Merc 4.3 MPIs will not get past 100 degrees

[Jimmy Buoy]

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.

The brass plug on yours right near the blue drain plug was replaced using a brass tapered hose fitting. Inspecting mine with the t-stats removed reveals the "upper chamber" above the t-stat where the hot water exits via the open t-stat and is pushed into the two smaller hoses which terminate at the lower exhaust manifolds. There is also a smaller hole from the back side of the tubes for those manifolds which allows some water to flow between that "upper chamber" above the t-stat and the rest of the t-stat housing.

The area where the blue plug is is completely open from bottom to top other than a partial "wall" where the large hose (just inside the housing). I could actually see if from the bottom of the area where the t-stat is installed. Water has to go around that wall, but it is clearly open to the rest of the housing. Not sure if that brass plug is part of the "upper chamber" or not, but there is a separation wall somewhere in the housing where it becomes and "upper and lower" section like where the t-stat is located.

Where did you source your housings? The ones I've seen can vary wildly in price. Also, I take it you didn't have a low temp issue with yours before replacing?

 

[Fill Ess]

I got them on Amazon from GLM Marine. I paid $250 for one, but it was the last one. WHen they came back in stock it was up to $350

Now it's back in stock but they only have 1

https://www.amazon.com/gp/product/B084BNTTM2

 

[Fill Ess]

Oh and right, they always ran around 160-165

 

[Jimmy Buoy]

I got them on Amazon from GLM Marine. I paid $250 for one, but it was the last one. WHen they came back in stock it was up to $350

Now it's back in stock but they only have 1

https://www.amazon.com/gp/product/B084BNTTM2

I really don't want to be that guy that starts throwing parts at the problem - especially at that price! I would like to find out if my bypass hole is normal in size though. If it's too big I guess it could be allowing too much water to purge out of the system via those two lower exhaust manifold hoses. Any water purged out will be replaced by the raw water pump's flow. Personally, I can't see both of these manifolds mysteriously developing too big a bypass hole at the same time...

I'm going to take a better look at the bypass hole using a borescope by taking the large hose off and seeing if I can snake in to that part of the housing.

Knowing yours works, it would be a good test to know if the t-stat housings are my issue by swapping one of mine out. Let me know if you'd be interested in lending or selling them to me (I would have to fix the plug of course).

 

[Fill Ess]

I wouldn't replace them either unless there's some reason why they let so much water bypass that it never gets up to operating temp

I'll take a look at the inside of the old ones

 

[Fill Ess]

I took a look inside the old housings. I have all the hoses/plugs/sensors/stat/etc remove

On mine, the only way that water can get to the top hoses (red below) is if the thermostat is open. I don't see any bypass. Inside that very top (plugged on mine) there is a tiny bypass hole -- maybe 1/8" diameter -- but that is plugged and goes to the part of the housing where water is flowing when the stat is closed. Same with all the other holes.

 

[Fill Ess]

Have you had your thermostats out? I guess it's possible that the rim )where the edge of the stat sits) is all corroded and some water is getting by. But with the low temps your seeing it's almost as if there were no stats at all and you're getting a constant stream of cold raw water. If I start my engines cold and let them idle, they get to 130F in a few minutes. Then right up to 160 and stay there.

 

[Jimmy Buoy]

I took a look inside the old housings. I have all the hoses/plugs/sensors/stat/etc remove

On mine, the only way that water can get to the top hoses (red below) is if the thermostat is open. I don't see any bypass. Inside that very top (plugged on mine) there is a tiny bypass hole -- maybe 1/8" diameter -- but that is plugged and goes to the part of the housing where water is flowing when the stat is closed. Same with all the other holes.

View attachment 130310

Thanks for the description of yours which does differ from mine. The passageway marked in red circles is the key here. I originally thought like you, that the only way to get water there is via the open t-stat. Scoflaw challenged that and said water would have to be always flowing out of those top hoses to feed the bottom of the exhaust manifolds.

