This must be one of the most interesting (and unluckiest)engine failures we’ve seen in the workshop for a while…

We recently received this 2012 Mazda R2 diesel engine into the workshop for a rebuild. It wouldn’t turn over and had obviously seized up. So we bolted it up onto a stand and pushed it into the stripping bay for disassembly and inspection. We noticed a few interesting things during the process.

Firstly, when we removed the valve cover, there were some large lumps of burnt oil around number two injector. There weren’t the soft lumps that you find in sludged-up engines. These ones you can squash between your fingers. These were hard and brittle, like old lumps of plastic.

Not surprisingly number two injector was seized into the head, and was quite difficult to remove. Once we finally got is out, it was black and tarred with strong evidence of blow-by. Next out came the seal, and as you can see in the pic, combustion gases had been escaping past the seal and into the injector tube. We think that these gases effectively cooked the engine oil in the valve train area of the head, creating the hard lumps.

  

Next, we removed the head. You can see from the pic that one piston had been hitting the cylinder head, leaving the marks on the head. This can only mean a conrod bearing failure. You see, as the conrod bearing fails, the clearance between the conrod and crankshaft journal increases, effectively lengthening the stroke. This is what allowed the piston to contact the cylinder head.

  So what caused the bearing failure? This became obvious once we had removed the sump. As you can see in the pics, the oil pump pick-up was completely blocked by, you guessed it, small hard lumps of burnt oil.

  

So here’s what happened. A failed injector seal allowed combustion gases into the valve train area of cylinder head. Over time, these gases cooked the engine oil and created hard lumps of burnt oil. The lumps broke up and made their way down into the sump as part of the engine’s normal oil circulation. Once they were in the sump, they became jammed in the oil pump pick-up, blocking oil circulation. Lack of oil caused the conrod bearing failure, seizing the engine.

A classic case of a two-dollar part causing a catastrophic failure…