The vast majority of the failed diesel fuel pumps, clogged injectors and restricted fuel systems are caused by water in the fuel. Basically the fuel is changed by both age and water. Diesel water damage can occur with regular diesel, and was a big issue decades ago. But the filling stations have long since fixed most of that problem. Rarely, we see a diesel vehicle with the fuel vent system damaged, and water has gotten into the fuel. Old biodiesel fuel undergoes chemical changes that create different contaminants. Just leave a sealed glass jar of good biodiesel on a top shelf in your kitchen for a year. Monitor the changes over time. Spider webs and the black and ruddy dots will slowly form. Imagine those rascals running through your hot engine and forming the contaminants that clogged your fuel sender check valves and your fuel filter. Water content adds to the problem. Biodiesel also pulls water out of the air. We occasionally find that microbes growing in the water have dissolved the paper fuel filter element. We sometimes see contaminants formed after the fuel filter. Now we have a fuel pump and injectors loaded with (bad word). The injector then sprays incorrectly, and the fuel does not explode correctly. Cylinder damage happens due to piston and ring wear from cylinder wall contamination and possible excessive side force. Damage occurs slowly, sometimes taking many thousands of miles, depending on the nature of the bad spray pattern.
If an injector starts to leak from diesel water damage and/or contamination, the fuel sprays out early, before the piston is near the top. The resultant explosion is now trying to drive the piston back down the way it came, making a knocking noise called nailing. Slight nailing can be hard to detect, and the piston and cylinder wears slowly. Damage occurs over some inestimable thousands of miles. Medium leakage causes nailing that is quite audible to the trained ear, and the piston and cylinder wears rapidly. Damage occurs in some hundreds of miles. Large injector leakage sounds like jack hammer hitting your motor many times per second. It is not a matter of cylinder wear, the engine breaks completely in a handful of miles. Karmakanix saw one older IDI diesel engine that would not longer drive for this reason. When we pulled off the head, one piston was no longer there, just some fragments attached to the rod’s wrist pin. The rest of the piston was just chunks in the oil pan. The guy’s neighbors had mentioned some noise, but he thought it was just a diesel. We have only seen one.
Almost every Volkswagen or Audi Tdi engine that we have ever seen worn down to the point of damage, smoking and running rough was killed by a damaged fuel injector or pump. The pump and/or injector was damaged by either contamination that formed in the fuel after the filter, or got through the filter because the water content damaged the paper filter element. And almost every rebuilt or replaced engine that failed within years was caused by not changing out the damaged fuel system that caused the first motor to fail. In the earlier VNT VW Mark IV models, injectors would fail to spray properly, causing nailing. In the next Pump Duse (PD) models, the corrosion from water would cause the fuel ports in the sides of injectors to close up, restricting the fuel supply to the injectors and closing down the fuel return ports. Since these injectors are driven by the camshaft and regulated electronically, the first indication is that the engine ECU has to give a longer injection open signal to get enough fuel in the cylinder to run properly, and there will be some acceleration power loss. Eventually, the engine will get to the point of stalling when returning to idle, and taking much long cranking times to start, as much as 10 to 15 seconds. In those engines, the transfer pump in the fuel tank, the steel fuel pipes, the tandem pump on the side of the head, and the passageway inside the head may get corroded beyond reuse. Usually the fuel cooler under the car is still functioning well enough to reuse, but the fuel tank itself may need cleaning or replacement. Almost all these issues that we have seen are from using biodiesel that got way too old. After 6 months, biodiesel captures water, and forms acids and slimes. We have not seen this form of water damage in any Common Rail (CR) diesels, likely since biodiesel should not be used in the engines except for a 5%, or BD 5, mixture.
Rarely do we see the pitted piston problem caused by water injected in large enough quantities to cause that form of diesel water damage. That was a problem last century. But we have seen glow plugs shattered from slight amounts of water getting injected with the fuel. Glow plug disintegration can also result from the nailing problem described above. Once the chunks of the broken glow plug finish rattling around inside the combustion chamber of the motor, they exit through the turbocharger. Sometimes the turbo is instantly finished off. Sometimes the damaged blades create a siren like howl. Makes you feel like the cops are following you everywhere.
Just a dozen times, we have seen severe diesel water damage from the results of a car hitting a big puddle and sucking water all the way to the combustion chamber. The situation is called hydrostatic lock. Those combustion chambers are the volume between the piston and the head, which is not squat in a diesel motor. The rod connected to that piston bends, shortening it. That cylinder now has very low power or does not even work at all. Sometimes the connecting rod gets bent enough that the piston hits the crankshaft rotating below it. The vibration is intense. With just a bit more water, the motor can explode completely. Karmakanix once saw a head blown completely off an early Rabbit diesel, lifted by almost 1/2″. So much for the exposed racing air filters and the low lying Cold Air Intakes that the Hot Rod magazines think you need. Not really recommended for the average Volkswagen and Audi Tdi owner.