Merc 4.3 MPIs will not get past 100 degrees

[Jimmy Buoy]

Checked one of the circulation pumps today by removing the belt then quickly turning the pulley back and forth while listening for water sloshing. It was smooth to turn and there was no sloppiness switching directions. Also, I could hear water sloshing back and forth which coincided with the stop and go motion of the pulley. It seems to be fine, so I've ruled out the circulation pump as an issue.

In my case, I'm becoming more convinced that my belt driven pump along with the modifications done to the plumbing by the previous owner has created too much raw water volume at too high a pressure which is causing the issue. It seems there are some delicate balances at work with these cooling systems. I've taken steps to reduce the available water volume to the pump and will see if that makes a difference once the thermostat housings are back in the boat.

 

[Wayward Sun]

Checked one of the circulation pumps today by removing the belt then quickly turning the pulley back and forth while listening for water sloshing. It was smooth to turn and there was no sloppiness switching directions. Also, I could hear water sloshing back and forth which coincided with the stop and go motion of the pulley. It seems to be fine, so I've ruled out the circulation pump as an issue.

In my case, I'm becoming more convinced that my belt driven pump along with the modifications done to the plumbing by the previous owner has created too much raw water volume at too high a pressure which is causing the issue. It seems there are some delicate balances at work with these cooling systems. I've taken steps to reduce the available water volume to the pump and will see if that makes a difference once the thermostat housings are back in the boat.

Roger that sir. I wish I had the time to pull mine. I'm getting into my busy time at the job and I'm just not sure when I can get to it. When I do, I'll post back as well what I find. Thanks again for staying on this....

 

[scoflaw]

Checked one of the circulation pumps today by removing the belt then quickly turning the pulley back and forth while listening for water sloshing. It was smooth to turn and there was no sloppiness switching directions. Also, I could hear water sloshing back and forth which coincided with the stop and go motion of the pulley. It seems to be fine, so I've ruled out the circulation pump as an issue.

In my case, I'm becoming more convinced that my belt driven pump along with the modifications done to the plumbing by the previous owner has created too much raw water volume at too high a pressure which is causing the issue. It seems there are some delicate balances at work with these cooling systems. I've taken steps to reduce the available water volume to the pump and will see if that makes a difference once the thermostat housings are back in the boat.

If that theory was correct the engine would come up to the t-stat temp initially. The blocks water is sealed until the stat opens, higher than normal water pressure would not effect this.

 

[Jimmy Buoy]

If that theory was correct the engine would come up to the t-stat temp initially. The blocks water is sealed until the stat opens, higher than normal water pressure would not effect this.

Inspecting the cast iron thermostat housing design, it is evident that there is clear passage for water to flow from the large opening on top of the intake manifold, past the underside of the t-stat where the plastic retainer has its cutout openings, and into the #4 hose. That #4 hose heads downward and has a "Y" junction with the smaller diameter Raw Water Supply #12 hose and a large hose directly to the water circulation pump.

The t-stat does not stop water from exiting that large opening so the #4 hose in essence becomes part of the "sealed block/heads". I'll call this the "Mercruiser Circulation Circle". All the t-stat seems to be able to do is allow some of the water coming out of the top of the engine, flowing into that cast iron housing to exit out of the circulation circle and into the exhaust manifolds. But before the water gets hot enough to trigger that, the circulation circle continues driven by the engine circulation pump. Raw water is for the most part unable to push it's way through the "Y" junction because it is "full" with no or little space for more water (until the t-stat allows some of that water to be flushed down the manifold "drain".

I hope my theory is sound or else it's back to square one on this strange phenomena. Will know is a few days when I can get the housings reinstalled and fire her up!

Thanks for you comment. It makes me question my theory more, but when I hold my thermostat housing that still has the closed t-stat in it under a running tap, water will flow freely from the #4 hose opening to the opening under the thermostat which is normally attached directly to the large opening on top of the intake manifold. It will of course flow the opposite direction if I turn the unit upside down and run water into the bottom of the t-stat housing it will flow out of the #4 hose connection.

This IMHO demonstrates that even with the t-stat closed, the block is not "water sealed". Correct?

