Mercruiser 5.7 overheating at idle

[Mlevy]

I have a 1994 stingray 719zp that originally came with the 4.3 mercruiser. I have re-powered this summer with the 5.7 vortec long block. Everything is new, water circ pump, impeller in stern drive, intake manifold, exhaust manifolds, risers, thermostat housing with 140f thermostat, sensors in housing.

When the motor is running bellow 1500rpm the temps climb until it alarms. Above 1500rpm the temps start to come back down to normal. I have checked all the cooling hoses and they are obstruction free and I have great flow to the circ pump on the motor.

This problem existed with the previous motor, it seams as though there is a lot of back pressure in the exhaust and it is forcing exhaust back into the thermostat housing or I should say keeping water from flowing through the housing and spilling into the exhaust. I have replaced the exhaust flappers as well and there are no pieces stuck in the y pipe. With the exhaust elbows removed looking into the y-pipe the water sits almost to the top of the y pipe.

Could this be part of the reason why it has so much back pressure and does not allow water to flow out of the thermostat housing? I have gone through every past discussion on the site and others coming up with nothing. Any help would be greatly appreciated.

Thanks in advance!

 

[Scott Danforth]

First, welcome aboard

Second, yes, water will be near the top of the y-pipe until you are on-plane. thats normal on all boats that is where the water-line is.

next, if you are over-heating, you have a lack of flow somewhere.

when you had the motor out, did you flush your power steering cooler?

did the old impeller have bits missing? if so, did you go looking for them? if not, now is the time to do so.

the fact that the problem is also with the old motor means that its either in the drive, the hose between the drive and the transom plate, the power steering cooler or anything else you kept in the boat.

are you using the same thermostat housing from the 4.3? how is the bypass?

 

[Mlevy]

Thanks!

Power steering cooler was removed and checked for obstructions when I pulled the 4.3. I checked all the hoses when I removed the old motor. The Old impeller was good but replaced since I had it apart.

The only parts not replaced were the gimble housing that mounts to the transom and exhaust and y pipe. The stern drive is a new sei alpha 1 hen 2. Today I plan on picking up a scope camera and checking my inlet hose.

I am using a new to me thermostat housing that is a different style than my last one. Still has the t balls on the hoses to the risers. They seat nice and water flows good.

I was so frustrated this past weekend I pulled all the outlet hoses off the t stat housing and started the boat. I was getting good flow out of them all, but the exhaust coming in the hoses still attached to the risers and manifolds was so strong I can’t see how water would ever be able to travel into them.

 

[Rick Stephens]

You're plugged up somewhere.

I like to start with the impeller. It is so easy to check that you have full flow by pulling the line coming into the thermostat housing from the oil cooler, giving full water supply, either to muffs or by the boat being dunked, then firing up and checking how much water you get in 30 seconds. Should easily fill a 5 gallon bucket in under 30 seconds. To spec is 20 seconds, if I remember right.

If the impeller and plumbing to therm housing passes, then it is time to look at the thermostat housing and how you're plumbed past it.

 

[Mlevy]

Ok I will check that when I get home from work and repost with my findings, thanks Rick!

 

[Mlevy]

Update:

I checked the flow from the inlet hose, filled a 5gal bucket in around 23sec. I took a video of it filling the bucket and sent it to a boat shop mechanic I have been buying all my parts from. They are an engine builder with a very good reputation in the area and get me all of my Merc parts. They think it has to do with my thermostat housing.

I borrowed the second housing from the shop thinking it would solve my problem. Both t stat housing’s I have used are the 4 hose type with two running to the manifolds and two running to the risers. The shop thinks that the water is taking the path of least resistance and is not circulating through the t stat to the sensors. They think a two hose t stat housing would help eliminate some stagnant hot water in the housing.

So basically the sensors think they are overheating but the rest of the motor isn’t.

What is everyone’s thoughts on this? Would this be a reasonable thing to look into? I know that housing has to be pricy to just try out.

TIA

 

[Rick Stephens]

One of the best and cheapest ways to figure this out is run down to an auto parts store, or order online, am infrared temp gun. They are very reasonable and always useful, especially in cases like this. Shoot the thing around and see what you get. Is the side of the block hot? Maybe the gaskets for the new Manifolds are incorrect? Maybe the check balls are checking too much? Maybe water is flowing like the mechanic is guessing. All of those will show with the gun. Certainly a LOT cheaper than taking it to someone to do the same.

 

[Mlevy]

Update:

I have figured out my issue and thought I should post incase anyone has the same issue down the road.

I used an infrared camera that I have access to from work and watched the motor come up to temp. I could clearly see on the camera that the cool water was traveling through the hoses to the risers and water was staying stagnant where the t stat is and the hoses that go to the manifold. I removed the hoses that were getting cool water and checked the t stat balls or check balls. The springs did not apply enough force on the balls and water was dumping out the risers and not the manifolds.

I removed the balls and stretched the springs. Re-assembled, let the motor cool down. Watched on the camera again and it was better, some water was moving in the t stat. Took apart again, stretched the spring some more and now it is even heat (150f) throughout the whole t stat and hoses going to the risers and manifolds.

Thanks for your help everyone!

-Mitch

 

[Rick Stephens]

That makes sense. I'd order up new springs tho. They are not expensive, a couple bucks each. #24-75533

 

[Mlevy]

Ordered them this morning, thanks again!