Mercruiser 454 Overheating

[jasonhaywood]

Help please!! I have a 1998 454 Mag MPI completely rebuilt with a Bravo 1. Regular temp at idle. Overheats as soon as I get any rpms then cools down when I slow back down. Interestly I have bubbles in the clear flush "T". The T is right before the raw water impeller on the engine. I use the T to flush the engine because it is on a lift. The impeller is brand new and didn't fix the problem. Is there a seal or anything that could allow air to get in the system between the raw water pump and the foot. Thanks in advance for your thoughts!!

 

[Bondo]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

Is there a seal or anything that could allow air to get in the system between the raw water pump and the foot.

Ayuh,... There's a Couple of Hoses,+ several Fittings...
Start by follow 'em,+ doing a Visual Check...

There's factory Manuals for your motor,+ drive up in the Adults Only section at the top of this forum,...

Welcome Aboard,...
Oh, btw.... Drop the HELP thing,... Everybody here is lookin' for Help,.. That's Why they've all here...

 

[Don S]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

Assuming you are not getting air bubbles in your clear hose when at idle, there are 2 seals that could be under water when going slow, but above water when on plane.
1 is between the drive and the bellhousing. the other is an oring that seals the water passage between the upper and lower units.

 

[jasonhaywood]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

Hey guys!! Thanks for the fast response! I will drop the "help". Darn amatures!! The bubbles are in there all the time. They get worse as the rpms increase. I had the problem last year and it has gotton worse. My mechanice took off the foot and hose at the raw water pump and pressurized it and it held pressure. But the system has negative pressure put on it by the sucking of the raw water pump. So I think he might have been going down the wrong ally. Is there a diagram of this sytem anywhere. I do better with pictures! I'm an ex-aircraft mechanic in the AF, so I take instuctions very well. Thanks again for you help!!

 

[Bondo]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

so I take instuctions very well.

Ayuh,... This is where the Pictures are,....

There's factory Manuals for your motor,+ drive up in the Adults Only section at the top of this forum,...

 

[Don S]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

You aren't going to find any schematics of the cooling system from the outdrive to the pump. But if your mech. pressure checked from the pump inlet hose on the pump to the drive and it held pressure, then it's probably not sucking air either. Of course if he just pressureized it with an air compressor and listend for a leak, you may still have one and just can't hear it.
Assuming the system from the bellhousing where the drive hooks to the pump is good, then it has to be a leak in the drive.

This tee you have for flushing, are you sure it's not leaking when you have it capped off?

 

[jasonhaywood]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

I did make sure the T was not the issue. We also switched out the foot to a different unit ( for different reason). So the air is somewhere between where the foot hooks to the system and the clear T. When he pressurized it, he said he had a gauge on it and it held pressure for hours. I just thought of something. I will put the flush muff on the foot and apply water pressure. There should be no air. Start the engine and see if the air starts. Maybe? Can you hear me scratching my head? I now have a bald spot there. Although my wife says it's always been there!

 

[Don S]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

I will put the flush muff on the foot and apply water pressure. There should be no air

Wrong thinking, you will get a lot of air that way.
Put the drive in a large tub so the water is over the pickups and try it, then you should have no air.

 

[Fun Times]

Re: Mercruiser 454 Overheating- Please Help!

Re: Mercruiser 454 Overheating- Please Help!

Have you recently added or Installed anything new to the bottom or Transom area of the boat that could be in front of the sterndrives water flow that could create air bubbles while the boat is up on plane?

Does the drive have both the low water pick up holes at the front of the nose on the lower unit, Along with the side water pick up holes?

Or just one set of water pick up holes?