Could a bad water pump w metal impeller cause engine to overheat when ac is on?

A friend has asked me help him repair his dodge hemi.  The engine is overheating when the ac is turned on.  He says he spoken to an expert mechanic who says he needs a new water pump.  I have  explained that there is nothing to wear out in a water pump except the bearings or the seal. When water starts getting through the seal it comes out the weep hole  in the bottom of the pump. Because the impeller is made out of metal, it never wears out so the pump always works even when it is leaking.  The  Engine has a new thermostat. But he's also told me that he regularly has to stop and put water in the radiator.

Therefore one of two things are happening. Either the leak is so bad that the radiator  isn't holding enough water to satisfy the engine needs and AC needs at the same time.  I believe the AC coil is integrated into the radiator core itself.  Low water would quickly cause the engine to overheat.  Or, the radiator is clogged up  and needs to be flushed out.  

He has been putting distilled water in the radiator to save money. I suggested that he stop that and start putting 100% antifreeze in the radiator. I believe antifreeze has a much higher boiling point. I might be wrong about that.   In any case, as long as the water pump is turning and the belt isn't slipping, the water pump, in my opinion, is pumping water like it should and the problem is elsewhere.  Am I correct? Am I missing anything?
Steve MeyerSystem Analyst and DeveloperAsked:
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Mal OsborneAlpha GeekCommented:
I had a water pump lose most of the "arms" on the impeller a few years ago. This was in a 2003 Holden Commodore, with a  "Buick V6" engine. So certainly can happen.

Coolant does indeed have a higher boiling point than water, HOWEVER it also has a lower specific heat.  Thus, for the same amount of heat put into it, coolant will increase more in temperature. For an engine, this means that 100% coolant will run hotter, and not cool the engine as well, at a given flow rate. Depending on the type of coolant, it will have around HALF the specific heat of water.

For racing purposes, engines often use straight, distilled water. This means far more corrosion and engine damage if it freezes, but racing motors usually do not live long enough to corrode out, and don't get left in snow drifts.

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Mal OsborneAlpha GeekCommented:
David Johnson, CD, MVPRetiredCommented:
Antifreeze is best in a 50/50 ratio with water.  
The question is where is this water going?
Don't forget that modern engines use a pressurized cooling system (15PSI is the norm)
Pure antifreeze (glycol) will reduce its anti-corrosion effectiveness since it needs oxygen to be effective.
The water pump may need new seals.. Again back to where is this water going to.. Hopefully not into the engine due to a bad gasket
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
This all helpful, but the argument i am having is that a water pump may leak, but unless the impeller is damaged, or the belt is slipping, the pump will always pump the same amount of water, contingent of course, on the pulley/engine RPM, and assuming there are no obstructions in the circuit.
Steve MeyerSystem Analyst and DeveloperAuthor Commented:
Why does distilled water cause corrosion.
David Johnson, CD, MVPRetiredCommented:
Because of the mixture of metals brass, iron, copper, aluminum used in the engine construction

There has to be water to pump.. Pressure test the system.
Mal OsborneAlpha GeekCommented:
Pure water does not contain the corrosion inhibitors that coolant does. Impurities get in, creating free ions, a "natural battery" is created, and any dissimilar metals rapidly corrode. On most engines, this will mean anything made out of aluminium having a very short life span. Engines without any aluminium parts in contact with coolant are far better in this regard, but this is unusual in post WWII vehicles. Usually heads and water pumps are alloy, and rot out quickly running straight water.

Only way to tell if the water pump impeller is damaged is to remove it and take a look. It is certainly possible.
Mal OsborneAlpha GeekCommented:
Here are a couple of pics of what you might see if you remove the water pump. This is not all that common, but perhaps the mechanic knows his stuff, and these engines might be prone to that.

Dr. KlahnPrincipal Software EngineerCommented:
A friend has asked me help him repair his dodge hemi.

Without wanting to seem callous:  If he's got the money to own, maintain and pay insurance on a Hemi, then he must have enough money to take the car into a Dodge shop and have them diagnose and fix the problem properly.  Some things are best done by professionals in that field.
Mal OsborneAlpha GeekCommented:
Bill PrewIT / Software Engineering ConsultantCommented:
How old is the vehicle?  
How old is the water pump?  
How long has the problem existed?

A relatively cheap diagnostic test is to replace the radiator cap as well.  If it's not holding the right amount of pressure then that will certainly result in loss of coolant.

Back to your original question, about impeller corrosion, it's certainly possible that the impeller could be corroded, especially if it's been run with 100% water for a long time.  Only way to know is either to pull it apart, or do a flow test with engine at operating temp.  Some shops used to have a short length of radiator "hose" they could insert inline and have a "propeller" in it that measured flow rate, but I haven't seen one of those in years.

Depending on the car and engine configuration, you may be able to feel the input and output hoses of the radiator to get a sense of if water is flowing well by their temps, after thermostat has opened.  But with many of today's engine compartments this is nearly impossible to get at without danger of physical injury.

Logically, if it was just an impeller problem, then that on it's own won't cause water leakage.  It will cause overheating, but that would usually exceed the pressure of the radiator cap and you'd see the water escaping into the reserve tank and / or out an overflow tube from the radiator.  Things to look for.

Steve MeyerSystem Analyst and DeveloperAuthor Commented:
This was helpful, we had a leaky pump, no corssion in the pump, no lost veins, the impeller is nylon, we replaced pump, had a bad fan clutch, replaced it, and added a gallon on antifreeze.  I think the main problem was the clutch, and I still think the radiator needs a flush although the old pump shown no corrosion and looked New on the inside.
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