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MerCruiser 228 V8 305 - Overheating Issue

J

Jean1899

New member
Hello everyone,

Since last year, I've been struggling with an overheating issue on my MerCruiser 228 V8 (5.0 liter). During one trip, the engine suddenly overheated—90°C (194°F)—without any prior warning. Since then, the problem keeps recurring.

Parts that have already been checked or replaced:​

  • Thermostat
  • Impeller
  • Both exhaust manifolds (risers)
  • Port side exhaust
  • Water pump
Unfortunately, none of this solved the problem.

What the mechanic did:​

Since I was out of ideas, I brought the boat to a mechanic. He:

  • Removed both port and starboard exhaust systems
  • Flushed all cooling passages with acid
  • Also flushed the engine block with acid
  • Performed a test run and said everything was fine—no overheating

Real-world test:​

Yesterday, I launched the boat again. Result:

  • The engine overheated again
  • It happened more slowly than before, but:
    • During extended cruising at around 3000 RPM, the temperature climbs steadily
    • If I briefly reduce throttle and then accelerate again, the temperature suddenly drops

My thoughts & question:​

I'm at a loss. The cooling system seems clean, yet the issue persists. Does anyone have an idea what else could be causing this? Possibly:

  • Air in the cooling system?
  • Faulty thermostat (even though replaced)?
  • Blockage inside the engine block?
  • Vacuum issue or backpressure in the exhaust?
Any advice or suggestions would be greatly appreciated!

Best regards,
Jean
 
Salt or fresh water?

When you replaced the impeller were there any missing pieces?

Plastic inserts at water intake broken/loose/clogged?

Solid stream of water being delivered to thermostat housing after power steering cooler?

Hose from lower unit to transom broken/cracked/kinked?

What year/serial number?

I ran across same issue years ago on salt water boat brought to Wisconsin. Found thermostat housing corroded thru. Had to lightly sand blast it to make it visible.

As far as air in system, it is an open system. By the nature of the system some air is always likely to be introduced via the lower unit pick ups from waves and such. But that air will flow back out. However, if I recall there was a service bulletin about the plastic inserts deforming & inducing turbulance/creating excess air to be injested.
 
1. What does the water hose from gimbal housing to transom assembly look like?

2. Any change when trimming drive up or down?

3. Air bubbles will cause temp issues so you can replace incomming water hose from inside transom to thermostat housing with a clear cheap hose purchase at say Home depot and look for bubbles (when in the water, not on garden hose). There may be a few bubbles but it should be 90% or more, No bubbles.

4. Get a different gage and confirm temp rise.

5. I have found chunks of cast iron floating inside of the thermostat housing causing temp fluxuations.

6. On top of impeller housing there should be a 1/4 inch thick rubber oring that should be in contact with housing. This help keep exhaust from entering impeller housing
 
The boat has only been operated in freshwater.
When I replaced the impeller, no parts were missing or damaged – the old impeller was completely intact.

I haven’t yet checked the water flow right after the power steering cooler.
The hose from the lower unit to the transom assembly appears to be in good condition – no visible cracks, kinks, or other signs of damage.

The plastic inserts at the water intake also seem to be intact at first glance, but I will inspect them more thoroughly to be sure.

I haven’t yet tested whether trimming the drive up or down causes any change. If there is a difference – what could that indicate?

As for the hose from inside the transom to the thermostat housing, I can’t easily replace it with a transparent one.
Would the engine need to be removed to access that hose, or is there another way to get to it?
 
Try not to generalize your answers.

How are we to know if all parts are in place if you dont specify a specific part asked about?

We do not know your expertise level.

So,

Is that 1/4 inch thick oring in place on top of impeller housing.

Can you 100% confirm that the gaskets used for the impeller housing are NOT torn?

Did you replace the impeller Base assembly? That has seals which keep gear oil in and water out of lower unit..

Unless your engine is burried you should be able to get the incoming water hose off the thermostat housing and follow it back to power steering cooler. Or add a 2-3 ft clear section to that hose for visual comfirmation of Little to no bubbles
 
If you reused the water pump housing, did you inspect it to make sure it wasn't grooved or otherwise damaged??
 
I`m thinking your sucking air when on plane because there is no water in the exaust cavity.Anytime you replace the impeller always install muffs to check the gaskets sealing properly. Once the cavitation plate is above water you can suck in air.
 
I installed a transparent hose into the cooling system.
At idle, only a few air bubbles are visible, but when running at around 3000 RPM, the bubbles increase significantly.
 
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