2003 5.0 MPI stubborn overheat issue

Good Morning! I recently bought a 2003 Mercruiser 5.0L MPI with a Bravo 3 (prior use was all dry stack, last used 2 years ago), at the time it wasn't flowing any water through the exhaust and overheating within a few minutes on the hose so I took a gamble... I took it home, pulled the sea water pump and it only had a half a fin left. So recovered as much as I could and replaced the impeller. (There were several fins not found.) Put it back together and it ran for about 20 minutes on the hose with strong water flow from the exhaust relief ports. After 20 minutes the temps started to climb so I shut it down. I have since replaced the thermostat and pulled many hoses looking for any debris that might be clogging the cooling system. I found nothing so I put it in the water, got up on plane and 5 minutes later started overheating. We shut it down, anchored, let it cool and then headed back to the ramp at slightly above idle. It made it back the whole way sitting at 175F, so I let it sit at the dock to see what it would do and it ran another 30 minutes at idle sitting right at 175F. Then within 60 seconds it spiked to 200 and started alarming. So again shut it down and here we are. I'm going to pull the T fitting again (by the thermostat, I hear it often catches pieces of the impeller) and see if any pieces shifted under use. After that I'm kind of at a lost aside from pulling the manifolds and risers (which is a task I'm hoping not to have to tackle).

Anyone have any advice that might save me a ton of time and money?
TYIA

back flush the system and find the fins.

you may have to pull most of the hoses off to find where they went.

overheating is a lack of water flow in or out of the engine

Scott Danforth said:

back flush the system and find the fins.

you may have to pull most of the hoses off to find where they went.

overheating is a lack of water flow in or out of the engine

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Thank you Scott, excuse my ignorance on the backflushing. I've tried to dig around for info but it seems all over the place. What is the primary hose to pull for backflushing? I don't believe my engine has an oil cooler. I'm seeing recommendations to disconnect and shove the garden hose down the right (port) hose of the thermostat, best I can tell that is just going to circulate water in the normal route. The "T" I referred to above appears to come from the power steering cooler and up to the T which splits to the manifolds. I could also try flushing there back to the raw water discharge (which I presume I disconnect and leave in the bilge to catch everything).

Start at the hoses from the manifolds and flush water thru the system backwards to the raw water pump. Backflush

Trying to push water thru normally just keeps the bits of impeller stuck where they are

Could be pieces of impeller in the system but the problem I've had in the past (20+ years with a B3) is that the sea water pump housing becomes scored and is not effective at keeping the engine cool even after installing a new impeller.

I'm actually dealing with that now on a new (new to me) boat I bought last summer. Just had the same issue you described. I took the pump apart and the housing was very scored. I installed a different pump I had and hoping that it solves the issue. Weather hasn't agreed this week for a test.

Well I took every hose off that I could relatively easily get to and found a completed clogged line running from the bottom of the port manifold back to the water distribution pump, clogged completely full of sand. No impeller pieces found yet. I haven't been able to reach the power steering or fuel cooler yet but I'm going to try and contort myself enough to get those later this week.

However I did run the boat after clearing that line and was able to run for over an hour with no overheating. The catch was I couldn't go over 2200 rpm for more than 10 seconds or temps would start to push 200. I'm guessing those fins are still stuck in the coolers somewhere and allowing enough water to cool at lower speeds but restricting enough at higher speeds to cause the overheating? Let's hope I can get to those dang hoses!

If there is a plastic section near the circulation pump on the port side check that there are no pieces in it. I found some in it on the Donzi I had.
Most likely you have Bravoitis due to some of the sealing grommets on the incoming water passages
The ultimate cure is to put a sea suction fitting through the bottom of the hull. Then merge the hose from the drive with the incoming line before the suction side of the pump.

Morning everyone, just as an update and rehash: I have replaced the impeller, removed every cooling hose, and backflushed. We were out over the weekend and it's still running GREAT at 2200-2300 RPM (temps 165-170). When I get on the throttle temps climb quickly until I come off the throttle back to ~2200 and temps quickly drop.

I'm left with 3 thoughts of the remaining issues:
1. Risers and manifolds (however they are the dry joint style and appear *from the exterior* to be newer and it great condition)
2. The sea water pump housing itself (however when replacing the impeller I saw not obvious issues)
3. Bravoitis (corrected by the addition of a raw water intake instead of pulling the engine/drive)

Are there any other possibilities I'm missing? We didn't spend a ton on the boat so I don't want to sink an insane amount of time and money into only to be left with the same issue.

I'm still dealing a with a similar issue. The only difference being that I just switch the outdrive to Bravo 3.

I bought the boat last year and went out 1 time. Ran perfect with the original Bravo 1.

A few weeks ago swapped a Bravo 3 outdrive. Went out and had a similar overheating issue. I checked the pump and it looked to be a little scored so I installed the pump from my other boat that I recently rebuilt. Flow looked great on the ear muffs. I went for a test run last week and it overheated over 3k rpm again. It didn't immediately get colder at idle so I tapped on the thermostat housing. That seemed to lower the temp immediately. But I then went for another lap and the temp went back up. Again, I tapped on the housing and it cooled down.

My conclusion is that the thermostat is sticking. Got a new one to install. Weather hasn't been nice so I'm not sure when I can install and test again.

Jesse Walter said:

I'm still dealing a with a similar issue. The only difference being that I just switch the outdrive to Bravo 3.

I bought the boat last year and went out 1 time. Ran perfect with the original Bravo 1.

A few weeks ago swapped a Bravo 3 outdrive. Went out and had a similar overheating issue. I checked the pump and it looked to be a little scored so I installed the pump from my other boat that I recently rebuilt. Flow looked great on the ear muffs. I went for a test run last week and it overheated over 3k rpm again. It didn't immediately get colder at idle so I tapped on the thermostat housing. That seemed to lower the temp immediately. But I then went for another lap and the temp went back up. Again, I tapped on the housing and it cooled down.

My conclusion is that the thermostat is sticking. Got a new one to install. Weather hasn't been nice so I'm not sure when I can install and test again.

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I hope it's that simple because that's an easy fix! I have unfortunately already replaced the thermostat with a new 160F one and am still tackling the same problem :\

heathmc said:

I hope it's that simple because that's an easy fix! I have unfortunately already replaced the thermostat with a new 160F one and am still tackling the same problem :

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Not as familiar with your B3, but if you're still pulling your water through the drive you might be sucking air esp on plane. Might try a section of clear hose before the tstat and look for bubbles.

It may be possible that you have a leaking head gasket. The leak can overcome the pressure from the raw water pump.