Overheating at Low RPMs

[GabeNalley]

I have a 1993 4.3 Merc w/ Alpha 1 gen 2. I replaced the impellor at beginning of this season. Since then I have had a problem with overheating at low rpm. When RPM gets above 1000 engine begins to cool down. <br /><br />Can you put the vanes of the impellor in backwards? Would they turn themselves around w/ rotation? Any other suggestions before I'm elbow deep in gear lube again?

 

[Bondo]

Re: Overheating at Low RPMs

Nope,........ There Ain't No Backwards...........<br /><br />As Soon as you Touch the Key,........ There All pointed the Right Way............. <br /><br />When you changed it out,.. Were All the blades intact,..???.......<br />Missing blades always Go Somewhere,...... Usually Bad Places.........<br /><br />How Old are your Risers/ Elbows,..???.......<br />A Restriction on the Out end of the system is as Bad as Not pumping it In.........

 

[Richard Petersen]

Re: Overheating at Low RPMs

Well? Did you remove a complete impellor with all the blades the same full lenght? Or did you install the wrong type into your boat? Wrong one supplied? We really like to know.

 

[GabeNalley]

Re: Overheating at Low RPMs

Ran w/ no water. Didn't lose any blades, just rubber particles off the tips of the blades. The replacement was the one that it should have been, right size and # of blades. I tried to pump some antifreeze through it today and it didn't even get to the thermostat. Just came back out the exhaust.<br /><br />I assume all risers/elbows are original. I have only replaced grease seal, bellows, and impellor.<br /><br />When in the water, I can be in idle position or running position, either one will cool as long as it is above 1000 RPM. I tested the thermostat today as well, Not 100% accurate but it did open in boiling water.

 

[Richard Petersen]

Re: Overheating at Low RPMs

Boiling water test is not accurate. There should be 3#'s stamped in the thermostat. That is the temperature the thermostat STARTS to open very slightly. You need a thermometer to check it. There is also a small hole in the thermostat that lets the water circulate and open the thermostat quicker. Look for a tiny bit of stuff in the hole.

 

[rodbolt]

Re: Overheating at Low RPMs

fix the pump problem. leave the t-stat alone before you create one with that to. did you replace the entire kit? did you use the correct pump housing alignment tools and procedures. and that pump can be installed backwards, I have seen many that failed to flip. most shaft mounted or pulley driven pumps are bi directional, most outboard and stern drive pumps are not. your just not flowing enough water at enough pressure to kep the motor cool at low speeds.

 

[GabeNalley]

Re: Overheating at Low RPMs

I can't see where alignment is the issue. I has a housing around it that bolts in 3 places w/o agustment holes. Impellor locks to shaft w/ key.<br /><br />I guess I'll have to drop lower unit to inspect.<br /><br />T-stat was rated at 140.

 

[Don S]

Re: Overheating at Low RPMs

Recheck your pump impeller. The Alpha I Gen II impellers will not flip back to the proper direction. <br />When you are putting the housing over the impeller, turn the shaft clockwise. That will have the vanes going the right direction.<br />I assume you put new gaskets on housing when you had it apart.<br /><br />Look at page 75 on the following link.<br /><br /> http://www.boatfix.com/merc/Servmanl/14/14b3.pdf

 

[GabeNalley]

Re: Overheating at Low RPMs

Dropped the lower unit (waited for a warm day obvoiusly) and inspected. I found that the water tube couple didn't sit right when I put it back together. I assymbled as the diagram showed, but the line-up wasn't right. The housing was off center from the coupler. I toyed with it today and put it back together only to come back and look at the diagram and see that the couple is upside-down. Looking at the design of the thing it sat good on both ends and had more pressure then it would pushing the intake tube down into the coupler. What are the reprocations of this?

 

[Don S]

Re: Overheating at Low RPMs

What happens when that coupler is upside down is the seals aren't doing anything. You are just loosing a lot of the water that would normally be going to the pump. When the engine is under load and trying to produce power, it creates a LOT more heat that the pump (because of the leaks) can't push enough water to the engine for cooling. When you slow down, less heat buildup and it cools down.<br />Put it together correctly and take a test run. Will probably work fine.

 

[LuckyPenny]

Re: Overheating at Low RPMs

I hate to say this, but you may have two unrelated problems. First, it may not be the water pump at all. At higher RPM your engine will run hotter and require more water to cool it down. Since you are cooling at high RPM the water pump is providing water to your system. Check out the circulator pump. At low RPM this pump picks up the load that the lower unit pump cannot. At higher RPM a functioning lower unit pump can provide enough pressure to overcome a weak circulator pump.