block deck was checked. The cylinder heads were cleaned checked by the machine shop. I assembled the engine with all new parts. cam lifters, pistons, valve springs, and gaskets. All Marine parts were used from a boat that hit some rocks and totaled the boat. All components are marine starter, alternator, distributor, coil , carb, and all exhaust components.

So my issues is when then engine is running under load or high idle it over heats. At idle it runs about 150ish and under load it it peg the temp gauge at 240 in a few minutes. I checked all the components i could think to check by checking water flow. The riser, manifold, power steering cooler, and thermostat housing. The outdirve water impeller is new and id flowing good water. The engine water pump is new and pushing good water. with muff the engine can idle no issues at 2500rpm after about 10 to 20min. The temp would start to get hot. i have a temp gun and its reading the same as the gauge. both manifold were about 200 degrees, the riser about 120. i boiled the thermotat and at 142 it open by 156 its completely open. The heads are factory 400 with steam holes the head gaskets are felpro with steam holes. basically i went thru every option i could think of but cant figure out my issue hoping someone can give ideas to check.

any help would be appreciate. Thanks Tom

 

[poconojoe]

Sorry, I can't help, but someone here will help, I'm sure. I would like to follow your issue. A very interesting story and seems like you did a great rebuild. Good luck and hope you get that beast to cool down

 

[achris]

Did you put a Mercruiser V8 Thermostat housing on, and plumb the hoses according to the water flow diagrams for that era of drive?

Chris......

 

[Scott Danforth]

welcome aboard

as Chris asked, how is the motor plumbed?

as for over heating, that is generally a sign of no water flow. did you replace the raw water pump after your over-heat? you mention it is flowing good. at 1500 RPM, you should be able to fill a 5 gallon bucket in about 16-18 seconds.

200 degrees is way too hot in a boat

at 240 degrees you have no water in the motor and you burned up your exhaust flappers and the rubber exhaust hoses.

 

[kenny nunez]

You need to add a raw water pump to the front of the engine and a thru hull water supply using 1” Id hose and a swing valve. Then add a “Y” connection so that the drive water pump will still pump into the system and makes it easier to run the engine on a flush out. This is a setup that a Mercruiser recommends for the 6.2 MPI .

 

[achris]

You need to add a raw water pump to the front of the engine and a thru hull water supply using 1” Id hose and a swing valve. Then add a “Y” connection so that the drive water pump will still pump into the system and makes it easier to run the engine on a flush out. This is a setup that a Mercruiser recommends for the 6.2 MPI .

Not so. This is a 350, not a 6.2. The 6.2s run a Bravo drive, which is why they have a engine mounted raw water pump. There is no pump in the drive.... The MC1 drive has a pump that is more than adequate to supply enough water for the 350... It was in the 70s and 80s, and it still is. It's just a matter of having the right thermostat housing and hose plumbing.....

Chris.......

 

[kenny nunez]

A point well taken. Since small block engines needs a minimum of 17 gallons per minute@ 11psi @ 4K rpm. I wonder if the drive ratio is still 1.65 verses 1.5 could be the problem. Needless to say a 400 has to generate more heat than a 350.
As far as raw water pumps go you cannot beat the old style of drawing through the hull. The cooling system on my Donzi with the 6.2 / Alpha 1 works perfect with the engine driven pump.

 

[kenny nunez]

My bad on the 1.65 - 1.5. He may have to go to a 1.32 upper gear set to really turn the drive pump fast enough. Otherwise a belt or crank nose pump.

 

[achris]

My bad on the 1.65 - 1.5. He may have to go to a 1.32 upper gear set to really turn the drive pump fast enough. Otherwise a belt or crank nose pump.

Also wrong. The '165' drive is a 1.65:1 reduction, but I have had 1.84:1 drives on V8s in big boats, and they still provide enough water.

There is nothing wrong with the drive pump, it WILL provide enough water. The problem is elsewhere, plumbing....

Chris......

 

[kenny nunez]

If his water supply is adequate the next thing he should do is to add some sections of clear hose to see if there are any combustion gas bubbles coming from the engine. While the engine was professionally machined there still could be a small leak. Or seal off the engine and put around 20 lbs in the water jacket and let it sit.

 

[GA_Boater]

We have a lot of off the wall suggestions and the OP really has to answer the questions asked.

Thomas - Is the motor plumbed per Mercruiser specs with a Merc V8 thermostat housing?

 

[Thomas A Ross]

This is the water diagram i used.

 

[Thomas A Ross]

Also i didn't measure water flow at 1500 rpm i can do that tomorrow if that helps. I under stand it not a 350 but its all stock and using 60 degree fresh water it should be able to cool it self in my opinion. It has a mercrusier 140 thermostat. All component came from a donor marine engine that ran good but hit a rock and cracked the hull. I stripped manifold and intake but some how i didn't get enough water out of the block before it got cold out and it cracked. So i pulled the 400 out of our camper and rebuilt it with marine parts. Also the cam is the same spec as extreme marine comp cam. I just order a new 160 degree sierra brand thermostat with new gaskets. I was told that marine 350 should have 160 not 140 so i was going to replace that. But i did check the thermostat by boiling it to check temp it starts to open and is fully open. Does that help is there any other thing i need to test?

 

[achris]

This is the water diagram i used.

Ahhhh.... That diagram is for Ford engines.... Not GM....

Find yourself a diagram for 198, 228, 230, 250, 260....
You also need to make sure you have a diagram that matches the manifolds you have. You'll have either 'log' style or centre rise, the diagram must be for which one you have... And the manifold/elbow gasket needs to be checked to ensure it's the right one too. (block off/full flow)

Chris.........

 

[achris]

If you post photos of the thermostat housing and the exhaust manifolds (with the elbows), I'll look for a flow diagram for you...

 

[arabian 200]

Just another thought, the chec 400 block has no cooling space between the cylinders because of the large bore in the 350 type block high rpm causes higher temps internally with this engine.just my thoughts.

 

[Thomas A Ross]

This is the one i used actually i put the incorrect one here. And i did flow check in 10 seconds i had half of a five gallon bucket at 1800 rpms.

I do know all about the 400 small blocks. My machinist at local shop is my friend. He races modified race car with a 400 says he has no issues and he expected with the cool water intake it should stay cool. Thats the plan at least.

 

[Thomas A Ross]

the only picture i have Right now i can get more tomorrow

 

[achris]

Given that everything looks right as per the diagram and the photo, and you have enough water from the pump to the engine, what about the separator plates between the manifold and the elbow? If they are full flow, then you'll not have the water circulating properly. Also, are the passages in the manifolds and elbows clear? I've seen more than a few overheats exactly as you have described caused by plugged up elbows. The other thing worth checking is the water pocket cover in the drive....

Chris........

 

[Thomas A Ross]

achris do you have any way to test the elbows or manifolds. I removed the water hoses and feed my garden hose in water ports and they all flow just like the hose. It still only over heats under load or higher rpm. Also i adding a picture of my thermostat housing wondering if i using the correct gasket ?