Just replaced my almost 3 year old water pump due to a failing bearing.
A new radiator was put in the same day as the pump, and the system was flushed thoroughly before they were installed in 8/10.

yesterday, when I got the pump off, aside form an almost seized bearing, I noticed black pitting all over the impeller blades' surface and they felt quite rough.

All the coolant I drained out looked great aside from some bits of nonmetallic particulate matter here and there.

Granted I hadn't changed the coolant since the pump and rad were put in almost 3 years ago.
Was running 70% distilled water / 30% coolant fwiw.

What would pit/corrode the impeller like that?

I'd like to avoid it in the future.

 

Your answer is in your post. Not enough coolant concentration. Need to be 50/50.

 

This is a common problem with long-life antifreeze (OAT,HOAT & Dexcool) if not maintained, overheated,air in system tap water or wrong mix ratio. These coolants will cause several issues if not maintained.
When the coolant breaks down for any of the reasons I mentioned it will become highly corrosive and EAT metal and in the process of doing that it forms gas bubbles that will cause cavitation which will beat the hell out of the impeller.
It is prudent that you religiously maintain your coolant system. If you want to you can drain and flush and switch over to the old green conventional antifreeze and change coolant every 2 years and these "Dexcool" ( yes, they are all dexcool emulations) problems will go away.

 

Electrolysis is also a major factor.

 

Like cruiser said ! You NEED to have the right mixture. 50/50

 

I have been running 70/30 (More coolant) in the past 10 years or so in my vehicles. On my last flush/fill on the XJ about 3 years ago I ran 80/20. Used distilled water only.

The green stuff isn't like it was in the 80s. I think they removed some of the "love" in the 90s and later. I run it strong.

So far my coolant is a vivid green still. 250K miles. Planning on a new motor this summer.

 

Air in your system will cause this more than your ratio of water/coolant.

 

Please explain how air is more likely to cause this than the water/coolant ratio.

 

dexcool was reformulated to get rid of the issues it used to have. just sold a gm diesel prior to buying the jeep so had system flushed and did some research on this... you do need the 50/50 as stated above. get a meter and check the mixture too. it will tell you how far down it will protect you from freezing.. water alone is not a good coolant for cars. antifreeze helps with cooling as well as from freezing. its a protectant for your metal parts too and anti corrosion inhibitor.. lesson learned for ya

 

It's been my understanding the anti-freeze isn't good at cooling, it's purpose is to increase boiling point and decrease freezing point for the water in the system as well as reducing corrosion. Pure glycol (anti-freeze) in your system wouldn't cool your engine as well as pure water in the same system. But I could have misunderstood.

 

it could have been a bad impeller on the pump to begin with. ie poor metal used. the glycol itself will actually break down and turn into an acid with age. air is a bigger problem for the pitting described as it creates a cavitation like issue in which the air bubbles compress the explode casing the pits and eventual failure of the impeller much like a boat prop.

 

I'm not sure about increasing the boiling point but your other statements are correct. If your concentration of coolant is too high, your vehicle will run hotter since it doesn't transfer heat as well as water. The coolant is used to protect from corrosion and keep the water from freezing.

I have an old Porsche that I run on the track. We don't run coolant in track cars due to safety concerns. Dumped coolant on the track is as bad as dumped oil. If you hit it, you're in for a ride. I run distilled water and Redline Water Wetter in that car. It cools better and isn't nearly as slick as a coolant mixture. Of course I have to add coolant at the end of the season (winter) to keep things from freezing.

 

In the final reduction, you can't avoid it. The best you can do is delay it.

The problem is Galvanic corrosion - it's what happens when you have dissimilar metals in the presence of a liquid electrolyte for an extended period. (In fact, an old-school way of "quick-checking' your coolant status is to put one voltmeter probe in the coolant and touch the other to a clean spot on the radiator - more than 1.0VDC, flush & change coolant. Yes, it's an electrical process, which gains ground as the potential difference between coolant and "system" increases.)

You can delay it by flushing your system and renewing your coolant - there are additives in the coolant that are supposed to reduce Galvanic action and cut down on the PD/EMF in the system (which will slow the corrosive action.)

Since the impeller is mild steel, is usually goes first. At least it's cheaper to replace than just about everything else.

Another potential "delay" method would be to have sacrificial anodes, like in marine applications. This is easy enough to do - zinc will corrode first, so zinc slugs are typically used in cases like this. Then, you pull the zincs about once a year - if they're all but gone, replace them.

(You'll see these in home water heaters as well - zinc rods that screw in from the top. You shut off water flow to the heater, drain an inch or two worth to get some air space at the top, then unscrew the rods and check. If they're gone - or close to it - they get replaced. You can run without them for a little while, just put the plugs back in at the top while you're waiting for the replacements to show up. But, it's easier than dealing with a tank that has its weldments corrode...)

 

The radiator cap is the main item that raises the boiling point.....3 degrees for each pound. Antifreeze/coolant is for corrosion protection, pump seal lube and freeze protection.

 

Anybody else here run soluble oil, I do and have not had a problem with this.