I'm sure everyone out there religiously takes oil samples at every change so you'll know if you have a drop of coolant in your oil, Right? Yeah, Right!

So, here's a couple of more likely scenarios: You go to change your oil and find it's turned to milk. Maybe your just topping off your oil and find goopy snot under the fill cap. Or, worst case, you park your truck for the night and come out in the morning to find it won't roll over. What's going on you ask? Well, there's a good chance you developed a pin hole coolant leak in your liner. Sure, there maybe something else going on, but today's topic is pitted liners. How does this happen you ask? Why me?

Here's the basics:

1. Cylinder liners are made of Iron
2. Iron rusts
3. A running diesel engine causes tiny bubbles to form in the coolant around a liner
4. The bubbles pop and eat away at the soft rust (See figure 1 & 2)
5. Repeat steps 3&4 about a billion times and Ta-da! You now have a pin hole.
6. Coolant leaks through the hole and runs down the inside of the liner into your oil pan. It also can spray into the cylinder and sit on top of the piston

Here's the science:

1. Cylinder liners are made of Iron (FE)
2. When water, highly water diluted coolant, or non-diesel grade coolant is used, it causes the liner to turn into ferris oxide or rust
3. When the engine is running the pistons hit the liner and cause a high frequency vibration. As the liner vibrates it moves outwards towards the coolant and then back away from it. This causes vapor bubbles to form as the liner moves away. When the liner moves outwards again it then compresses and implodes these bubbles. The implosion will erode small holes in liner wall. This process is called Cavitation Corrosion or Erosion
4. The Cavitation Corrosion / Erosion can be found anywhere that coolant contacts the liner. It is usually found most heavily on the side of the liner that is contacted by the piston during the firing stroke
5. The high cylinder pressures will keep coolant from entering the cylinder while the engine is running and may force small amount of oil into the coolant. When the engine is not running the coolant pressure will force coolant into the cylinder. The coolant may sit on top of the piston, run down the cylinder wall into the oil reservoir or both

Time frame and Prevention:

There are too many factors to consider to determine how long it may take to erode a liner. I know of cases where it's happened in less than 300,000 miles. Some estimates say it can in less than 500 hours under severe conditions.

The are a couple of things you can do to help prevent liner pitting. Far and away the best method is to use a diesel specific coolant that contains a liner protective additive. You can also buy liner protective additive separately and add it to standard coolant. Liner protective additive can be called by a few different names. Supplemental Coolant Additive (SCA) or Diesel Coolant Additive (DCA) are a couple of the more popular. What SCA or DCA does is coat the liners with a protective coating. This helps to prevent the liner from rusting and it also transforms rust into FE3O5. FE3O5 is very hard compared to soft rust and protects against the imploding bubbles. If the concentration of DCA or SCA is correct to the amount of coolant, it will continue to re-coat the liners further preventing pitting. Be careful however, as too much additive can cause problems such as water pumps seals leaks. Several additive manufacturers offer test strips to help determine the level of additive in a system. They also provide recommendations on maintaining the correct level.

Other preventative measures that can help prevent liners from pitting are:

1. Plumbing the system to prevent hot spots. The hotter the coolant the easier the formation of vapor bubbles
2. Prevent air leaks into the cooling system. The less air in the coolant, the more difficult it is for vapor bubbles to form


Pitted liner cavitation corrosion  Pitted sleeve, liner corrosion erosion
Figure 1. Pitted liner from cavitation corrosion / erosion                        Figure 2. Imploding vapor bubbles erode the liner