Mercruiser 5.7L Closed Cooled Radiator Cap Problem

[trelil]

I?m having an overheating at max load problem with my 1996 23? Trophy with a single closed cooled Mercruiser 5.7. I purchased the boat with a bad motor that had been partially disassembled and took it directly to my local marina and had a crate motor installed with new manifolds, risers and riser extensions. I can take the boat out all day long with one other person and never have a problem. The coolant temperature stays between 170-180 with a 160 degree thermostat installed. I never run over 4000 RPM. If I load up the boat with 4 or 5 guys, at 4000 RPM I get the temperature alarm and the gauge reads 190-200. If I drop the engine speed to under 3000 RPM?s the alarm quickly goes off. I have replaced the Alpha one, Gen 2 impeller and 160 degree thermostat with no change. I even sent the heat exchanger to a radiator shop to have it cleaned and pressure tested with no change. I believe my problem may lie with the radiator cap as the heat exchanger will expel coolant to the reservoir when the motor is running but never transfer it back. The radiator cap that came with the boat is rated for 14 psi but only had one rubber gasket installed, the small one that seats below the spring. I replaced it with an automotive cap also rated for 14 psi with both gaskets. Neither is allowing water to transfer from the reservoir back to the heat exchanger. Any ideas?

 

[Bt Doctur]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

The GEN II impeller may not be enough , most FWC systems use the engine mounted raw water pump ,not the one in the drive.

 

[trelil]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

Thanks for the response Doc. The GEN II impeller provides raw water thru the power steering cooler, heat exchanger and out the risers. The engine mounted pump recirculates coolant from the HX, thru the engine, therm housing and manifolds.

 

[wca_tim]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

Thanks for the response Doc. The GEN II impeller provides raw water thru the power steering cooler, heat exchanger and out the risers. The engine mounted pump recirculates coolant from the HX, thru the engine, therm housing and manifolds.

What he was meaning is that the water pump in the drive may not supply enough flow through the heat exchanger to cool the coolant you use to cool the engine, etc... especially if you're cooling the manifolds too. The belt driven raw water pumps push a lot more water through the heat exchanger providing a lot greater cooling capacity...

 

[NHGuy]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

Also, you should want to learn if the closed side of the system has a full load of coolant after it has completely cooled down. Any air? If the system is still full you can put a pressure tester on it to see if there is a leak elsewhere. You can also use a coolant strength checker to see if it has too much water, more water raises the freeze temperature.
If you find that it is "manufacturing" coolant you can get a chemical Freeze check kit. Those kits check for the presence of hydrocarbons in the coolant. We used them in the desert to find engine cracks, head problems & head gasket leaks on automobiles.
How does the coolant look and smell?

These steps are not as likely to be needed as the ones suggested with the better flowing raw water pump, but you can't know til you go through things.

Maybe your new motor produces more heat than the original one. If it has more power it does, and if the OE motor was the most that the sterndrive water pump could support that could be your reason why.

 

[trelil]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

Guys I really appreciate all of the assistance. After checking the boat this morning, it actually did siphon coolant from the reservoir and totally refilled the heat exchanger. Cleaning the HE may have done the trick. I will grab a few friends and test it today.

BTW, my cooling system is set up as indicated below. My original motor was rated at 250HP and my replacement at 260HP, could that have anything to do with it?

 

[Bt Doctur]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

That uses the impeller in the outdrive and like I said, it may not move enough water thru the system to cool the heat exchanger. most use a engine driven raw water pump.
http://www.boatfix.com/merc/Techbk/97/97hga6.pdf

 

[wca_tim]

Re: Mercruiser 5.7L Closed Cooled Radiator Cap Problem

one thing to consider is that if there is any crap in the hose between the drive and the heat exchanger, it can hamper water flow. I have gotten in the habbit of backflushing the hose when the drive is off, or disconnecting it from the transom assembly to clean it out. something always seems to come out... and water flow often noticably increases as a result..