I tested it by holding the housing upside down under a slowly running tap and tilted it a bit so that water would run in via the large hose port. I tilted the housing downward to see if any water would drain out those "red circled" top hoses in your pic. Sure enough - water did flow out. Closer inspection inside that tube (top hose outlet) at first didn't show anything, but using a small flexible wire, I discovered a hole in the middle of that tube on both housings going to the part of the housing where water is flowing with the t-stat closed.

With your hole plugged it would seem that those hoses would get zero water until the stat opens. Would that be long enough to cause hotspots in the cast iron exhaust manifold?

I could probably plug the hole myself with some two part epoxy but it would be nice to know for sure before doing so.

Are both your manifolds the same in this regard? Could you do a water test like I did on mine?

 

[Jimmy Buoy]

Have you had your thermostats out? I guess it's possible that the rim )where the edge of the stat sits) is all corroded and some water is getting by. But with the low temps your seeing it's almost as if there were no stats at all and you're getting a constant stream of cold raw water. If I start my engines cold and let them idle, they get to 130F in a few minutes. Then right up to 160 and stay there.

Thermostats have been out, tested ok, new ones put back in. The rim was a bit corroded, but cleaned up with a screwdriver and the old gasket (looked like crushed rubber) removed, then a new cork gasket applied. It sat in the groove just fine and the stat sat well on it. The plastic spacers had to be broken to get them out so they were replaced. I had to Dremel the hole out a bit since corrosion had made the hole smaller than normal. Fit fine with the spacer and even felt between the stat edge and cork gasket with a dental pick to see if there were gaps - none were evident. Looked like a snug fit.

I think there is too much water bypassing through that hole to the two exhaust hoses. As this water is being dumped into those two exhaust tubes, more cold water is constantly being introduced into the system from the raw water pump. That's all I can think of, but can't be sure unless I plug one up and start'r up.

 

[Fill Ess]

Ok now I wonder if your hoses are all hooked up properly

For the 2 hoses that go into the risers (red below), where does the water come from?

 

[Fill Ess]

And refresh my memory ...........

Are you the one who has the straight pipe next to the water distribution housing instead of the manifold?

 

[Jimmy Buoy]

Ok now I wonder if your hoses are all hooked up properly

For the 2 hoses that go into the risers (red below), where does the water come from?

View attachment 130345

The risers are fed from the water distribution housing (plastic ball type thing just after the cool fuel unit). The hose is a vertical one that has a T-Joint at the top of the engine with one going to the left riser and one to the right side riser.

 

[Jimmy Buoy]

And refresh my memory ...........

Are you the one who has the straight pipe next to the water distribution housing instead of the manifold?

Correct sir. The stock manifold would have two small hoses and two larger hoses. On mine the two smaller block drain hoses just hang below the front of the engine with brass shut off valves on the end which drain both sides of the block. The other two hoses have been removed and blue drain plugs installed on the bottom of each exhaust manifold.

 

[Jimmy Buoy]

With summer in full swing, I made an executive decision to buy 2 new t-stat housings from Amazon while they were available. The ones in the boat are quite rusty anyway and it's not worth it to mess about with trying to partially plug an existing bypass hole. Knowing my luck, the epoxy would come loose, be flushed down an exhaust tube and plug up part of the exhaust manifold!

Thanks for taking a look at yours so I can compare what I've seen in my stat manifolds. If that's not the issue, I'll be totally stumped (but at least I'll have new shiny t-stat housings)

 

[Fill Ess]

Hardin Marine does make a nice s/s aftermarket housing that would fit.

The only problem is -- it has another hose outlet where we have a drain plug. So you would either need to cap it off or change your hose routing.

It's actually cheaper and it's lower profile and won't rust out

https://www.hardin-marine.com/p-125...mostat-housing-replaces-mercury-860256a4.aspx

Depending upon how you winterize a raw water cooled engine, if you're the type that likes you take off the hoses at the t-stat housing and pour in antifreeze there, having an extra capped off hose would make that easier because you wouldn't have to pull the big one that goes to the circ pump.

So are you saying the PO removed the hoses that feed water to the bottom of the exhaust manifolds and replaced them with drain plugs? In that case how are they cooled?

