Logo
FULL SITE MENU

Mercruiser 4.3 MPI overheating

S

sdwazup

New member
I've got the 4.3 (sn# 1A054765) and I can't figure out why its overheating. I've had several "marine mechanics" look at it. On they're advice I've replaced the sea water pump, circulation pump and put in a thermostat kit. I also opened up the heat exchanger (I did find some of sea water pump impeller in the intake). I thought I had the problem solved but thats not the case. The boat will run all day and not overheat with the hose hooked up. It feeds into the sea water pump intake hose (which is normally fed from a thru hull intake on the bottom of the boat. I put the boat in the water and removed the hose to the intake of the heat exchanger. Planned on getting 7.1L in 15 sec. Didn't get anything. Any suggestions ???
 
Alpha
Look
real close at the hose that goes between the bell housing and the transom assembly outside. There is a 3/4 inch black hose between the two. see if it's round and smooth with drive up and down, they have been known to develop a flat
 
sdwazup said:
........The boat will run all day and not overheat with the hose hooked up. It feeds into the sea water pump intake hose (which is normally fed from a thru hull intake on the bottom of the boat.......
Click to expand...

Ok, I've taken just a few sentences from your opening post above.

Alpha Drives take water in thru the lower unit and that is where the sea water pump is located also... Not using a thru hull fitting to a belt driven sea water pump.
So there is an inconsistency in what you are posting. Bravo Drive maybe? Some DIY aftermarket cooling system installed?
 
I looked up your engine parts by s/n and it says "Alpha/Bravo". And yes it does show a sea water pump on the engine. So now I'm confused.

You say water is picked up via a thru hull fitting. So what happens to the pump in the lower unit (Alpha)? The pump base is what keeps oil in the gearcase.
Are you sure that your water doesn't come in via the lower unit? Maybe there is a 2nd sea water pump belt driven, IDK. Maybe the pump shown is only applicable to Bravo installations? Maybe you have a Bravo?

Your posting suggests you have the closed cooling (heat exchanger/antifreeze in block). Maybe some type of weird hose routing and splitting the cooling duties between 2 water sources?

Bottom line; Assuming there is a lower unit pump too, that impeller may be junk.
 
Last edited:
Here's what I'm working with
1. the thru hull pick up
2. pick up under boat
3. alpha one outdrive
4. garden hose feed on the sea water intake line
5. garden hose hook up on transom
 

Attachments

  • b2.jpg
    b2.jpg
    613.5 KB · Views: 48
  • b3.jpg
    b3.jpg
    649.9 KB · Views: 48
  • b4.jpg
    b4.jpg
    381.5 KB · Views: 50
  • b5.jpg
    b5.jpg
    402.4 KB · Views: 45
  • b6.jpg
    b6.jpg
    305.1 KB · Views: 133
I was told there is no pump in the outdrive. There are two open hoses under the outdrive that would've come from the pump if I had one.
 
sdwazup said:
I was told there is no pump in the outdrive. There are two open hoses under the outdrive that would've come from the pump if I had one.
Click to expand...

Like I said above, ALL Alpha as well as earlier #1 drives to the beginning of time have a water pump in them. The 2 seals that keep oil in the drive are an integral part of the water pump base. The base is held in place by the cover. Now it is possible the water pump is assembled without the impeller & it would hold the oil seals in place. But why?????

The water is pumped up thru the drive, thru the bell housing to the 3/4" hose talked about by AllDodge in post #4 above. That hose is routed up the left side of the bellows to a "U" shaped tube that passes thru the transom plate. It appears just below the steering tiller arm inside the boat. From there it typically routes forward thru the power steering cooler to the water distribution at the front of the engine.

With that 3/4" hose disconnected inside, the boat will sink if it is below the water line & it is on most deep V hulls.

How long have you owned this thing?
 
Droid said:
Like I said above, ALL Alpha as well as earlier #1 drives to the beginning of time have a water pump in them. The 2 seals that keep oil in the drive are an integral part of the water pump base. The base is held in place by the cover. Now it is possible the water pump is assembled without the impeller & it would hold the oil seals in place. But why?????

The water is pumped up thru the drive, thru the bell housing to the 3/4" hose talked about by AllDodge in post #4 above. That hose is routed up the left side of the bellows to a "U" shaped tube that passes thru the transom plate. It appears just below the steering tiller arm inside the boat. From there it typically routes forward thru the power steering cooler to the water distribution at the front of the engine.

With that 3/4" hose disconnected inside, the boat will sink if it is below the water line & it is on most deep V hulls.

How long have you owned this thing?
Click to expand...
I've had it about a year and haven't taken it offshore once because I'm not comfortable with it. The first day I was getting a low water pressure alarm. So I rebuilt the sea water pump and it ran pretty good for a couple trips then the overheating started. Starting at the sea water pump I started looking for some kind of blockage. Clear thru the power steering cooler, fuel cooler, water distribution valve. When I got to the heat exchanger I found this (pic) and thought my troubles were behind me
 

Attachments

  • 20250802_091635 (1).jpg
    20250802_091635 (1).jpg
    779.5 KB · Views: 33
looks like impeller chunks...

you would be better off with a good ball valve in the 'flush hose', vs just relying on the cap to seal the inlet side off. make sure the flush hose is in good shape and its clamps are tight...the raw water pump creates a good bit of suction and will pull air there a marginal connection...and air in cooling water stream will cause an overheat.
 
Ok, I put in the ball valve which was a great idea. I've checked the sea water line from the raw pump to the heat exchanger for obstructions. The ball valve is closed and I still have no water flow. I don't get it ?
 
With the boat sitting in the water you have no flow.
Ok, only way that can happen is either the hoses are swapped on the pump, the impeller is trashed, or something is clogged
 
My engine running hot over 200 degrees at wide open throttle with closed cooling system, it's a sbc 383 with volvo penta drive. I changed water circulation pump and raw water pump but can't find my issue. It runs at 175 at 3500 rmp but over 4500 I'm running hot.
 
The hoses aren't switched on the pump because the inlets are different sizes. The sea water pump is new. The only part of the line that wasn't backflushed for obstructions is the check valve after the steering cooler. I'll do that now. I just wanna say that all you guys that know your s#@t
are really appreciated by the average joe like myself ! I'm so tired of getting burned by "marine mechanics"
 
AllDodge said:
With the boat sitting in the water you have no flow.
Ok, only way that can happen is either the hoses are swapped on the pump, the impeller is trashed, or something is clogged
Click to expand...
Your right again ! The check valve had impeller pieces in it. It didn't have hose clamps so it had to be cut out. How do I get it back in ?
 
Back
Top