 

[scoflaw]

I think it impossible to reach 160* with any amount of raw water being introduced. My engine before I added closed cooling would not go over 100* when a tiny piece of block rust was holding the stat open about an 1/8"

 

[Jimmy Buoy]

New t-stats are back in the boat and the hoses/sensors are all hooked up - ready to test!

First, the Port Engine. Starts up fine and idles @732rpm but temps STILL won't get over mid 80's. Port Engine is virtually the same. Running out of ideas on how to get the engine to normal operating temps.

Talked to a mechanic that had worked on boats previous to his work on cars and he suggested checking the belt orientation to ensure that the circulation pump pulley was rotating in the right direction. I had already confirmed that it was correct based on the Merc Manual and mentioned that the PO had replaced the two pumps so they should be good. He then suggested the possibility that the wrong rotation pumps may have been provided and installed without being aware of the error. I didn't realize that these pumps were sold in either RH or LH rotation?

I'm going to remove the smaller hose which is part of the water heater loop to the engine. One end is on the t-stat housing while the other is at the circulation pump housing. I'll remove it at the circulation pump and add a section of clear hose with a piece of ribbon laying inside it so I can verify the water flow direction when the engine is started. The ribbon should indicate water flowing from the hose to the circulation pump - I shall see!

Not quite ready to surrender yet...

 

[Bill Curtis]

Seeing a couple so slight flow differences between your diagram and the one from manual #31 for small block Bravo MPI engines.

Arrows at 11 and 12 should be pointing other way. I would bypass that drain manifold (with the 4 drain hoses on it) and the plastic drain valve (being sure to provide cooling water to exhaust elbows) and troubleshoot from there. Would be a far simpler system to "see".

The dashed lines are drains. The lines you have as green and blue are drains only

 

[Jimmy Buoy]

Seeing a couple so slight flow differences between your diagram and the one from manual #31 for small block Bravo MPI engines.

Arrows at 11 and 12 should be pointing other way. I would bypass that drain manifold (with the 4 drain hoses on it) and the plastic drain valve (being sure to provide cooling water to exhaust elbows) and troubleshoot from there. Would be a far simpler system to "see".

The dashed lines are drains. The lines you have as green and blue are drains only

View attachment 130249

Thanks for the info Bill. I've been searching google for a proper flow diagram, but the only one I found seems to be a diagram of how the water drains out of the 3 point drain system. My boat has been "customized" by the PO and the drain manifold has been replaced with a straight open hose. The two block hoses just terminate with brass shut off drains on them and the bottom of the manifolds each have a blue drain plug.

I think I've narrowed the problem down to one of two things.

1. The engine's water circulation pump (replaced by the PO) may have the opposite directional impeller than this engine requires and is therefore pushing water out of the block instead of pulling it into the block. (should be checking this today using some clear hose/ribbon).

2. The cast iron thermostat housings may have rusted enough internally to enlarge the small bypass hole which allows some of the pressurized water in the housing to escape into the two tubes feeding the bottom of the exhaust manifolds. Without this bypass these hoses would not have any cooling water flowing in them until the thermostat opens - the system wouldn't be designed to allow that. I have no idea what the size of this bypass hole would be on a new thermostat, so this is just a theory. Like others have indicated, there must be water being flushed from the system to allow more cold water to be introduced.

The thing to remember is that this is affecting BOTH engines the same at the same time. It's difficult to think that something so rare would happen to two engines at the same time. I'm thinking its something that the PO did to both engines that is causing this underheat scenario.

 

[Bill Curtis]

If your thermostat housing is like the one in the diagram, they did make S/S ones. I have one in my parts bin.
Take a look at the closed cooling 2 piece thermostat housings on line. There is notched area in housing and cap and that is the bypass, to give you an idea. I would say mine is 3/8" square, or something like that.

The fact yours happened at the same time, on both previously properly running engines is really baffling.

Most of the marine circ pumps I have seen are bi directional.

 

[Jimmy Buoy]

If your thermostat housing is like the one in the diagram, they did make S/S ones. I have one in my parts bin.
Take a look at the closed cooling 2 piece thermostat housings on line. There is notched area in housing and cap and that is the bypass, to give you an idea. I would say mine is 3/8" square, or something like that.