 

[Fill Ess]

 

[Jimmy Buoy]

Hardin Marine does make a nice s/s aftermarket housing that would fit.

The only problem is -- it has another hose outlet where we have a drain plug. So you would either need to cap it off or change your hose routing.

It's actually cheaper and it's lower profile and won't rust out

https://www.hardin-marine.com/p-125...mostat-housing-replaces-mercury-860256a4.aspx

Depending upon how you winterize a raw water cooled engine, if you're the type that likes you take off the hoses at the t-stat housing and pour in antifreeze there, having an extra capped off hose would make that easier because you wouldn't have to pull the big one that goes to the circ pump.

So are you saying the PO removed the hoses that feed water to the bottom of the exhaust manifolds and replaced them with drain plugs? In that case how are they cooled?

No removal of hoses feeding the manifolds. Those are still attached to the t-stat housing and lower manifolds. He just re-located the two block drain hoses (capping them off with a brass drain valve fitting) and removed the two manifold DRAIN hoses (which are placed below the feed hose on the bottom of the manifold) Those drain hoses were replaced with simple blue drain plugs for manual removal/draining of the manifolds come fall. The manifold drain hose would normally be attached to a fitting just under the manifold water feed hose with the other end attached to a special tube just after the water distribution manifold, but there would have to be a one way valve (cap and floating little white ball) to only allow water to drain out of the manifold once the blue plug is removed from the water distribution housing.

My manifolds were replaced by the PO and he probably didn't want the extra expense and complexity of another fitting for something so simple to simply unscrew to drain. Less chance of obstructions blocking the manifold drain hole too since I always run a bit of hanger wire up into the drain hole to ensure no rust flakes blocked off the water flow while draining.

I thought about SS t-stat housings but didn't want to take the chance that they were different internally as well as the obvious external difference so I went to a stock design. Looking at the pic, I don't see threaded holes where the two temp sensors would screw into the housing. Hopefully what I've ordered will work as stock and there are no surprises. Should know next week if these are my cooling issue

 

[Fill Ess]

My bad I just took a screen shot from the 3d view

It has threads for the temp sensor and sender

 

[Jimmy Buoy]

I've received one of two new distributor housings. The bung hole on the top of the thermostat was bare so I could see that it terminated just under the threads, but there were two small (about 1/8") holes drilled at the bottom of the hole. One drilled into the back side into the section of the unit open to the bottom of the thermostat, temp sensors, and large hose which goes to the circulation pump. The other is drilled into the front side of the housing and is visible from inside the tube that has the two smaller hoses which feed the bottom of the exhaust manifold. This is the "bypass" that provides some constant flow of cooling water to the bottom of the exhausts even before the t-stat opens.

I opened up the top of my old housings and WOW - the bypass hole was rusted out to about 5x the size. That seems to have been what was allowing the system's water to flush out the exhaust more or less unrestricted. That water being flushed out was constantly being replaced by new seawater and the engine would not get over 81 degrees.

Now, after idling for 15 minutes at the dock in neutral the hottest it gets is 117 degrees. Not 160, but when I get the other engine's t-stat housing back on, I'll water test the boat and see what the temps are under load.

Here's a pic of the old port unit - the bottom of the bung hole is totally rusted out.

The stb unit isn't quite rusted right through yet, but it's well on its way.

Port unit from the inside of the exhaust tubing mounts - you can see the light from the bypass hole

I'd say that was the over-cooling issue (or at least a large part of the issue)

Water pressure at idle was 1.1 psi prior and now sits at 4.5 psi with the new t-stat housing. Low psi isn't always a faulty pressure sensor!

 

[Jimmy Buoy]

Problem SOLVED!!

Installed the stb t-stat housing and ran both engines dockside at idle (active thunderstorms in the area + big winds) and ran them both up to 3000 rpm in neutral and back - NO STALLING (so far). In forward at the dock, increased from 640rpm idle to 900rpm and back - NO STALLING. Temps went to 130 degrees at the dock, so I assume it will go over 160 once underway with a load on the engines.