We’ve got a couple of engine rebuilds in the workshop at the moment, and both came in badly corroded on the inside. The cause was the same for each. They’d been run on water instead of coolant.
These are stationary engines that run constantly at high RPM, and they were overheating out on site. When that happened, the operators topped them up with tap water from the hose. That part is fine. Water will get you out of trouble on the day. The problem was that the water stayed in there for months instead of being drained and replaced with coolant.
Tap water has no anti-corrosion or anti-rust inhibitors in it. So over three months of running hot at high RPM, it started eating away at everything it touched: the block, the heads, the water pump, and the oil cooler. The rust it creates then washes through the system and blocks the radiator from the inside.
Water causes a second problem too. It doesn’t carry heat away as well as coolant, and it boils at a lower temperature, so the engine overheats sooner than it should. On the water side of the piston, the heat forms tiny air pockets that build up and pop in the same spot over and over. Every time one pops, it takes a little metal with it. Left long enough, this eats right through the cylinder liner, the sleeve the piston runs inside. We’ve got photos on our Facebook page of liners corroded nearly all the way through from exactly this.
Once corrosion gets that far, a rebuild stops being a straightforward rebuild. It turns into block machining and crank polishing, and the bill climbs a long way.
The fix is simple. If you ever lose your coolant out on a job, run water to get yourself home. But the moment you’re back at the yard, drain it out and put proper coolant back in. That one habit will save you a fortune over the life of the engine.
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