The fact yours happened at the same time, on both previously properly running engines is really baffling.

Most of the marine circ pumps I have seen are bi directional.

I just bought the boat late last fall and only had it out once, but with getting 2 beeps/min I decided to cut the run short and sort things out in the spring. Also, it was in need of some outdrive refinishing so that took much of my spring (including new upper steering pins) so I just got it in the water end of June. I first thought that both of the temp gauges didn't operate, but they don't register under 100 degrees. I also have a Rinda tool that shows me the engine's digital temp reading while running + Vessel View Mobile + Laser Infrared Temp Gun now so I know what the true engine temps are - and they are incredibly LOW.

I'm interested in seeing your S/S housing. Can you provide a pic? Starting to think that both my t-stat housings are shot. Hard to believe.

 

[Bill Curtis]

Looks just like this, but in black paint

 

[Fill Ess]

I'm not sure if the circulation pumps can be directional but when I was shopping for new ones I made sure they were marked as marine with brass impeller and stainless bac cover. They were marked as bidirectional

 

[Fill Ess]

Hard to believe the actual engine Temps are that low without the thermostats being missing, stuck open, drilled out or bypassed

 

[Fill Ess]

I just replaced both housings so I'll take a look at the condition of the insides

I have the 3 port on with the blue drain plug where the big hose is on the 4 port ones

 

[Jimmy Buoy]

Looks just like this, but in black paint

Thanks. Not sure why I thought they'd be any different looking...

 

[Jimmy Buoy]

I'm not sure if the circulation pumps can be directional but when I was shopping for new ones I made sure they were marked as marine with brass impeller and stainless bac cover. They were marked as bidirectional

Any of the replacements I saw online were bidirectional too for this engine.

 

[Jimmy Buoy]

I did a test yesterday on flow. Port engine has two 5/8" hoses going from the engine to the water heater. One end of the loop is fitted to the t-stat housing, the other fitted to the engine circulation pump housing.

Removed the hose to the circulation pump and added a length of clear PVC hose between the pump and original hose and added a piece of ribbon dangling inside the clear hose to easily see the flow direction and started the engine. The clear section had water in it from the pump housing and up the hose about 3" then air above that. The engine was running, but the water just sat there. Not what I expected. I thought it would have been sucked into the water circulation pump and would start to draw water from the thermostat housing. The hose never changed to flow water while running the engine.

I then separated the clear hose from the original while the engine was running and held my thumb over the original hose (coming from the t-stat housing). Water then flowed out of the clear hose attached to the circulation pump. I could easily place my finger over the end and stop the flow, but I thought this would have suction - not flow out.

Then I blocked the clear hose and let my finger off of the hose from the t-stat housing. Water flowed out of it, but again it could easily be blocked with my finger without any real pressure behind it. I did expect positive water flow out the the t-stat housing.

Not sure if this test is giving me any confirmation that the water circulation pump is doing its job or not.

Thoughts?

 

[Jimmy Buoy]

I just replaced both housings so I'll take a look at the condition of the insides

I have the 3 port on with the blue drain plug where the big hose is on the 4 port ones

Can I ask why you replaced them? Have the same 3 port type - two above the t-stat for lower exhaust manifolds and one bigger one on the opposite end to feed the engine water circulation pump. Water temp sensors, one per side and a blue drain plug.

 

[Fill Ess]

Where on the thermostat housing is the connection to the heat exchanger on your HW heater?

If it's the top then that area is only getting new water when the thermostat opens -- at least that's how it is on mine.

 

[Fill Ess]

Can I ask why you replaced them? Have the same 3 port type - two above the t-stat for lower exhaust manifolds and one bigger one on the opposite end to feed the engine water circulation pump. Water temp sensors, one per side and a blue drain plug.

Last fall when I winterized, both drain plug heads broke off and I found those threads to be all chewed up. I guess I could have cleaned the threads but the housings were already rusty and the thermostats were older. I was cleaning up surface rust on the front of the engine and pulleys anyways plus adding S/S Hardin sea pumps. I hunted around for a stainless thermostat housing but it seems like they make it in every configuration except ours

These are the new ones before they went in. Luckily I saved all the old hardware because the bolts they sent were the wrong size