1999 Mercruiser 7.4 (454) MPI Engine overheated today...

[Robert Marine]

I have a 1999 Mercruiser 7.4 (454) MPI Engine Connected to a Mercruiser Bravo I outdrive. Today while at 3/4 throttle I heard a whistling sound from the engine bay and it started to over heat. I raised the engine cover and traced the sound to the throttle body area. The flame arrestor had a layer of dark brown material adhered to it that I could scrape away with my finger. The noise continued and seemed to sound like an engine starving for air from my auto mechanic days gone by. Could a restriction at the throttle body/flame arrestor cause the noise and the engine to Overheat? This has never happened before. For now I am going to tear down the throttle body and get it cleaned up. Any other suggestions or similar experiences? Any help greatly appreciated.

 

[Robert Marine]

This is the flame arrestor with crud scraped off. The edges still indicate the material that was coating it when I looked at it.

 

[tpenfield]

Maybe an "IAC Whistle", but doubtful it was causing the overheat. I would clean the flame arrestor (remove it and clean it) and check to see if there is dirt/clogs in the duct of the IAC. (can't remember on that year 7.4 how the IAC is set up.)

Then see if you are still overheating and address it accordingly.

 

[alldodge]

Agree the overheat is not the TB
When was the last time the impeller was changed?

 

[Robert Marine]

Agree the overheat is not the TB
When was the last time the impeller was changed?

6 mos ago

 

[Robert Marine]

Maybe an "IAC Whistle", but doubtful it was causing the overheat. I would clean the flame arrestor (remove it and clean it) and check to see if there is dirt/clogs in the duct of the IAC. (can't remember on that year 7.4 how the IAC is set up.)

Then see if you are still overheating and address it accordingly.

IAC whistle? Identify IAC please...
Thanks, Bob

 

[Scott Danforth]

Idle Air Control

 

[alldodge]

Whats your motor serial number

 

[Robert Marine]

Whats your motor serial number

0L335439

 

[alldodge]

The IAC is part of the Throttle Body (TB), again not current issue

Get a short piece of clear hose, a hose clamp and double barbed fitting from the local hardware store. Remove hose going into thermostat housing that comes from the raw water pump. With boat in the water see if you see any bubbles.

Bubbles means there is an air leak

Would also suggest removing the hose between cool fuel module and oil cooler to see if the pump impeller may have come apart.

First time ever, I replaced my impeller and housing with brand new Merc pump and didn't get from the trailer to the dock before it overheated. The new impeller came out in pieces. Would not have believed it if I didn't do it my self

 

[Robert Marine]

The IAC is part of the Throttle Body (TB), again not current issue

Get a short piece of clear hose, a hose clamp and double barbed fitting from the local hardware store. Remove hose going into thermostat housing that comes from the raw water pump. With boat in the water see if you see any bubbles.

Bubbles means there is an air leak

Would also suggest removing the hose between cool fuel module and oil cooler to see if the pump impeller may have come apart.

First time ever, I replaced my impeller and housing with brand new Merc pump and didn't get from the trailer to the dock before it overheated. The new impeller came out in pieces. Would not have believed it if I didn't do it my self

This great Info! I can easily do all this...One question though what will be in the hose between fuel module and oil cooler?
...Pieces of Pump!? yikes...

 

[alldodge]

Maybe, but the oil cooler has several small tunes that would stop impeller pieces if it did come apart.

Could angle the hose into a 5 gallon bucket. The pump should fill the bucket in about 5 or so seconds

 

[Robert Marine]

Maybe, but the oil cooler has several small tunes that would stop impeller pieces if it did come apart.

Could angle the hose into a 5 gallon bucket. The pump should fill the bucket in about 5 or so seconds

If I suspect pieces of impeller in the oil cooler or other components how do I fish them out?

 

[dubs283]

If I suspect pieces of impeller in the oil cooler or other components how do I fish them out?

Remove the hoses and dig them out with a pick tool. Might need a flashlight and mirror. Or just remove the cooler and clean on a bench.

If you do find pieces/debris in the cooler it's a good idea to backflush the hoses leading to it

 

[Robert Marine]

This is a great forum. Gotten much more useful information than I would’ve ever suspected. Thank you all above for all of your input I’m sure I’ll figure this out.

 

[Robert Marine]

Remove the hoses and dig them out with a pick tool. Might need a flashlight and mirror. Or just remove the cooler and clean on a bench.

If you do find pieces/debris in the cooler it's a good idea to backflush the hoses leading to it

Roger that…